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U N I T 2Cell Structure and Function

Unit Opener Copy and Image to Come

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Inside the Cell

KEY CONCEPTS

■ The structure of cell components is closelycorrelated with their function.

■ Inside cells, materials are transported totheir destinations with the help of molecular“zip codes.”

■ Cells are dynamic. Thousands of chemicalreactions occur each second within cells;molecules constantly enter and exit acrossthe plasma membrane; cell products areshipped along protein fibers; and elementsof the cell’s internal skeleton grow andshrink.

In Chapter 1 you were introduced to the cell theory, whichstates that all organisms consist of cells and that all cells arederived from preexisting cells. Since this theory was initially

developed and tested in the 1850s and 1860s, an enormousbody of research has confirmed that the cell is the fundamentalstructural and functional unit of life. Life on Earth is cellular.

In a very real sense, then, understanding how an organismworks is a matter of understanding how cells are structured andhow they function. To drive this point home, recall fromChapter 1 that many eukaryotic organisms and virtually allbacteria and archaea are unicellular. In number of individualspresent, unicellular organisms dominate life on Earth. For re-searchers who study these species, understanding the cell is syn-onymous with understanding the organism as a whole. Even inplants, animals, and other multicellular eukaryotes, complexbehavior originates at the level of the cell. For example, your

ability to read this page begins with changes in light-sensitivemolecules located in cells at the back of your eyes. When thesemolecules change shape, they trigger changes in the membranesof nerve cells that connect your eyes to your brain. To under-stand complex processes such as vision, then, researchers oftenbegin by studying the structure and function of the individualcells involved—the parts that make up the whole.

Chapter 6 introduced an essential part of the cell: theplasma membrane. Thanks to the selective permeability ofphospholipid bilayers and the activity of membrane trans-port proteins, this structure creates an internal environmentthat is different from conditions outside the cell. Our tasknow is to explore the structures that are found inside themembrane and analyze what they do. We’ll focus on severalparticularly dynamic structures and processes and introducesome of the experimental approaches that biologists use to

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This cell has been treated with fluorescing molecules that bind to its fibrous skeleton.Microtubules (large protein fibers) are green; microfilaments (smaller fibers) are red. The cell'snucleus has been stained blue.

Chapter 7 Inside the Cell 129

Cell wall

Chromosome

Plasmamembrane

Cytoplasm

Flagellum

Ribosomes

Plasmids

1 µm

FIGURE 7.1 A Prokaryotic CellProkaryotic cells are identified by a negative trait—the absence of amembrane-bound nucleus. Although there is wide variation in thesize and shape of bacterial and archaeal cells, all such cells contain aplasma membrane, chromosomes, and ribosomes; almost all have astiff cell wall. Some prokaryotes have flagella and/or innermembranes where photosynthesis takes place.

understand them. Let’s begin by surveying the basic types ofcells, cell structures, and cell processes that biologists havedocumented to date.

7.1 What’s Inside the Cell?In Chapter 1 you read about the two fundamental types of cellsobserved in nature. Recall that eukaryotic cells have a membrane-bound compartment called a nucleus, while prokaryotic cells do not. In terms of morphology (“form”), then, species fallinto the two broad categories: (1) prokaryotes and (2) eukary-otes. But in terms of phylogeny, or evolutionary history, organ-isms fall into the three broad groups called (1) Bacteria, (2) Archaea, and (3) Eukarya. Members of the Bacteria and Ar-chaea are prokaryotic; members of the Eukarya—includingalgae, fungi, plants, and animals—are eukaryotic.

In the late seventeenth century, biologists began studying thestructure of cells with microscopes. Over time, improvements inoptics and cell preparation techniques allowed researchers tocatalogue the structures reviewed in this section. When electronmicroscopes became widely available in the 1950s, investigatorsdescribed the internal anatomy of these structures in more de-tail. More recent advances in microscopy have allowed investi-gators to videotape certain types of cell processes in living cells.

What have anatomical studies based on microscopy re-vealed? Let’s look first at the general anatomy of prokaryoticcells and eukaryotic cells, and then consider how the structuresthat have been identified help cells function.

Prokaryotic CellsFigure 7.1 shows the general structure of a prokaryotic cell. Formost bacterial and archaeal species, the plasma membrane en-closes a single compartment—meaning that the cell has few orno substructures delimited by internal membranes. Closer ex-amination reveals a series of intricate structures, however. Let’stake a look at a typical prokaryotic cell, starting at the outsideand working in.

As Chapter 6 pointed out, the cell membrane, or plasmamembrane, consists of a phospholipid bilayer and proteins thateither span the bilayer or attach to one side. Inside the mem-brane, the contents of a cell are collectively termed the cytoplasm

(“cell-formed”). Because the cytoplasm contains a high concen-tration of solutes, in most habitats it is hypertonic relative to thesurrounding environment. When this is the case, water enters thecell via osmosis and makes the cell’s volume expand. In virtuallyall bacteria and archaea, this pressure is resisted by a stiff cell

wall. Bacterial and archaeal cell walls are a tough, fibrous layerthat surrounds the plasma membrane. In many species the cellwall is made of a carbohydrate-protein complex calledpeptidoglycan or related substances (Figure 7.2). The pressure ofthe plasma membrane against the cell wall is about the same asthe pressure in an automobile tire. The cell wall protects the organism and gives it shape and rigidity. In addition, many

Plasmamembrane

Cell wall

Cytoplasm

200 nm

FIGURE 7.2 The Bacterial Cell WallIn bacteria and archaea, the cell wall consists of peptidoglycan orsimilar polymers that are cross-linked into tough sheets. The inside ofthe cell wall contacts the plasma membrane, which pushes up againstthe wall. The outside of the cell wall makes direct contact with theoutside environment, which is almost always filled with competitorsand predators.

130 Unit 2 Cell Structure and Function

bacteria have an additional protective layer outside the cell wallthat consists of lipids with polysaccharides attached. Lipid mole-cules that contain carbohydrate groups are termed glycolipids.

In the cytoplasm of a prokaryotic cell, the most prominentstructure is the chromosome. The prokaryotic chromosome

consists of a large DNA molecule associated with a small num-ber of proteins. In most species there is a single, circular chro-mosome, but other species have several circular chromosomes,and a few species—including the bacterium that causes Lymedisease—have one to several linear chromosomes. In prokary-otes and all other organisms, the sequence of nitrogenous basesin DNA acts as a code that contains the genetic, or heredity, in-formation. Stated another way, the primary structure of DNAcontains the instructions for making the proteins and othermolecules needed by the cell. A gene is a segment of DNA thatcontains the information for building an RNA molecule or apolypeptide. Genes are components of chromosomes.

Prokaryotic chromosomes are found in a localized area ofthe cell called the nucleoid. The genetic material is not separat-ed from the rest of the cytoplasm by a membrane, however. Inthe well-studied bacterium Escherichia coli, the circular chro-mosome is 500 times longer than the cell itself. This situation istypical in prokaryotes. To fit into the cell, the DNA doublehelix coils on itself with the aid of enzymes to form the highlycompact, “supercoiled” structure shown in Figure 7.3. Super-coiled regions of DNA resemble a rubber band that has beenheld at either end and then twisted.

Depending on the species and population being considered,prokaryotic cells may also contain one to about a hundred small,usually circular, supercoiled DNA molecules called plasmids. Plas-mids contain genes but are physically independent of the main,

cellular chromosome. In most cases the genes carried by plasmidsare not required under normal conditions; instead they help cellsadapt to unusual circumstances, such as the sudden presence of apoison in the environment. As a result, plasmids can be consid-ered auxiliary genetic elements. They are copied independently ofthe main chromosome and are passed along to daughter cellswhen the parent cell divides. Certain plasmid genes also allow acopy of the entire plasmid to be transferred from one cell to an-other. As a result, plasmids can spread through a population oreven be passed between species. Plasmids have been studied inten-sively because some carry genes that confer resistance to anti-biotics. One recently characterized plasmid carries genes that provide resistance to seven distinct antibiotics.

Two other prominent cell structures found in prokaryotesare ribosomes, which manufacture proteins, and flagella (singu-lar: flagellum), which power movement. Ribosomes are ob-served in all prokaryotic cells and are found throughout thecytoplasm. Bacterial ribosomes are complex structures consist-ing of a total of three distinct RNA molecules and over 50 dif-ferent proteins. These molecular components are organizedinto two major structural elements, called the large subunit andsmall subunit (Figure 7.4). It is not unusual for a single cell tocontain 10,000 ribosomes. Both ribosomes and prokaryotic fla-gella lack a membrane. Not all bacterial species have flagella,however. When present they are usually few in number and arelocated on the surface of the cell. Over 40 different proteins areinvolved in building and controlling bacterial flagella. At topspeed, flagellar movement can drive a bacterial cell throughwater at 60 cell lengths per second. In contrast, the cheetahqualifies as the fastest land animal but can sprint at a mere 25body lengths per second.

Supercoiled DNAin chromosome

20 nm

Large subunitof ribosome

Small subunitof ribosome

FIGURE 7.3 Bacterial DNA Is SupercoiledThe circular chromosomes of bacteria and archaea must becoiled extensively, into “supercoils,” to fit in the cell. Forexample, the Escherichia coli chromosome would be more than1 mm long if it were linear. This is about 500 times the lengthof the cell itself.

FIGURE 7.4 The Bacterial RibosomeBacterial ribosomes are made of RNA and protein molecules that areorganized into large and small subunits.


Photosyntheticmembranes

0.5 µm

FIGURE 7.5 Photosynthetic Membranes in BacteriaThe green stripes in this photosynthetic bacterium are infoldings ofthe plasma membrane. They are green because they contain thepigments and enzymes required for photosynthesis.

Prokaryotes lack a nucleus, but it is not correct to say thatno membrane-bound structures ever occur inside these cells.Many species contain membrane-bound storage containers,and extensive internal membranes occur in bacteria and ar-chaea that perform photosynthesis. The photosynthetic mem-branes arise as invaginations of the plasma membrane. As theplasma membrane folds in, either vesicles pinch off or the typesof flattened stacks shown in Figure 7.5 form. The internal partsof this membrane contain the enzymes and pigment moleculesrequired to convert the kinetic energy in sunlight into chemicalenergy in the form of sugar.

In addition, recent research indicates that at least one bac-terial species has an internal compartment that qualifies asan organelle (“little organ”). An organelle is a membrane-bound compartment in the cytoplasm that contains enzymesspecialized for a particular function. The bacterial organellethat was just discovered has proton pumps in its membraneand an acidic environment inside, where calcium ions are stored.

Recent research has also shown that bacteria and archaeacontain long, thin fibers that serve a structural role in the cell.All bacterial species, for example, contain fibers made from theprotein FtsZ. These filaments are essential for cell division totake place. Some species also have protein filaments that helpmaintain cell shape. Protein filaments such as these form thebasis of the cytoskeleton (“cell skeleton”).

Even though internal membranes and some cytoskeletalcomponents are found in prokaryotic cells, their extent palesin comparison with that in eukaryotes. When typicalprokaryotic and eukaryotic cells are compared side by side,three outstanding differences jump out: (1) Eukaryotic cellsare usually much larger; (2) they contain extensive amountsof internal membrane; and (3) they feature a diverse and dy-namic cytoskeleton.

1Ca2+2

Eukaryotic CellsThe lineage called Eukarya includes forms ranging from unicel-lular species to 100-meter-tall redwoods. Brown algae, redalgae, fungi, amoebae, and slime molds are all eukaryotic, asare green plants and animals.

The first thing that strikes biologists about eukaryotic cellsis how much larger they are on average than bacteria and ar-chaea. Most eukaryotic cells range from about 5 to indiameter, while most prokaryotic cells vary between 1 and

in diameter. A micrograph of an average eukaryotic cell, at the same scale as the bacterial cell in Figure 7.5, wouldfill this page. This difference in size inspired the hypothesis that when eukaryotes first evolved, they made theirliving by ingesting bacterial and archaeal cells whole. Stated another way, the evolution of large cell size is thought to havemade it possible for eukaryotic cells to act as predators—organisms that kill and consume other organisms. Hundreds ofeukaryotic species alive today still make their living by sur-rounding and taking in whole bacterial and archaeal cells.

The evolution of large cells has a downside, however. Ions andsmall molecules such as adenosine triphosphate (ATP), aminoacids, and nucleotides cannot diffuse across a large volumequickly. If ATP is used up on one side of a large cell, ATP fromthe other side of the cell would take a long time to diffuse to thatlocation. Prokaryotic cells are small enough that ions and smallmolecules arrive where they are needed via diffusion. In fact, thesize of prokaryotic cells is probably limited by the distance thatmolecules must diffuse or be transported inside the cell.

How do eukaryotic cells solve the diffusion problem? Theanswer lies in the numerous organelles observed in eukaryoticcells. In effect, the huge volume inside a eukaryotic cell is com-partmentalized into a large number of bacterium-sized parts.Because eukaryotic cells are subdivided, the molecules requiredfor specific chemical reactions are often located within a givencompartment and do not need to diffuse long distances to beuseful. But solving the diffusion problem is not the only advan-tage conferred by organelles:

• Compartmentalization of the cell allows incompatible chem-ical reactions to be separated. For example, new fatty acidsare synthesized in one organelle while excess or damagedfatty acids are degraded and recycled in a different organelle.

• Compartmentalization increases the efficiency of chemical re-actions. First, the substrates required for particular reactionscan be localized and maintained at high concentration withinorganelles. Second, groups of enzymes that work together canbe clustered on internal membranes instead of floating free inthe cytoplasm. Clustering these molecules increases the speedand efficiency of the reactions, because reactants have shorterdistances over which to diffuse or be transported.

Based on their morphological differences, prokaryotic cellscan be compared to small machine shops while eukaryotic cells

10 mm

100 mm


(a) Generalized animal cell

(b) Generalized plant cell

Centrioles

Nuclear envelope

Ribosomes

Lysosome

Peroxisome

Plasma membrane

Nuclear envelope

Nucleolus Nucleus

Chromatin

Rough endoplasmicreticulum

Smooth endoplasmicreticulum

Golgi apparatus

Vacuole (lysosome)

Peroxisome

Mitochondrion

Cytoskeletal element

Ribosomes

Nucleolus

Chromatin

Rough endoplasmicreticulum

Smooth endoplasmicreticulum

Golgi apparatus

Mitochondrion

Cytoskeletal element

Plasma membrane

Structures thatoccur in animal cellsbut not plant cells

Cell wall

Chloroplast

Structures thatoccur in plant cellsbut not animal cells

On average, prokaryotes are about 10times smaller than eukaryotic cells indiameter and about 1000 times smallerthan eukaryotic cells in volume.

Nucleus

FIGURE 7.6 Animal and Plant CellsGeneralized or “typical” (a) animal and (b) plant cells. (Compare with the prokaryotic cell, shown at true relative size at bottom left.)QUESTION Which organelles are unique to animal cells, and which organelles are unique to plant cells? Which are common to both?


2 µm

Nucleolus

Nuclear envelope

Heterochromatin

Euchromatin

FIGURE 7.7 The Nucleus Is the Eukaryotic Cell’s InformationStorage and Retrieval CenterThe genetic, or hereditary, information is encoded in DNA, which is acomponent of the chromosomes inside the nucleus.

Ribosome

100 nm

FIGURE 7.8 Ribosomes Are the Site of Protein SynthesisEukaryotic ribosomes are similar in structure to bacterial andarchaeal ribosomes—though not identical. They are comprised oflarge and small subunits, each of which contains both RNAmolecules and proteins.

resemble sprawling industrial complexes. The organelles andother structures found in eukaryotes are analogous to highlyspecialized buildings that act as factories, power stations, ware-houses, transportation corridors, and administrative centers.Figure 7.6 shows how organelles are arranged in a typical animalcell and plant cell. What are these structures, and what do they do?

The Nucleus The nucleus is among the largest organelles and ishighly organized (Figure 7.7). It is enclosed by a unique structure—a complex double membrane called the nuclear envelope, whichis studded with openings called nuclear pores. The inside sur-face of the nuclear envelope is associated with fibrous proteinsthat form a lattice-like sheet called the nuclear lamina. The nu-clear lamina stiffens the envelope and helps organize the chro-mosomes. Each chromosome occupies a distinct area inside thenucleus and is attached to the nuclear lamina and the inner sur-face of the nuclear envelope in at least one location. In eukary-otes, chromosomes are linear and consist of DNA that is tightlycomplexed with a series of ball-shaped histone proteins, form-ing a structure called chromatin. Some sections of each chromo-some are condensed into a highly compact, supercoiledstructure called heterochromatin; other sections are unwoundinto long, filamentous strands called euchromatin. The nucleusalso includes a distinctive region called the nucleolus, where theRNA molecules found in ribosomes are manufactured and thelarge and small ribosomal subunits are assembled. Section 7.2

discusses the structure and function of the nucleus, and partic-ularly the nuclear envelope, in more detail.

Ribosomes In eukaryotes, the cytoplasm consists of every-thing inside the plasma membrane excluding the nucleus; thefluid portion of the cytoplasm is called the cytosol. Many ofthe cell’s millions of ribosomes are scattered throughout thecytosol. The ribosomes shown in Figure 7.8 are comprised oftwo subunits, one small and one large. Each subunit is com-posed of several different proteins and one large RNA mole-cule. In eukaryotes the large subunit also contains two smallRNA molecules. (In prokaryotes the large subunit has just onesmall and one large RNA molecule.) Neither ribosomal sub-unit is enclosed by a membrane. When the two subunits cometogether, they form a complex molecular machine that synthe-sizes proteins.

Rough Endoplasmic Reticulum In addition to the ribo-somes found free in the cytosol, many ribosomes are associatedwith membranes. More specifically, hundreds of thousands ofribosomes are attached to a network of membrane-bound sacsand tubules called the rough endoplasmic reticulum, or rough

ER. Translated literally, endoplasmic reticulum means “inside-formed network.” Notice in Figure 7.6 that the ER is continu-ous with the outer membrane of the nuclear envelope. Fromthere, the layers of sacs extend into the cytoplasm.


The ribosomes associated with the rough ER are responsiblefor synthesizing proteins that will be inserted into the plasmamembrane, secreted to the cell exterior, or shipped to an organellecalled the lysosome. As they are being manufactured by ribo-somes, these proteins move to the interior of the sac-like compo-nent of the rough ER (Figure 7.9). The interior of any sac-likestructure in a cell or body is called the lumen. In the lumen of therough ER, newly manufactured proteins undergo folding andother types of processing.

The proteins produced in the rough ER have a variety of func-tions. Some carry messages to other cells; some act as membranetransport proteins or pumps; others are enzymes. The commontheme is that rough ER products are destined for transport to adistant destination—often to the surface of the cell or beyond.

Golgi Apparatus In many cases, the products of the rough ERpass through the Golgi apparatus before they reach their finaldestination. The Golgi apparatus consists of flattened, membra-nous sacs called cisternae (singular: cisternum), which arestacked on top of one another (Figure 7.10). The organelle alsohas a distinct polarity, or sidedness. The cis (“this side”) surfaceis closest to the rough ER and nucleus, and the trans (“across”)surface is oriented toward the plasma membrane. The cis side re-ceives products from the rough ER, and the trans side ships themout toward the cell surface. In between, within the cisternae, the

rough ER’s products are processed and packaged for delivery.Micrographs often show “bubbles” on either side of the Golgistack. These are membrane-bound vesicles that carry proteins orother products to and from the organelle. Section 7.3 analyzesthe intracellular movement of products in more detail.

Smooth Endoplasmic Reticulum Not all of the ER is asso-ciated with transport of material to Golgi sacs, and not all ERhas ribosomes attached. While parts of the ER that contain ribo-somes look dotted and rough in electron micrographs, the por-tions of the organelle that are free of ribosomes appear smoothand even. Appropriately enough, these parts of the ER are calledsmooth endoplasmic reticulum or smooth ER (Figure 7.11). Thesmooth ER membrane contains enzymes that are required forreactions involving lipids. Depending on the type of cell, theseenzymes may be involved in synthesizing specialized types oflipids needed by the organism or in breaking down hydropho-bic molecules that are poisonous to the cell. Smooth ER is themanufacturing site for phospholipids required for the plasmamembrane, and smooth ER also functions as a reservoir for cal-cium ions that act as a signal inside the cell.

The structure of endoplasmic reticulum correlates closelywith its function. Rough ER has ribosomes and functions pri-marily as a protein-manufacturing center; smooth ER lacks ribosomes and functions primarily as a lipid-processing center.

1Ca2+2

100 nm

cis face ofGolgi apparatus

trans face ofGolgi apparatus

Cisternae

Lumen of Golgiapparatus

Vesicle

Vesicles

FIGURE 7.10 The Golgi Apparatus Is a Site of Protein Processing,Sorting, and ShippingThe Golgi apparatus is a collection of flattened vesicles calledcisternae. The organelle has a cis face oriented toward the rough ERand a trans face oriented toward the plasma membrane.

200 nm

Roughendoplasmicreticulum

Lumen ofrough ER

Ribosomes

Freeribosomesin cytoplasm

FIGURE 7.9 Rough ER Is a Protein Synthesis and ProcessingComplexRough ER is a system of membrane-bound sacs and tubules withribosomes attached. It is continuous with the nuclear envelope andwith smooth ER.


200 nm

Smoothendoplasmicreticulum

Lumen ofsmooth ER

FIGURE 7.11 Smooth ER Is a Lipid-Handling Center and a Storage FacilitySmooth ER is a system of membrane-boundsacs and tubules that lacks ribosomes.

Together with the Golgi apparatus and lysosomes, the endo-plasmic reticulum forms the endomembrane system. The endomembrane (“inner-membrane”) system is the primarycenter for protein and lipid synthesis in eukaryotic cells.

Peroxisomes Peroxisomes are globular organelles that arefound in virtually all eukaryotic cells (Figure 7.12). They havea single membrane and grow and divide independently ofother organelles. Although different types of cells from thesame individual may have distinct types of peroxisomes, theseorganelles are united by a common function: Peroxisomes arecenters for oxidation reactions. In many cases the products ofthese reactions include hydrogen peroxide which ishighly corrosive. If hydrogen peroxide escaped from the per-oxisome, the would quickly damage organelle mem-branes and the plasma membrane. This is rare, however. Insidethe peroxisome, the enzyme catalase quickly converts hydro-gen peroxide to water and oxygen.

The various types of peroxisomes that exist contain differentsuites of enzymes. As a result, each is specialized for oxidizingparticular compounds. For example, the peroxisomes in yourliver cells contain enzymes that oxidize an array of toxins, in-cluding the ethanol in alcoholic beverages. The products of theseoxidation reactions are usually harmless and are either excretedfrom the body or used in other reactions. Other peroxisomescontain enzymes that catalyze the oxidation of fatty acids. Thesereactions result largely in the production of a molecule calledacetyl CoA, which is used for the synthesis of important mole-cules elsewhere in the cell. In plant leaves, specialized peroxi-somes called glyoxisomes are packed with enzymes that convert

H2 O2

1H2 O22,

one of the products of photosynthesis into a sugar that can beused to produce energy for the cell. Seeds do not perform photo-synthesis, so they lack this type of peroxisome. Instead, theyhave peroxisomes with enzymes that oxidize fatty acids to yieldglucose. The glucose is then used by the young plant as it beginsto grow. In each case, there is a clear connection between struc-ture and function: The enzymes found inside the peroxisomemake a specialized set of oxidation reactions possible.

Lysosomes The major structures involved in solid-waste pro-cessing and materials storage in the cell are called lysosomes.The size and shape of these organelles vary widely, and in thecells of plants, fungi, and certain other groups they are referredto as vacuoles. In animal cells, lysosomes function as digestivecenters (Figure 7.13). The organelle’s interior, or lumen, is acidic

Peroxisomemembrane

Peroxisomelumen

100 nm

FIGURE 7.12 Peroxisomes Are the Siteof Fatty-Acid ProcessingPeroxisomes are globular organelles witha single membrane.

Lysosome

Material beingdigested

500 nm

FIGURE 7.13 Lysosomes AreRecycling CentersLysosomes are usually oval or globularand have a single membrane.


Cell or particle Damagedorganelle

Macromolecules

Lysosome Lateendosome

Early endosome(fusing withvesicle fromGolgi)

Phagocytosis Autophagy Receptor-mediatedendocytosis

Cell

because a proton pump in the lysosome membrane importsenough hydrogen ions to maintain a pH of 5.0. This organellealso contains about 40 different enzymes. Each of these proteins is specialized for breaking up a different type of macromolecule—proteins, nucleic acids, lipids, or carbohydrates—into its component monomers. These digestive enzymes are collectivelycalled acid hydrolases because they hydrolyze macromoleculesmost efficiently at pH 5.0. In the cytosol, where the pH is about7.2, these enzymes are less active.

Figure 7.14 illustrates three ways that materials are deliv-ered to lysosomes in animal cells:

1. When phagocytosis (“eat-cell-act”) occurs, the plasmamembrane of a cell surrounds a smaller cell or a food par-ticle and engulfs it. The resulting structure is delivered to alysosome, where it is taken in and digested.

2. During autophagy (“same-eating”), damaged organellesare surrounded by a membrane and delivered to a lyso-some. There the components are digested and recycled.

3. Materials can also find their way into lysosomes as a result ofreceptor-mediated endocytosis. This process begins whenmacromolecules outside the cell bind to membrane proteinsthat act as receptors. More than 25 distinct receptors havenow been characterized, each specialized for responding to adifferent macromolecule. Once binding occurs, the plasmamembrane folds in and pinches off to form a membrane-bound vesicle called an early endosome (“inside-body”).Early endosomes undergo a series of processing steps that in-clude the receipt of digestive enzymes from the Golgi appara-tus and the activation of proton pumps that gradually lowertheir pH. In this way, early endosomes undergo a gradual

maturation process that may lead to the formation of a late

endosome and eventually a fully functioning lysosome.

Regardless of whether materials are delivered to lysosomesvia phagocytosis, autophagy, or receptor-mediated endocytosis,the result is similar: Molecules are hydrolyzed. The amino acids,nucleotides, sugars, and other molecules that result from acidhydrolysis leave the lysosome via transport proteins in the or-ganelle’s membrane. Once in the cytoplasm, they can be reused.

It is important to note, however, that not all of the materialsthat are surrounded by membrane and taken into a cell end upin lysosomes. Endocytosis (“inside-cell-act”) refers to any pinch-ing off of the plasma membrane that results in the uptake of ma-terial from outside the cell. Endocytosis can occur in three ways: (1) phagocytosis, (2) receptor-mediated endocytosis, and (3)pinocytosis (“drink-cell-act”). Pinocytosis brings fluid into thecytoplasm via tiny vesicles that form from invaginations of theplasma membrane. The fluid inside these vesicles is not trans-ported to lysosomes, but is used elsewhere in the cell. In addi-tion, most of the macromolecules that collect in earlyendosomes are selectively removed and used long before thestructure becomes a lysosome.

Compared with the lysosomes of animal cells, the vacuolesof plant and fungal cells are large—sometimes taking up asmuch as 80 percent of a plant cell’s volume (Figure 7.15). Al-though some vacuoles contain enzymes that are specialized fordigestion, most of the vacuoles observed in plant and fungal

1 µm

Vacuole

FIGURE 7.15 Vacuoles Are Storage CentersVacuoles are variable in size and function. Some contain digestiveenzymes and serve as recycling centers; most are large storagecontainers.

FIGURE 7.14 Three Ways to Deliver Materials to LysosomesMaterials can be transported to lysosomes after phagocytosis orautophagy. Endosomes created by receptor-mediated endocytosismay mature into lysosomes.


0.2 µm

Matrix

Cristae

Outer andinner membranes

FIGURE 7.16 Mitochondria Are Power-Generating StationsMitochondria are variable in size and shape, but all have a doublemembrane with sac-like cristae inside.

cells act as storage depots. In many cases, the stored material iswater, which maintains the cell’s normal volume, or ions suchas potassium and chloride But inside seeds, cellsmay contain a large vacuole filled with proteins. When germi-nation occurs, enzymes begin digesting these proteins to pro-vide amino acids for the growing individual. In cells that makeup flower petals or fruits, vacuoles are filled with colorful pig-ments. Elsewhere in the plant, vacuoles may be packed withnoxious compounds that protect leaves and stems from beingeaten by predators. The type of chemical involved varies byspecies, ranging from bitter-tasting tannins to toxins such asnicotine, morphine, caffeine, or cocaine.

Mitochondria The chemical energy required to build all ofthese organelles and do other types of work comes fromadenosine triphosphate (ATP), most of which is producedin the cell’s mitochondria (singular: mitochondrion). AsFigure 7.16 shows, each mitochondrion has two mem-branes. The outer membrane defines the organelle’s surface,while the inner membrane contacts a series of sac-likecristae. The solution inside the inner membrane is called themitochondrial matrix. In eukaryotes, most of the enzymesand molecular machines responsible for providing chemicalenergy in the form of ATP from food molecules are embed-ded in the membranes of the cristae or suspended in the ma-trix. Depending on the type of cell, from 50 to more than amillion mitochondria may be present.

1Cl-2.1K+2

1 µm

Stroma

Thylakoids

Outer and innermembranes

Granum

FIGURE 7.17 Chloroplasts Are Sugar-Manufacturing CentersIn photosynthesis, sunlight is converted to chemical energy in the form of sugar. The enzymes and other molecules required forphotosynthesis are located in membranes inside the chloroplast.These membranes are folded into thylakoids and stacked into grana.

Each mitochondrion contains its own small chromosome,independent of the main chromosomes in the nucleus. Mito-

chondrial DNA is a component of a circular and supercoiledchromosome that is similar in structure to bacterial chromo-somes. Mitochondria also manufacture their own ribosomes.Like most organelles, mitochondria can grow and divide inde-pendently of nuclear division and cell division.

Chloroplasts Most algal and plant cells possess an organellecalled the chloroplast, in which sunlight is converted to chemicalenergy during photosynthesis. The chloroplast has a doublemembrane around its exterior, analogous to the structure of a mi-tochondrion (Figure 7.17). Instead of featuring sac-like cristaethat connect to the inner membrane, though, the interior of thechloroplast is dominated by hundreds of membrane-bound, flat-tened vesicles called thylakoids, which are independent of theinner membrane. Thylakoids are stacked like pancakes into pilescalled grana (singular: granum). Many of the pigments, enzymes,and molecular machines responsible for converting light energyinto carbohydrates are embedded in the thylakoid membranes.Certain critical enzymes and substrates, however, are found out-side the thylakoids in the region called the stroma.

The number of chloroplasts per cell varies from none to sev-eral dozen. Like mitochondria, each chloroplast contains a cir-cular chromosome. Chloroplast DNA is independent of the main


genetic material inside the nucleus. Chloroplasts also grow anddivide independently of nuclear division and cell division.

Cytoskeleton The final major structural feature that is com-mon to all eukaryotic cells is an extensive system of proteinfibers called the cytoskeleton. As we’ll see in Section 7.4, the cy-toskeleton contains several distinct types of proteins and fibersand has an array of functions. In addition to giving the cell itsshape, cytoskeletal proteins are involved in moving the cell it-self and in moving materials within the cell.

The Cell Wall In fungi, algae, and plants, cells possess an outercell wall in addition to their plasma membrane (Figure 7.18). Animals, amoebae, and other groups lack this feature. Althoughthe composition of the cell wall varies among species and evenbetween types of cells in the same individual, the general plan issimilar: Rods or fibers composed of a carbohydrate runthrough a stiff matrix made of other polysaccharides and pro-teins. In addition, some plant cells produce a secondary cellwall that features a particularly tough molecule called lignin.Lignin forms a branching, cagelike network that is almost im-possible for enzymes to attack. The combination of cellulosefibers and lignin in secondary cell walls makes up most of thematerial we call wood.

How Does Cell Structure Correlate with Function?The preceding discussion emphasized how the structure ofeach organelle fits with its role in the cell. As Table 7.1 indi-cates, an organelle’s membrane and its complement of enzymescorrelate closely with its function. The same connection be-tween structure and function occurs at the level of the entirecell. Inside an individual plant or animal, cells are specializedfor certain tasks and have a structure that correlates with thosetasks. For example, the muscle cells in your upper leg are ex-tremely long, tube-shaped structures. They are filled with pro-tein fibers that slide past one another as the entire muscleflexes or extends. It is this sliding motion that allows yourmuscles to contract or extend as you run. Muscle cells are alsojam packed with mitochondria, which produce the ATP re-quired for the sliding motion to occur. In contrast, nearby fatcells are rounded, globular structures. They consist of littlemore than a plasma membrane, a nucleus, and a fat droplet.Neither cell bears a close resemblance to the generalized ani-mal cell pictured in Figure 7.6a.

To drive home the correlation between the overall structureand function of a cell, examine the transmission electron mi-crographs in Figure 7.19. The animal cell in Figure 7.19a isfrom the pancreas and is specialized for the manufacture andexport of digestive enzymes. It is packed with rough ER and

100 µm

Cytoplasmof cell 1

Plasmamembraneof cell 1

Plasmamembraneof cell 2

Cell walls

Cytoplasmof cell 2

FIGURE 7.18 Cell Walls Protect Plants and FungiPlants have cell walls that contain cellulose; in fungi the majorstructural component of the cell wall is chitin.

(a) Animal pancreatic cell: exports digestive enzymes

(b) Animal testis cell: exports lipid-soluble signals

(c) Plant leaf cell: manufactures ATP and sugar

(d) Plant root cell: stores starch

FIGURE 7.19 Cell Structure Correlates with FunctionEXERCISE In each cell, label ribosomes, rough ER, chloroplasts, thenucleus, smooth ER, mitochondria, vacuole, plasma membrane, andcell wall if they are visible.


TABLE 7.1 A Summary of Cell Components

Structure Function

Membrane Components

Nucleus Double (“envelope”); openings Chromosomes Genetic informationcalled nuclear pores Nucleolus Assembly of ribosome subunits

Nuclear lamina Structural support

Ribosomes None Large/small subunits Protein synthesisComplex of RNA and proteins

Endomembrane system Rough ER Single; contains receptors Network of branching sacs Protein synthesis and processing

for entry of selected proteins Ribosomes associated

Golgi apparatus Single; contains receptors for Stack of flattened cisternae Processing of proteinsproducts of rough ER

Smooth ER Single; contains enzymes for Network of branching sacs Lipid synthesissynthesizing phospholipids Enzymes for synthesizing lipids

Peroxisomes Single; contains transporters for Enzymes that catalyze oxidation Processing of fatty acidsselected macromolecules reactions

Catalase (processes peroxide)

Lysosomes Single; contains proton pumps Acid hydrolases (catalyze Digestion and recyclinghydrolysis reactions)

Vacuoles Single; contains transporters Varies—pigments, oils, Varies—coloration; storage of oils, for selected molecules carbohydrates, water, or toxins carbohydrates, water, or toxins

Mitochondria Double; outer contains enzymes Enzymes that catalyze ATP productionfor processing pyruvate; inner oxidation-reduction reactions, contains enzymes for ATP ATP synthesisproduction

Chloroplasts Double, plus membrane-bound Pigments Production of ATP and sugars sacs in interior Enzymes that catalyze via photosynthesis

oxidation-reduction reactions

Cytoskeleton None Actin filaments Structural supportIntermediate filaments Movement of materialsMicrotubules In some species: movement of

whole cell

Plasma membrane Single; contains transport and Phospholipid bilayer with transport Selective permeability—maintains receptor proteins and receptor proteins intracellular environment

Cell wall None Carbohydrate fibers running Protection, structural supportthrough carbohydrate or protein matrix

Golgi, which make this function possible. The animal cell inFigure 7.19b is from the testis and synthesizes the lipid-solublesignaling molecule called testosterone. This cell is dominatedby smooth ER, where lipid processing takes place. The plantcell in Figure 7.19c is from the leaf of a potato and is special-ized for absorbing light and manufacturing sugar; the cell inFigure 7.19d is from a potato tuber (part of an undergroundstem) and functions as a starch storage container. The leaf cellcontains hundreds of chloroplasts, while the tuber cell has aprominent storage vacuole filled with carbohydrate. In eachcase, the type of organelles in each cell and their size and num-ber correlate with the cell’s specialized function.

The Dynamic CellBiologists describe the structure and function of organelles andcells by a combination of tools and approaches. Light microscopesand transmission electron microscopes have allowed researchers tosee cells at increasingly high magnification. Microscopy allowed

biologists to characterize the basic size and shape of organelles andwhere they occurred in the cell; a technique called differential cen-

trifugation made it possible to isolate particular cell componentsand analyze their chemical composition. As Box 7.1 (page 140)explains, differential centrifugation is based on breaking cells apartto create a complex mixture and then separating components in acentrifuge. The individual parts of the cell can then be purified andstudied in detail.

Although these techniques have led to an increasingly so-phisticated understanding of how cells work, they have a limi-tation. Transmission electron microscopy is based on a fixed“snapshot” of the cell that is to be observed, and differentialcentrifugation is based on splitting cells into parts that are ana-lyzed independently. Neither technique allows investigators toanswer directly questions about how things move from place toplace in the cell or how parts interact. The information gleanedfrom these techniques can make cells seem somewhat static. Inreality, cells are dynamic.

BOX 7.1 How Does a Centrifuge Work?

For decades, the centrifuge was among themost common tools used by biologistswho study life at the level of moleculesand cells. It was vital to early studies of or-ganelles and other cell structures becauseit can separate cell components efficiently.A centrifuge accomplishes this by spinningcells in a solution that allows moleculesand other cell components to separate ac-cording to their density or size and shape.

The first step in preparing a cell sam-ple for centrifugation is to release the or-ganelles and cell components by breakingthe cells apart. This can be done by put-ting them in a hypotonic solution, by ex-posing them to ultrasonic vibration, bytreating cells with a detergent, or bygrinding them up. Each of these methodsbreaks apart plasma membranes and re-leases the contents of the cells.

The pieces of plasma membrane bro-ken up by these techniques quickly resealto form small vesicles, often trapping cellcomponents inside. The solution that re-sults from the homogenization step is amixture of these vesicles, free-floatingmacromolecules released from the cells,and organelles. A solution such as this iscalled a cell extract or cell homogenate.

When a cell homogenate is placed in acentrifuge tube and spun at high speed, thecomponents that are in solution tend tomove outward, along the dashed line inFigure 7.20a. The effect is similar to amerry-go-round, which seems to push yououtward in a straight line away from thespinning platform. In response to this out-ward-directed force, the solution contain-ing the cell homogenate exerts a centripetal(“center-seeking”) force that pushes thehomogenate away from the bottom of thetube. Larger, denser molecules or particlesresist this inward force more readily thando smaller, less dense ones and so reach thebottom of the centrifuge tube faster.

To separate the components of a cellextract, researchers often perform a se-ries of centrifuge runs. Steps 1 and 2 ofFigure 7.20b illustrate how an initialtreatment at low speed causes larger,heavier parts of the homogenate to movebelow smaller, lighter parts. The materi-al that collects at the bottom of the tube

is called the pellet, and the solutionand solutes left behind comprise thesupernatant (“above swimming”). Thesupernatant is placed in a fresh tube andcentrifuged at increasingly higher speedsand longer durations. Each centrifugerun continues to separate cell compo-nents based on their size and density.

To accomplish even finer separation ofmacromolecules or organelles, researchersfrequently follow up with centrifugation

at extremely high speeds. One strategy isbased on filling the centrifuge tube with aseries of sucrose solutions of increasingdensity. The density gradient allows cellcomponents to separate on the basis ofsmall differences in size and shape. Whenthe centrifuge run is complete, each cellcomponent comprises a distinct band ofmaterial in the tube. A researcher can thencollect the material in each band for fur-ther study.

Motor

(a) How a centrifuge works

(b) DIFFERENTIAL CENTRIFUGATION

(c) DENSITY GRADIENT CENTRIFUGATION

The solution in the tubeexerts a centripetal force,which resists movement ofthe molecule to the bottomof the tube.

Very large or densemolecules overcome thiscentripetal force morereadily than smaller, lessdense ones. As a result,larger, denser moleculesmove toward the bottomof the tube faster.Top

view

Sideview

Centrifugetube

Macromolecule

1. Start withuniform cellhomogenatein centrifugetube.

1. Add sample totube of variable-density solution.

2. Run centrifuge.Cell componentsseparate by densityinto distinct bands.

3. To extract specific cellcomponents for analysis,poke tube with needle andwithdraw a specific band.

2. Subject tube to low-speed centrifugation.Large componentssettle out below thesupernantant.

3. Transfersupernatant to newtube, and subjectit to medium-speedcentrifugation.

4. Transfersupernatant to newtube, and subject itto high-speedcentrifugation.

Supernatant

Pelletcontains largecomponents

Pelletcontainsmediumcomponents

Pelletcontainssmallcomponents

SampleLower-densitysolution

Higher-densitysolution

When the centrifuge spinsfrom position A to B, themacromolecule tends tomove along the dashed line.This motion pushes themolecule toward the bottomof the tube.

BA

FIGURE 7.20 Cell Components Can Be Separated by Centrifugation(a) Overhead view of a centrifuge, illustrating why cell components separate by being spun.(b) Through a series of centrifuge runs made at increasingly higher speeds, an investigator canseparate fractions of a cell homogenate according to size by differential centrifugation. (c) A high-speed centrifuge run can achieve extremely fine separation among cell componentsby density gradient centrifugation.

The amount of chemical activity and the speed of molecularmovement inside cells is nothing short of fantastic. Bacterial ribosomes add up to 20 amino acids per second to a growingpolypeptide, and eukaryotic ribosomes typically add two persecond. Given that there are about 15,000 ribosomes in eachbacterium and possibly a million in an average eukaryotic cell,hundreds or even thousands of new protein molecules can befinished each second in every cell. In the same amount of time,a typical cell in your body uses an average of 10 million ATPmolecules and synthesizes just as many. It’s not unusual for acellular enzyme to catalyze 25,000 or more reactions per sec-ond; most cells contain hundreds or thousands of enzymes. Aminute is more than enough time for each membrane phospho-lipid in your body to travel the breadth of the organelle or cellwhere it resides. The hundreds of trillions of mitochondria in-side you are completely replaced about every 10 days, for aslong as you live. The plasma membrane is fluid, and its compo-sition is constantly changing.

Because humans are such large organisms, it is impossiblefor us to imagine what life is really like inside a cell. At thescale of a ribosome or an organelle or a cell, gravity is incon-sequential. Instead, electrostatic attractions between mole-cules and the kinetic energy of motion are the dominantforces. At this level, events take nanoseconds, and speeds aremeasured in micrometers per second. Contemporary meth-ods for studying cells, including those featured in Box 7.2,capture this dynamism by tracking how organelles and mole-cules move and interact over time.

The rest of this chapter focuses on this theme of cellulardynamism and movement. To begin, let’s look at how mole-cules move into and out of the cell’s control center—the nu-cleus. Then we’ll consider how proteins move from ribosomesinto the lumen of the rough ER and then to the Golgi appara-tus and beyond. The chapter closes by analyzing how cy-toskeletal elements help transport cargo inside the cell ormove the cell itself.


Contemporary methods for studyingcells allow researchers to see specificmolecules moving inside living cells. Oneof the most popular techniques for tag-ging molecules of interest relies on a fluorescent molecule called green fluo-

rescent protein, or GFP. GFP is naturallysynthesized in jellyfish that fluoresce, oremit light. By affixing GFP molecules toanother protein and then inserting it intoa cell, investigators can follow its fateover time and even videotape its move-

ment. For example, recent studies haveused GFP to tag proteins that are secret-ed from the cell. Control experimentsshow that GFP does not affect the be-havior of these proteins. Researchersthen videotaped the GFP-tagged pro-tein’s transport from the rough ERthrough the Golgi apparatus and out tothe plasma membrane. This is cell biolo-gy: the movie.

To produce extremely high resolutionstill images of proteins that are tagged

with GFP or other fluorescing tags, re-searchers often rely on confocal mi-

croscopy. This technique is based onmounting live cells on a microscope slideand then focusing a beam of ultravioletlight at a specific depth within the speci-men. The fluorescing tag emits visible lightin response. A detector for this light isthen set up at exactly the position wherethe emitted light comes into focus. The re-sult is a sharp image of a precise plane inthe cell being studied (Figure 7.21).

BOX 7.2 Techniques for Studying the Dynamic Cell

(a) Conventional fluorescence image (b) Confocal fluorescence image

FIGURE 7.21 Confocal Microscopy Provides Sharp Images of Living CellsThe same living cells from the intestine of a mouse at the same magnification. (a) The conventional image is blurred,because it results from light emitted by the entire group of cells. (b) The confocal image is sharp, because it results fromlight emitted at a single plane inside the cells.


Nuclear pores

(c) Cross-sectional view of nuclear pore

(b) Surface view of nuclear envelope

(a) The nuclear envelope has a double membrane.

Heterochromatinin nucleus

Inner membrane

Nuclear envelope

Outer membraneCytoplasm

Nuclear matrix

Cytoplasm

NucleusNucleus

DNA in nucleus

Inner membrane

Outer membrane

Cytoplasm

Nuclear pore complex

Nuclear envelope

Rough ER

Ribosome

Nuclear matrix

FIGURE 7.22 The Structure of the Nuclear Envelope(a) The nuclear envelope is continuous with the endoplasmicreticulum. (b) Electron micrograph showing that the surface ofthe nuclear envelope is studded with nuclear pores. (c) Thedrawing (bottom) is based on electron micrographs of the nuclearpore complex.

7.2 The Nuclear Envelope: TransportInto and Out of the Nucleus

The nucleus is the information center of eukaryotic cells. It is acorporate headquarters, design center, and library all rolledinto one. Appropriately enough, its interior is highly organized.The organelle’s overall shape and structure are defined by themesh-like nuclear lamina, which also helps anchor each chro-mosome. The remainder of each chromosome occupies a well-defined region in the nucleus, and specific centers exist wherethe genetic information in DNA is decoded and processed. Atthese locations, large suites of enzymes interact to produceRNA messages from specific genes at specific times. Mean-while, the nucleolus functions as the site of ribosome synthesis.

Consistent with its role as information repository and pro-cessing center, the nucleus is separated from the rest of the cellby the nuclear envelope. Biologists began to understand exactlyhow the nuclear envelope is structured when electron mi-croscopy became available in the 1950s. As Figure 7.22a shows,the nuclear envelope has two membranes, each consisting of alipid bilayer. The inner membrane and the outer membrane areseparated by a space that is continuous with the lumen of the en-doplasmic reticulum. Later, electron micrographs showed thatthe envelope contains thousands of openings called nuclear

pores (Figure 7.22b). Because these pores extend through bothinner and outer nuclear membranes, they connect the inside ofthe nucleus with the cytoplasm. The pore itself consists of over50 different proteins. These molecules form an elaborate struc-ture called the nuclear pore complex (Figure 7.22c).

A series of experiments in the early 1960s showed that mole-cules travel into and out of the nucleus through the nuclear porecomplexes. The initial studies were based on injecting tiny goldparticles into cells and then preparing them for electron mi-croscopy. In electron micrographs, gold particles show up asblack dots. One or two minutes after injection, the micrographsshowed that most of the gold particles were in the cytoplasm. Afew, however, were closely associated with nuclear pores. Tenminutes after injection, particles were inside the nucleus as wellas in the cytoplasm. These data supported the hypothesis thatthe pores function as the doors to the nucleus. Follow-up workconfirmed that the nuclear pore complex is the only gate be-tween the cytoplasm and the nucleus and that only certain mole-cules go in and out. Passage through the nuclear pore is selective.

What substances traverse nuclear pores? DNA clearly doesnot—it never leaves the nucleus. But information coded in DNAis used to synthesize RNA inside the nucleus. Several distinctivetypes of RNA molecules are produced, each distinguished by sizeand function. For example, most ribosomal RNAs are manufac-tured in the nucleolus, where they bind to proteins to form com-pleted ribosomal subunits. Messenger RNAs, in contrast, carrythe information required to manufacture proteins out to the cy-toplasm, where protein synthesis takes place. To perform theirfunction, all of the various types of RNA move out of the nucle-us. Traffic in the other direction is also impressive. Nucleotide

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triphosphates that act as building blocks for DNA and RNAhave to enter the nucleus, as do the proteins responsible for copy-ing DNA, synthesizing RNAs, extending the nuclear lamina, as-sembling ribosomes, or building chromosomes (Figure 7.23).

To summarize, ribosomal subunits and various types ofRNAs exit the nucleus; proteins that are needed inside enter it.In a typical cell, over 500 molecules pass through each of the3000–4000 nuclear pores every second. The traffic is intense.How is it regulated and directed?

How Are Molecules Imported into the Nucleus?The first experiments on how molecules move through the nu-clear pore focused on proteins that are produced by viruses.Viruses are parasites that use the cell’s machinery to make copiesof themselves. When a virus infects a cell, certain of its proteinsenter the nucleus. Investigators noticed that if a particular aminoacid in one of these proteins happens to be altered, the viral pro-tein is no longer able to pass through the nuclear pore. This sim-ple-sounding observation led to a key hypothesis: Proteins thatare synthesized by ribosomes in the cytosol but are headed forthe nucleus contain a “zip code”—a molecular address tag thatmarks them for transport through the nuclear pore complex. Theidea was that viral proteins enter the nucleus if they have thesame address tag as normal cellular proteins have. This zip codecame to be called the nuclear localization signal (NLS).

A series of experiments on a protein called nucleoplasminhelped researchers better understand the nature of this signal.Nucleoplasmin plays an important role in the assembly of chro-mosomes and happens to have a distinctive structure: It con-sists of a globular protein core surrounded by a series ofextended protein “tails.” When researchers labeled nucleoplas-min with a radioactive atom and injected it into the cytoplasmof living cells, they found that the radioactive signal quicklyended up in the nucleus.

Figure 7.24 outlines how the nuclear localization signal innucleoplasmin was found. Researchers began by using enzymescalled proteases to separate the core sections of nucleoplasminfrom the tails. After separating the two components, the researchers labeled each part with radioactive atoms and in-jected them into the cytoplasm of different cells. When they

Nuclear envelope

Cytoplasm

Proteins neededin nucleus

mRNAs tocytoplasm

Nucleus

Nuclearenvelope

FIGURE 7.23 Molecules Move Into and Out of the Nucleusthrough the Nuclear PoresMessenger RNAs are synthesized in the nucleus and must beexported to the cytoplasm. Proteins needed in the nucleus aresynthesized in the cytoplasm and have to be imported to the nucleus.EXERCISE Label the inner and outer membranes of the nuclearenvelope. Label the nuclear pore.

Question: Where is the “Send to nucleus”zip code in the nucleoplasmin protein?

Hypothesis: The “Send to nucleus” zip code is in either the tail region or the core region of the nucleoplasmin protein.

Experimental setup:

Results:

Null hypothesis: The zip code is not on the nucleoplasminprotein itself, or there is no zip code.

Prediction:

Prediction of null hypothesis:

Conclusion:

1. Use proteaseto cleave tails offof nucleoplasminprotein core.

2. Attachradioactivelabel.

3. Inject labeledprotein fragmentsinto cytoplasm ofcell.

4. Wait, thenlocate labeledfragments ....

Tail fragmentslocated in nucleus

Core fragments stilllocated in cytoplasm

Core“Tails”

FIGURE 7.24 Where Is the “Send to Nucleus” Zip Code in theNucleoplasmin Protein?EXERCISE Without looking at the text, fill in the predictions andconclusion(s) in this experiment.


examined the experimental cells with the electron microscope,they found that tail fragments were transported to the nucleus.Core fragments, in contrast, remained in the cytoplasm. Thesedata suggested that the zip code must be somewhere in the tailpart of the protein.

By analyzing different stretches of the tail section, the biol-ogists eventually found a 17-amino-acid-long section that hadto be present to direct proteins to the nucleus. The biologiststherefore concluded that instead of consisting of five num-bers, the NLS zip code consisted of 17 specific amino acids inthe tail.

Follow-up work confirmed that other proteins bound forthe nucleus have similar localization signals, and that thesesignals interact with proteins called importins. Figure 7.25summarizes the current model for how nuclear import takesplace. Several different molecules are involved: the proteinbeing transported, an importin, ATP, guanosine diphosphate(GDP) or guanosine triphosphate (GTP), and a protein calledRan. GTP is similar in structure to ATP and has high potentialenergy. If you think of the protein as cargo, the importin as adelivery truck, and ATP as gas, then Ran is the unloadingcrew and GTP is their supervisor. More specifically, data sug-gest that when an importin in the cytoplasm binds to a mole-cule that has a nuclear localization signal, the importin/cargocomplex enters the nucleus along with Ran that has GDPbound to it. The movement of the cargo requires ATP. Inside,an enzyme exchanges the GDP for GTP. When this reactionoccurs, Ran’s conformation changes. It binds to the im-portin/cargo complex, causing the cargo molecule to drop off.Ran then escorts the importin back out to the cytoplasm.There the import sequence starts anew.

Work on nuclear import carries two general messages:

1. Movement is highly regulated. Although small moleculescan diffuse freely into and out of the nuclear pore complex,larger molecules can enter only if they contain a nuclear lo-calization signal.

2. Movement of large molecules is an energy-demanding,active process.

Do the same general principles hold when RNAs and other ma-terials move out of the nucleus?

How Are Molecules Exported from the Nucleus?After several decades’ worth of experiments, biologists havecome to a satisfying conclusion: Export of ribosome subunits,proteins, and other materials from the nucleus is almost exact-ly the reverse of import. In almost all cases, Ran and GTP areinvolved, as are shuttle proteins called exportins. Further, pro-teins that leave the nucleus have a specific zip code—a nuclearexport signal. Distinct types of exportins are specialized forbinding to the different types of materials and ferrying them tothe cytoplasm. The Ran molecules and exportins responsiblefor nuclear export cycle back and forth, into and out of the nu-cleus, just as they do during import. Like nuclear import, nu-clear export is both highly regulated and energy demanding.

Currently, biologists are focused on understanding how Raninteracts with proteins inside the nuclear pore complex as cargo moves in and out. Investigators are also trying to un-ravel how traffic is regulated to avoid backups and head-oncollisions. The goal is to understand the precise physicalmechanisms responsible for moving cargo into and out of thenuclear pore complex.

Nuclear envelope

Cytoplasm

Protein

Nucleus

1. Nuclear localizationsignal (NLS) on proteinbinds to importin.

5. GTP is hydrolyzed toGDP; importin dissociates.

2. Protein-importincomplex enters nucleusand binds to Ran-GTP.

3. Protein dissociates.

4. Importin + Ran-GTP complexmoves to cytoplasm.

Importin

GDPGTP

GTP

GTP

NLS

HOW PROTEINS ARE IMPORTED INTO THE NUCLEUS

+ +

Protein has been imported into nucleusRan-GTP

FIGURE 7.25 An Importin, Ran, and GDP Are Required to Import Proteins into the NucleusImportin, Ran, and GDP are recycled to the cytoplasm after they deliver cargo to the nucleus.


RNA 1. Protein entersER while beingsynthesized byribosome.

2. Protein exits ER,travels to cis faceof Golgi apparatus.

3. Protein entersGolgi apparatus andis processed as itmoves throughcisternae.

4. Protein exits Golgiapparatus at transface and moves toplasma membrane.

5. Protein issecreted from cell.

Rough ER

Golgi apparatus

Plasma membrane

cis face ofGolgi apparatus

trans face ofGolgi apparatus

THE SECRETORY PATHWAY: A MODEL

FIGURE 7.26 The Secretory PathwayHypothesisThe secretory pathway hypothesisproposes that proteins intended forsecretion from the cell are synthesizedand processed in a highly prescribed setof steps.

7.3 The Endomembrane System:Manufacturing and Shipping Proteins

The nuclear membrane is not the only place in cells wherecargo moves in a regulated and energy-demanding fashion. Forexample, Chapter 6 highlighted how specific ions and mole-cules are pumped into and out of cells or transported across theplasma membrane by specialized membrane proteins. In addi-tion, proteins that are synthesized by ribosomes in the cytosolbut are used inside mitochondria or chloroplasts contain spe-cial signal sequences, analogous to the nuclear localization sig-nal, that target the proteins for transport to these organelles.

Perhaps the most intricate of all manufacturing and shippingsystems, however, involves proteins that are synthesized in therough ER and move to the Golgi apparatus for processing, andfrom there travel to the cell surface or other destinations. Theidea that materials might move through the endomembranesystem in an orderly way was inspired by a simple observation.According to electron micrographs, cells that secrete digestiveenzymes, hormones, or other types of products have particularlylarge amounts of rough ER and Golgi. This correlation led tothe idea that these cells have a “secretory pathway” that startsin the rough ER and ends with products leaving the cell(Figure 7.26). How does this hypothesized pathway work?

George Palade and colleagues did pioneering research on thesecretory pathway with an experimental approach known as a

pulse-chase experiment. The strategy is based on providing ex-perimental cells with a large concentration of a labeled moleculefor a short time. For example, if a cell receives a large amount oflabeled amino acid for a short time, virtually all of the proteinssynthesized during that interval will be labeled. This “pulse” oflabeled molecule is followed by a chase—large amounts of anunlabeled version of the same molecule, provided for a longtime. If the chase consists of unlabeled amino acid, then the pro-teins synthesized during the chase period will not be labeled.The general idea is to mark a population of molecules at a par-ticular interval and then follow their fate over time. This ap-proach is analogous to adding a small amount of dye to a streamand then following the movement of the dye molecules.

In testing the secretory pathway hypothesis, Palade’s team fo-cused on pancreatic cells that were growing in culture, or in vitro.1

These cells are specialized for secreting digestive enzymes into thesmall intestine and are packed with rough ER and Golgi. The basicexperimental approach was to supply the cells with a 3-minutepulse of the amino acid leucine, labeled with a radioactive atom,followed by a long chase with nonradioactive leucine. Because theradioactive leucine was incorporated into all proteins being pro-duced during the pulse, it labeled them. Then the researchers

1The term in vitro is Latin for “in glass.” Experiments that are performedoutside living cells are done in vitro. The term in vivo, in contrast, is Latin for“in life.” Experiments performed with living organisms are done in vivo.

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prepared a sample of the cells for electron microscopy and autora-diography (see Chapter 4). When they examined cells immediatelyafter the pulse, they found the newly synthesized proteins insidethe rough ER (Figure 7.27a). Seven minutes later, most of the labeled proteins were in a Golgi apparatus or inside structurescalled secretory vesicles on the trans side of a Golgi apparatus(Figure 7.27b). After 80 minutes, most labeled proteins were in secretory vesicles or actually outside the cell (Figure 7.27c).

These results were consistent with the hypotheses that a secretory pathway exists and that the rough ER and Golgi apparatus function as an integrated endomembrane system.Clearly, proteins produced in the rough ER don’t float aroundthe cytoplasm aimlessly or drift randomly from organelle toorganelle. Instead, traffic through the endomembrane systemis highly organized and directed. Now let’s break the systemdown and examine four of the steps in more detail. The ribo-somes in rough ER are bound to the outside of the membrane.How do the proteins that they manufacture get into the lumenof the ER? How do they move from the ER to the Golgi ap-paratus? Once they’re inside the Golgi, what happens tothem? And finally, how do the finished proteins get to theirdestination? Let’s consider each question in turn.

Entering the Endomembrane System: The Signal HypothesisHow do proteins enter the endomembrane system? The signal

hypothesis, proposed by Günter Blobel and colleagues, predict-ed that proteins bound for the endomembrane system have azip code analogous to the nuclear localization signal. The ideawas that these proteins are synthesized by ribosomes that areattached to the outside of the ER and that the first few aminoacids in the growing polypeptide act as a signal that brings theprotein into the lumen of the ER.

This hypothesis received important support when researchersmade a puzzling observation: When proteins that are normallysynthesized in the rough ER are manufactured by naked ribo-somes in vitro—with no ER present—they are 20 amino acidslonger than usual. Blobel seized on these data. He claimed thatthe 20 amino acids are the “Send to ER” signal and that the sig-nal is removed inside the organelle. His group went on to identi-fy the exact sequence of amino acids in the ER signal sequence.

More recent work has documented the mechanisms respon-sible for receiving the send-to-ER signal and inserting the pro-tein into the rough ER (Figure 7.28). The action begins when aribosome synthesizes the ER signal sequence, which then bindsto a signal recognition particle (SRP) in the cytosol. An SRP is acomplex of RNA and protein that acts as a receptor for the ERsignal sequence. The com-plex then attaches to an SRP receptor in the ER membrane it-self. You can think of the SRP as a key that is activated by anER signal sequence. The receptor in the ER membrane is thelock. Once the lock and key connect, the rest of the protein issynthesized, and then the signal sequence is removed. The fin-ished polypeptide has one of two fates: (1) proteins that willeventually be shipped to an organelle or secreted from the cell enter the lumen of the rough ER; or (2) membrane proteinsthat remain in the rough ER membrane as they are being manufactured.

Once proteins are inside the rough ER or inserted into itsmembrane, they fold into their three-dimensional shape with

ribosome + signal sequence + SRP

(a) Immediately after labeling

(b) 7 minutes after end of labeling

(c) 80 minutes after end of labeling

Rough ER

Labeled proteins

Rough ER

Golgi apparatus

Secretory vesicles

Secretory vesicles

Labeled proteins

Secretory vesicles

Labeled proteins

Destination for secretedproteins (duct carriesmolecules away)

FIGURE 7.27 Results of a Pulse-Chase ExperimentThe position of labeled proteins immediately after the pulse of aminoacids, then 7 and 80 minutes later. QUESTION Do the data supportor contradict the secretory pathway hypothesis? Explain your answer.


2. Signal sequencebinds to signalrecognition particle(SRP).

3. Signal recognitionparticle binds toreceptor in ERmembrane.

4. Protein synthesiscontinues. Proteinenters ER. SRP isreleased.

5. Protein synthesisis complete. Signalsequence isremoved.

1. Signal sequenceis synthesized byribosome.

Protein

Receptor

SRPSignal sequence

Ribosome

Lumen ofrough ER

THE SIGNAL HYPOTHESIS

Cytosol

RNA

FIGURE 7.28 The Signal Hypothesis Explains How Proteins Destined for Secretion Enter the Endomembrane SystemAccording to the signal hypothesis, proteins destined for secretion contain a short stretch of amino acids that interactwith a signal recognition particle (SRP) in the cytoplasm. This interaction allows the protein to enter the ER.

Pro

tein

Car

boh

ydra

te g

roup

NH2

COOH

Asn

N-acetyl-glucosamine

Mannose

Glucose

This amino acid isusually asparagine

FIGURE 7.29 Glycosylation Adds Carbohydrate Groups to ProteinsWhen proteins enter the ER, most acquire the 14 sugar residuesshown here. Some of these sugars may be removed or others addedas proteins pass through the Golgi apparatus.

the help of chaperone proteins. In addition, proteins that enterthe lumen are acted on by enzymes that catalyze the addition ofcarbohydrate side chains. Because carbohydrates are polymersof sugar monomers, the addition of one or more carbohydrategroups is called glycosylation (“sugar-together”). The result-ing molecule is called a glycoprotein (“sugar-protein”). AsFigure 7.29 shows, proteins that enter the ER often gain a

specific carbohydrate that consists of 14 sugar residues. Thus,proteins are not only synthesized in the rough ER, they arefolded and modified by glycosylation. The completed glyco-proteins are ready for shipment to the Golgi apparatus.

Getting from the ER to the GolgiHow do proteins travel from the ER to the Golgi apparatus?Palade’s group thought they had the answer, based on datafrom the pulse-chase experiments that first confirmed the exis-tence of the endomembrane system. When labeled proteins ap-peared in a region between the rough ER and the Golgiapparatus, they appeared to be inside small membrane-boundstructures. Based on these observations, the biologists suggest-ed that proteins are transported between the two organelles invesicles. The idea was that vesicles bud off of the ER, moveaway, fuse with the membrane on the cis face of the Golgi ap-paratus, and dump their contents inside.

This hypothesis was supported when other researchers useddifferential centrifugation to isolate and characterize the vesi-cles that contained labeled proteins. Using this approach, inves-tigators have established that distinctive types of vesicles carryproteins from the rough ER to the Golgi apparatus and fromlayer to layer within the Golgi apparatus.

What Happens Inside the Golgi Apparatus?Recall from Section 7.1 that the Golgi apparatus consists of astack of flattened vesicles called cisternae, and that cargo entersone side of the structure and exits the other. It is still not clear,however, exactly how material moves through the stack. Thereis strong evidence that at least some molecules move among the


cisternae inside vesicles. But other data suggest that the cister-nae themselves mature and change over time, meaning that newcisternae are created at the cis face and old cisternae breakapart at the trans face. If so, then cisternae would have tochange in composition and activity over time. Figure 7.30 illus-trates these two hypotheses. Is each cisterna static except foroccasional additions or subtractions via vesicle delivery andshipment, or is the entire structure dynamic? The answer is notyet known, and the dichotomy is not necessarily absolute—both processes may occur to some degree.

Although the structure of the Golgi apparatus is stillsomewhat uncertain, its function is not. By separating indi-vidual cisternae and analyzing their contents, researchershave found that each cisterna contains a different suite of en-zymes that catalyze glycosylation reactions. As a result, pro-teins undergo further modification as they move from onecisterna to the next. Some proteins have sugar groups thatare phosphorylated in a vesicle near the cis face. Later, thecarbohydrate group that was added in the rough ER is re-moved. In other cisternae, various types of carbohydratechains are attached that may protect the protein or help it at-tach to surfaces.

How Are Products Shipped from the Golgi?The rough ER and Golgi apparatus are like an assembly line.Some of the products stay in the endomembrane system itself, re-placing worn-out molecules. But if proteins are processed to theend of the line, they will be sent to one of several destinations, in-cluding lysosomes, the plasma membrane, or the outside of thecell. How are these finished products put into the right shippingcontainers, and how are the different containers addressed?

Studies on enzymes that are shipped to lysosomes have pro-vided some answers to both questions. A key finding was thatlysosome-bound proteins have a phosphate group attached to aspecific sugar subunit on their surface, forming the compoundmannose-6-phosphate. If mannose-6-phosphate is removed fromthese proteins, they are not transported to a lysosome. This isstrong evidence that the phosphorylated sugar serves as a zipcode, analogous to the nuclear localization and rough ER signalsanalyzed earlier. More specifically, data indicate that mannose-6-phosphate binds to a protein in membranes of certain vesicles.These vesicles, in turn, have proteins on their surface that interactspecifically with proteins in the lysosomal membranes. In thisway, the presence of mannose-6-phosphate targets proteins forvesicles that deliver their contents to lysosomes.

(a) VESICLE TRANSPORT HYPOTHESIS: Cisternae are fixed, vesicles move.

(b) CISTERNAL MATURATION HYPOTHESIS: Cisternae move.

1. ER

2. Cluster ofvesicles

3. Cisternae don'tmove. Vesiclestransport materialsthrough cisternae.

4. Vesicles todestination

cis

trans

1. ER

2. Cluster ofvesicles

3. Cisternaemove—theymigratefrom cis totrans position.

4. Vesicles todestination

cis

trans

FIGURE 7.30 Two Hypotheses for How Materials Move through the Golgi ApparatusQUESTION Are these hypotheses mutually exclusive? Explain your answer.


2. Proteins are sortedin the Golgi when theybind to differentreceptors.

3. Transport vesiclesbud off the trans face ofthe Golgi and travel totheir destinations.

4. Proteins on vesiclesurface interact withreceptors at destination.

5. Vesicle deliverscontents.

1. In the endomembranesystem, proteins boundfor lysosomes or roughER are given differentcarbohydrate “tags.”Proteins bound forsecretion have built-inexport signal.

PROTEIN SORTING AND VESICLE TRANSPORT

Lumen ofGolgi apparatus

Return to the ERTo plasma membranefor secretion

“Tags”

Lysosome

Receptors

Transportvesicles

Cytosol

FIGURE 7.31 In the Golgi Apparatus, Proteins Are Sorted into Vesicles That Are Targeted to a DestinationSummary of the current model for how proteins are sorted into distinct vesicles in the Golgi apparatus and howthese vesicles are then targeted to their correct destination.

Figure 7.31 pulls these observations together into a com-prehensive model explaining how the products of the en-domembrane system are loaded into specific vesicles andshipped to their correct destination. Notice that transportvesicles bound for the plasma membrane secrete their con-tents to the outside. This process is called exocytosis (“out-side-cell-act”). When exocytosis occurs, the vesicle membraneand plasma membrane make contact and fuse. The two sets oflipid bilayers rearrange in a way that exposes the interior ofthe vesicle to the outside of the cell. The vesicle’s contentsthen diffuse away from the cell into the space outside the cell.In this way, cells in your pancreas deliver the hormone insulinto your bloodstream.

The general message of this section is that cells have sophisti-cated cargo production, sorting, and shipping systems. Proteinsthat are synthesized in the cytoplasm also have zip codes direct-ing them to mitochondria, chloroplasts, or other destinations.

If vesicles function like shipping containers for productsthat move between organelles, do they travel along some sortof road or track? What molecule or molecules function as thedelivery truck, and does ATP or GTP supply the gas? In gen-eral, what physical mechanisms are responsible for movingvesicles to their destination?

�CHECK YOUR UNDERSTANDING

Ions, ATP, amino acids, and other small molecules diffuserandomly throughout the cell, but the transport of proteinsand other large molecules is energy demanding and tightlyregulated. Proteins must have the appropriate molecular zipcode to enter or leave the nucleus, enter the lumen of therough ER, or become incorporated into vesicles destined forlysosomes or the plasma membrane. In many cases, proteinsand other types of cargo are shipped in vesicles that containmolecular zip codes on their surface. You should be able to(1) propose a hypothesis for how proteins are targeted tochloroplasts, and (2) outline an experiment that would testyour hypothesis.

7.4 The Dynamic CytoskeletonBased on early observations with light microscopes, biolo-gists viewed the cytoplasm of eukaryotic cells as a fluid-filledspace devoid of structure. As microscopy improved, however,researchers realized that the cytoplasm contains an extremelydense and complex network of fibers. This cytoskeletonhelps maintain cell shape by providing structural support. It’s


important to recognize, though, that the cytoskeleton is not astatic structure like the scaffolding used at construction sites.The fibrous proteins that make up the cytoskeleton move andchange to change the cell’s shape, to move materials fromplace to place, and to move the entire structure. Like the restof the cell, the cytoskeleton is dynamic.

As Figure 7.32 shows, there are several distinct types of cy-toskeletal elements: actin filaments (also known as microfila-ments), intermediate filaments, and microtubules. Each of theseelements has a distinct size, structure, and function. Let’s lookat each one in turn.

Actin FilamentsActin filaments are sometimes referred to as microfilaments be-cause they are the cytoskeletal element with the smallest diame-ter. As Figure 7.32 indicates, actin filaments are long, fibrousstructures made of a globular protein called actin. In animalcells, actin is often the most abundant of all proteins—typicallyit represents 5–10 percent of the total protein in the cell. Each ofyour liver cells contains about half a billion of these molecules.

Actin filaments form when individual actin moleculespolymerize. The completed structure resembles two strandsthat coil around each other. Because each actin monomer inthe strand is asymmetrical, the structure as a whole has a dis-tinct polarity. The two ends of an actin filament are differentand are referred to as plus and minus ends. Actin filamentstend to grow at the plus end, because polymerization occursfastest there.

Figure 7.33a shows a fluorescence micrograph of the actinfilaments in a mammalian kidney cell. Note that groups ofactin filaments are organized into long bundles or dense net-works and that actin filaments are particularly abundant justunder the plasma membrane. Whether they are arranged inparallel as part of bundles or crisscrossed in networks, individ-ual actin filaments are linked to one another by other proteins.In combination, the bundles and networks of actin filamentshelp stiffen the cell and define its shape.

Although actin filaments are an important part of the cell’sstructural support, it would be a mistake to think that they arestatic. Instead, actin filaments grow and shrink as actin sub-units are added or subtracted from each end of the structure.This phenomenon is called treadmilling, because the dynamicsof the fibers resemble those of a treadmill.

In addition, many cells have actin filaments that interactwith the specialized protein myosin. When ATP that is bound tomyosin is hydrolyzed to ADP, the “head” region of the myosinmolecule binds to actin and moves. The movement of thisprotein causes the actin filament to slide (Figure 7.34a). AsFigure 7.34b shows, the (ATP-powered) interaction betweenactin and myosin is the basis for an array of cell movements:

• Cell crawling occurs in amoebae, slime molds, and certaintypes of human cells. Cell crawling is based on three process-es: a directional extension of actin filaments that pushes theplasma membrane into bulges called pseudopodia (“false-feet”), adherence to a solid substrate, and a myosin-drivencontraction of actin filaments at the cell’s other end. In com-

Actin Keratin, vimentin, lamin, others

Strands in double helix Fibers wound into thicker cables

Protein subunits

Structure

Functions

α-tubulin and β-tubulin dimers

Hollow tube

maintain cell shape byresisting tension (pull)

motility via muscle contractionor cell crawling

cell division in animals

movement of organelles andcytoplasm in plants, fungi,and animals

MicrotubulesActin Filaments (Microfilaments) Intermediate Filaments

•

•

•

•

maintain cell shape byresisting compression (push)

motility via flagella or cilia

move chromosomesduring cell division

formation of cell plateduring plant cell division

move organelles

growth of plant cell walls

•

•

•

•

•

•

maintain cell shape byresisting tension (pull)

anchor nucleus and some other organelles

•

•

25 nm10 nm7 nm

Tubulin dimerKeratin subunitsActin subunit

FIGURE 7.32 The Cytoskeleton Comprises Three Types of FilamentsThe three types of filaments found in the cytoskeleton are distinguished by their size and structure, and by theprotein subunit of which they are made.


(a) Fluorescence micrograph of actin filaments in mammalian cells

(b) Fluorescence micrograph of intermediate filaments in mammalian cells

(c) Fluorescence micrograph of microtubules in mammalian cells

Nucleus

FIGURE 7.33 How Are Cytoskeletal Elements Distributed in the Cell?To make these micrographs, researchers attached a fluorescent compound to (a) actin, the protein subunit ofactin filaments, to (b) a protein found in intermediate filaments, and to (c) tubulin dimers.

Cytoplasmic streaming in plants

“Head”region

Myosin

Cell crawling Cell division in animals

Actin

(b) Actin-myosin interactions produce several types of movement.

(a) Actin and myosin interact to cause movement.

Actin-myosin interactionsmove cytoplasm around cell

When myosin's “head”attaches to actin and moves,the actin filament slides

Actin-myosin interactionspush cytoplasm forward

Actin-myosin interactionspinch membrane in two

Actin polymerizationcreates pseudopodia

FIGURE 7.34 Many Cellular Movements Are Based onActin-Myosin Interactions(a) When the “head” region of the myosin protein bindsto ATP or ADP, myosin attaches to actin and changesshape. The movement causes the actin filament to slide.(b) Actin-myosin interactions can move cells, divide cells,and move organelles and cytoplasm.

bination, the three events result in directed movement bywhole cells.

• Cytokinesis (“cell-moving”) is the process of cell division in an-imals. For these cells to divide in two, actin filaments that arearranged in a ring under the plasma membrane must slide pastone another. Because they are connected to the plasma mem-brane, the movement of the actin fibers pinches the cell in two.

• Cytoplasmic streaming is the directed flow of cytosol and or-ganelles around plant and fungal cells. The movement oc-curs along actin filaments and is powered by myosin.

In addition, extension of actin filaments is responsible forthe expansion of long, thin fungal cells into soil or rottingwood. The same mechanism causes structures called pollen

tubes to grow toward the egg cells of plants, so sperm can bedelivered prior to fertilization.

Intermediate FilamentsUnlike actin filaments and microtubules, intermediate filaments

(Figure 7.33b) are defined by size rather than composition.Many types of intermediate filaments exist, each consisting of adifferent protein. In many cases, different types of cells in thesame organism contain different types of intermediate fila-ments. This is in stark contrast to actin filaments and micro-tubules, which are made from the same protein subunits in alleukaryotic cells. In addition, intermediate filaments are notpolar; instead, each end of these filaments is identical. As a re-sult, intermediate filaments do not treadmill, and they are not


involved in directed movement driven by myosin or relatedproteins. Intermediate filaments serve a purely structural role ineukaryotic cells.

The intermediate filaments that you are most familiar withbelong to a family of molecules called the keratins. The cellsthat make up your skin and that line surfaces inside your bodycontain about 20 types of keratin. The presence of these inter-mediate filaments provides the mechanical strength requiredfor these cells to resist pressure and abrasion. Skin cells secreteanother 10 distinct forms of keratin. Depending on the locationof the skin cell and keratins involved, the secreted filamentsform fingernails, toenails, or hair.

Nuclear lamins, which make up the nuclear lamina layer in-troduced in Section 7.1, also qualify as intermediate filaments.These fibers form a dense mesh under the nuclear envelope. Re-call that in addition to giving the nucleus its shape, they anchorthe chromosomes. They are also involved in the breakup andreassembly of the nuclear envelope when cells divide. Some in-termediate filaments project from the nucleus through the cyto-plasm to the plasma membrane, where they are linked tointermediate filaments that run parallel to the cell surface. Inthis way, intermediate filaments form a flexible skeleton thathelps shape the cell surface and hold the nucleus in place.

MicrotubulesMicrotubules are composed of the proteins and

and are the largest cytoskeletal components in termsof diameter (Figure 7.33c). Molecules of and

bind to form dimers (“two-parts”), compoundsformed by the joining of two monomers. Tubulin dimers thenpolymerize to form the large, hollow tube called a microtubule.Because each end of a tubulin dimer is different, each end of amicrotubule has a distinct polarity. Like actin filaments, micro-tubules are dynamic and more likely to grow from one end thanthey are from the other. Microtubules grow and shrink inlength as tubulin dimers are added or subtracted.

Microtubules are similar to actin filaments in function aswell as structure. Both cytoskeletal elements provide structuralsupport, and both are involved in cell division. Although mi-crotubules are not involved in the physical division of the cell,they are essential for the directed movement of chromosomesto each of the two resulting cells. In animals and fungi, the mi-crotubules involved in chromosome movement emanate from astructure called the centrosome. Distinctive structures calledcentrioles are found inside centrosomes (Figure 7.35). Centri-oles may help organize microtubules; however, they are not es-sential for cell division to occur. In plants and many othereukaryotes, a region called the microtubule organizing center

performs the same function as the centrosome.Microtubules are involved in many other types of cellular

movement as well. For the remainder of this chapter, we’llfocus on how microtubules function in moving materials insidecells and in moving the entire cell.

b-tubulina-tubulin

b-tubulina-tubulin

Studying Vesicle Transport Materials are transported to awide array of destinations inside cells. To study how this move-ment happens, Ronald Vale and colleagues focused on a cellcalled the giant axon that is found in squid. The giant axon isan extremely large nerve cell that runs the length of a squid’sbody. If the animal is disturbed, the cell signals muscles to con-tract so the individual can jet away to safety.

The researchers decided to study this particular cell for threereasons. First, the giant axon is so large that it is relatively easyto see and manipulate. Second, signaling molecules are synthe-sized in the cell’s ER and then transported in vesicles down thelength of the cell. As a result, a large amount of cargo moves along distance. Third, the researchers found that if they gentlysqueeze the cytoplasm out of the cell, vesicle transport still oc-curs in the cytoplasmic material.

In short, the squid giant axon provided a cell-free systemthat could be observed and manipulated efficiently. What didthe biologists find out?

Microtubules Act as “Railroad Tracks” To watch vesicletransport in action, researchers mounted a video camera to amicroscope. As Figure 7.36 shows, this technique allowedthem to document that vesicle transport occurred along a fil-amentous track. A simple experiment convinced the groupthat this movement was an energy-dependent process. If theydepleted the amount of ATP in the cytoplasm, vesicle trans-port stopped.

To identify the type of filament involved, the biologists mea-sured the diameter of the tracks and analyzed their chemicalcomposition. Both types of data indicated that the tracks consist-ed of microtubules. Microtubules also appear to be required formovement of materials elsewhere in the cell. If experimental cellsare treated with a drug that disrupts microtubules, the movementof vesicles from the rough ER to the Golgi apparatus is impaired.

The general message of these experiments is that transportvesicles move through the cell along microtubules. How? Dothe tracks themselves move, like a conveyer belt, or are vesiclescarried along on some sort of molecular truck?

200 µm

FIGURE 7.35 Centrosomes Are a Type of Microtubule OrganizingCenterMicrotubules emanate from microtubule organizing centers, which inanimals are called centrosomes. The centrioles inside a centrosomeare made of microtubules.


Microtubule

tracks

Microtubule

tracks

(a) Micrograph image

(b) Video image (at higher magnification)

Vesicles

FIGURE 7.36 Vesicles Move along Microtubule TracksTransport vesicles moving along microtubules. The images are ofextruded cytoplasm from a squid giant axon. (a) An electronmicrograph that allowed researchers to measure the diameter of thefilaments and confirm that they are microtubules. (b) A slightly fuzzybut higher-magnification videomicroscope image, in whichresearchers actually watched vesicles move.

(a) Structure of kinesin

Tail

Stalk

Head

(b) Kinesin “walks” along a microtubule track.

Transportvesicle

Kinesin

Microtubule

ATP ATP

+ end− end

ADP + Pi+ADP Pi

FIGURE 7.37 A Motor Protein Moves Vesicles along Microtubules(a) Kinesin has three major segments. (b) The current model depicting how kinesin “walks”along a microtubule track to transport vesicles. The two head segments act like feet thatalternately attach and release in response to the gain or loss of a phosphate group.

A Motor Protein Generates Motile Forces To study the wayvesicles move along microtubules, Vale’s group set out to tear thesquid axon’s transport system apart and then put it back together.To begin, they assembled microtubule fibers from purified

and Then they used differential centrifuga-tion to isolate transport vesicles. But when they mixed purifiedmicrotubules and vesicles with ATP, no transport occurred.

b-tubulin.a-tubulin

Something had been left out—but what?To find the missing element or elements, the researchers pu-

rified one subcellular part after another, using differential cen-trifugation, and added it to the system. Through trial and error, they found something thattriggered movement. After further purification steps, the re-searchers finally succeeded in isolating a protein that generat-ed vesicle movement. They named the molecule kinesin, fromthe Greek word kinein (“to move”). Like myosin, kinesin is amotor protein that converts chemical energy in ATP into me-chanical work, just as a car’s motor converts chemical energyin gasoline.

Biologists began to understand how kinesin works whenX-ray diffraction studies similar to those that revealed thehelical nature of DNA revealed the three-dimensional struc-ture of kinesin. As Figure 7.37a shows, the protein consistsof two intertwined polypeptide chains. It has three major re-gions: a head section with two globular pieces, a tail, and astalk that connects the head and tail. Follow-up studies con-firmed that the two globular components of the head bind tothe microtubule. The tail region binds to the transport vesi-cle. The kinesin molecule is like a delivery person who car-ries transport vesicles along microtubule tracks. Cellscontain a number of different kinesin proteins, each special-ized for carrying a different type of vesicle.

How does kinesin move? More detailed studies of thisprotein’s structure indicated that each of the globular com-ponents of the molecule’s head has a site for binding ATP aswell as a site that binds to the microtubule. To pull these ob-servations together, biologists propose that kinesin trans-ports vesicles by “walking” along a microtubule. The idea isthat each part of the head region undergoes a conformation-al change when it binds ATP. As Chapter 3 showed, thesetypes of shape changes often alter the activity of a protein. AsFigure 7.37b shows, the ATP-dependent conformationalchange in kinesin results in a step forward. As each head

microtubule + vesicle + ATP


(a) TEM of axoneme

(b) Diagram of axoneme

Microtubule doublet

Plasma membrane

Central pair

Outer doublet

Dynein

Bridge

Spoke

75 nm

FIGURE 7.39 The Structure of Cilia and Flagella(a) Transmission electron micrograph of a cross section through anaxoneme. (b) The major structural elements in cilia and flagella. Themicrotubules are connected by bridges and spokes, and the entirestructure is surrounded by the plasma membrane. EXERCISE Labelthe arrangement of microtubules"9 + 2"

alternately binds and hydrolyzes ATP, the protein and itscargo move down the microtubule track.

In short, kinesins move molecular cargo to destinationsthroughout the cell. They are not the only type of motor pro-tein active inside cells, however. Recall that myosin causes actinfilaments to slide, resulting in the movement of cells or cyto-plasm. Myosin is also involved in the movement of organellesalong tracks made of actin. And a third motor protein, dynein,powers the transport of certain organelles as well as swimmingmovements that move the entire cell. Let’s take a closer look athow cells swim.

Cilia and Flagella: Moving the Entire CellFlagella are long hairlike projections from the cell surface thatfunction in movement. Flagella are found in many bacteria andeukaryotes. The structure of flagella is completely different inthe two groups, however. Bacterial flagella are made of a pro-tein called flagellin; eukaryotic flagella are constructed frommicrotubules (tubulin). Bacterial flagella move the cell by rotat-ing like a ship’s propeller; eukaryotic flagella move the cell byundulating. Eukaryotic flagella are surrounded by plasmamembrane; bacterial flagella are not. Based on these observa-tions, biologists conclude that the two structures evolved inde-pendently—even though their function is similar.

To understand how cells move, we’ll focus on eukaryotic fla-gella. Eukaryotic flagella are closely related to structures calledcilia (singular: cilium), which are short filamentous projectionsthat are also found in some eukaryotic cells. Unicellular eukary-otes may have either flagella or cilia, while some multicellularorganisms have both. In humans, for example, the cells that linethe respiratory tract have cilia; sperm cells have flagella.

Flagella are generally longer than cilia, and cells typically havejust one or two flagella but many cilia (Figure 7.38). But when re-searchers examined the two structures with the electron micro-scope, they found that their underlying organization is identical.

How Are Cilia and Flagella Constructed? In the 1950s,anatomical studies established that both cilia and flagella havea characteristic arrangement of microtubules. AsFigure 7.39a shows, nine microtubule pairs, or doublets, sur-round two central microtubules. The doublets, consisting ofone complete and one incomplete microtubule, are arrangedaround the periphery of the structure. The entire struc-ture is called the axoneme (“axle-thread”). The axoneme at-taches to the cell at a structure called the basal body. The basalbody is derived from the centrioles found inside the centro-some. The basal body has a arrangement of micro-tubules and plays a central role in the growth of the axoneme.

As electron microscopy improved, biologists gained a more de-tailed view of the structure. As the sketch in Figure 7.39b illus-trates, spoke-like structures connect each doublet to the centralpair of microtubules. In addition, molecular bridges connect thenine doublets to one another. Finally, each of the doublets has a setof arms that project toward an adjacent doublet. Microtubules arecomplex. How do their components interact to generate motion?

“9 + 0”

9 + 2

“9 + 2”

50 µm 1 µm

Cilia Flagella

FIGURE 7.38 Cilia and Flagella Differ in Length and NumberCilia are relatively short and large in number; flagella are relativelylong and few in number.


Dynein arms on this sideof flagellum walk, causingflagellum to bend

Dynein arms are atrest: Flagellum isstraight

FIGURE 7.40 How Do Flagella Bend?Researchers attached a pair of gold beads to a flagellum andphotographed its movement over a short time sequence. As theflagellum bends and beats back and forth, the sperm cell swimsforward. When dynein arms walk along the microtubule doublets onone side of a flagellum, the structure bends.

A Motor Protein in the Axoneme In the 1960s Ian Gibbonsbegan studying the cilia of a common unicellular eukaryotecalled Tetrahymena, which lives in pond water. Gibbons foundthat by using a detergent to remove the plasma membrane thatsurrounds cilia and then subjecting the resulting solution to dif-ferential centrifugation, he could isolate axonemes. Further, theisolated structures would beat if Gibbons supplied them withATP. These results confirmed that the beating of cilia is an ener-gy-demanding process. They also provided Gibbons with a cell-free system for exploring the molecular mechanism of movement.

In an early experiment with isolated axonemes, Gibbonstreated the structures with a molecule that affects the ability ofproteins to bind to one another. The axonemes that resultedfrom this treatment could not bend or use ATP. When Gibbonsexamined them in the electron microscope, he found that thearms had fallen off. This observation led to the hypothesis thatthe arms are required for movement. Follow-up work showedthat the arms are made of a large protein that Gibbons nameddynein (from the Greek word dyne, meaning “force”).

Like myosin and kinesin, dynein is a motor protein. Struc-tural and chemical studies have shown that dynein undergoesa conformational change when a phosphate group from ATPattaches to it. More specifically, the end of a dynein moleculechanges shape when it is phosphorylated. This shape changemoves the molecule along the nearby microtubule. When theprotein reattaches, it has succeeded in walking up the micro-tubule. This walking motion allows the microtubule doubletsto slide past one another. But because each of the nine doubletsin the axoneme is connected to the central pair of microtubulesby a spoke, and because all of the doublets are connected toeach other by molecular bridges, the sliding motion is con-strained. So if dynein arms on just one side of the axonemewalk while those on the other side are at rest, the result of theconstrained, localized movement is bending (Figure 7.40). Theresult of the bending of cilia or flagella is a swimming motion.

Scaled for size, flagellar-powered swimming can be rapid. Interms of the number of body or cell lengths traveled per second,a sperm cell from a bull moves faster than a human world-record-holder does when swimming freestyle. At the level of thecell, life is fast paced.

�CHECK YOUR UNDERSTANDING

Each component of the cytoskeleton has a unique structureand set of functions. In addition to providing structural sup-port, actin filaments and microtubules work in conjunctionwith motor proteins to move the cell or materials inside thecell. Intermediate filaments provide structural support. Mostelements of the cytoskeleton are dynamic—they grow andshrink over time. You should be able to predict what willhappen when experimental cells are treated with drugs thatinhibit formation of each type of cytoskeletal filament.


■ The structure of cell components is closely correlated withtheir function.

Because all organisms consist of cells, many questions in biolo-gy can be answered by understanding the structure and func-tion of cells and cell components. There are two basic cellulardesigns: prokaryotic and eukaryotic. Eukaryotic cells are usual-ly much larger and more structurally complex than prokaryoticcells. Prokaryotic cells consist of a single membrane-boundcompartment in which nearly all cellular functions occur. Eu-karyotic cells contain numerous membrane-bound compart-

ments called organelles. Organelles allow eukaryotic cells tocompartmentalize functions and grow to a large size.

Eukaryotic organelles are specialized for carrying outdifferent functions, and their structure is often correlatedclosely with their function. Mitochondria and chloroplastshave extensive internal membrane systems, where the en-zyme machines responsible for ATP generation and photo-synthesis reside. Rough ER is named for the ribosomes thatattach to it. Ribosomes are protein-making machines, andrough ER is a site for protein synthesis and processing.

What happens when organelles malfunction? Given the impor-tance of organelles in the function of eukaryotic cells, it’s notsurprising that abnormalities in organelles cause disease. In hu-mans, some of the best-understood organelle-based diseases in-volve lysosomes.

Lysosomes are the cell’s recycling center. Defunct organellesand old or damaged RNA, protein, carbohydrate, and lipidmolecules are transported to lysosomes. Enzymes inside theseorganelles then degrade the molecules to simpler componentsthat can be reused. It is essential that these degradative enzymesbe confined to the interior of the lysosome. If they escaped intothe cytosol, they would threaten to destroy the cell’s contents.

Several human diseases result from leakage of lysosomal en-zymes into the cytoplasm or the cell exterior. Rheumatoidarthritis is an inflammatory joint disease that is partly causedby the leakage of lysosomal enzymes into the joint fluid. Thedebilitating lung disease called asbestosis also results from en-zyme leakage, even though the population of cells involved isdifferent. Asbestosis is observed in asbestos miners and in con-struction workers who handled large quantities of asbestos-containing insulation and fireproofing material during theircareers. When these individuals breathed asbestos fibers intotheir lungs, the fibers were localized to the lysosomes of lungcells. But because lysosomal enzymes cannot degrade asbestosfibers, the fibers gradually accumulate inside the organelles.Eventually the buildup of fibers results in damage to the lysoso-mal membrane and leakage of enzymes into the cell cytoplasm.When many lung cells are harmed in this way, the result is se-vere coughing and shortness of breath.

In addition to disorders caused by enzyme leakage, about 40human illnesses are caused by deficiencies in specific lysosomalenzymes. The most severe of these so-called lysosomal storagediseases is inclusion-cell disease, which causes facial and skele-

tal abnormalities as well as mental retardation. Inclusion-celldisease occurs when most of the degradative enzymes found inlysosomes are missing. Their absence causes the organelles toswell with undigested materials, resulting in structures called

inclusions. The swollen lyso-somes ultimately cause cell dam-age, leading to disease symptoms.

Inclusion-cell disease is causedby a deficiency in a single en-zyme—the one that is required

for attaching mannose-6-phosphate to proteins. Recall fromSection 7.3 that this phosphorylated sugar serves as the zipcode that targets proteins to lysosomes from the Golgi appara-tus. In the absence of the mannose-6-phosphate zip code, en-zymes that are normally shipped to lysosomes are insteadsecreted from the cell. In fact, researchers originally discoveredthat mannose-6-phosphate serves as a zip code by comparinglysosomal proteins from healthy individuals with the same pro-teins secreted by individuals with inclusion-cell disease.

Tay-Sachs disease is another lysosomal storage disease. Un-like inclusion-cell disease, Tay-Sachs results from the absence ofone particular enzyme from lysosomes. In normal individuals,this protein degrades a type of glycolipid that is abundant inbrain cells. In Tay-Sachs patients, the glycolipid accumulates inthese cells and disrupts their function. Symptoms include rapidmental deterioration after about 6 months of age, followed byparalysis and death within 3 years.

Although most organelle diseases cannot yet be cured, drugtherapies are beginning to offer some hope. For example, thesymptoms associated with certain lysosomal storage diseasesshow improvement when the normal enzyme is provided in pillform. Research continues on this and other strategies for allevi-ating the symptoms caused by dysfunctional organelles.

What happenswhen organellesmalfunction?

ESSAY Organelles and Human Disease

CHAPTER REVIEW

Summary of Key Concepts


Content Review1. Which of the following best describe the nuclear envelope?

a. It is continuous with the endomembrane system.b. It is continuous with the nucleolus.c. It is continuous with the plasma membrane.d. It contains a single membrane and nuclear pores.

2. What is a nuclear localization signal?a. a stretch of amino acids that directs proteins from the nucleus to

the ERb. a molecule that is attached to nuclear proteins so that they are re-

tained inside the nucleusc. a signal built into a protein that directs it to the nucleusd. a component of the nuclear pore complex

3. Which of the following is not true of secreted proteins?a. They are synthesized in ribosomes.b. They are transported through the endomembrane system in

membrane-bound transport organelles.c. They are transported from the Golgi apparatus to the ER.d. They contain a signal sequence that directs them into the ER.

4. To find the nuclear localization signal in the protein nucleoplasmin,researchers separated the molecule’s core and tail segments, labeledboth with a radioactive atom, and injected them into the cytoplasm.

Why did the researchers conclude that the signal is in the tail regionof the protein?a. The protein reassembled and folded into its normal shape

spontaneously.b. Only the tail segments appeared in the nucleus.c. With a confocal microscope, tail segments were clearly visible

in the nucleus.d. The tail and head segments appeared together in the nucleus.

5. Molecular zip codes direct molecules to particular destinations inthe cell. How are these signals read?a. They bind to receptor proteins.b. They enter transport vesicles.c. They bind to motor proteins.d. They are glycosylated by enzymes in the Golgi apparatus.

6. The number and size of organelles in a cell correlates with thatcell’s function. Propose a function for cells that contain extensiverough ER.a. rapid cell division in growing bones or muscle tissuesb. production and processing of fatty acids and other lipidsc. movement via cell crawlingd. production of proteins that are secreted from the cell

Questions

Smooth ER lacks ribosomes because it is a center for lipidsynthesis and processing.

■ Inside cells, materials are transported to their destinationswith the help of molecular “zip codes.”

The defining organelle of eukaryotic cells is the nucleus, whichcontains the cell’s chromosomes and serves as its control cen-ter. For a cell to function properly, the movement of moleculesinto and out of the nucleus must be carefully controlled. Traf-fic across the nuclear envelope occurs through nuclear pores,which contain a multiprotein nuclear pore complex that servesas gatekeeper. Both passive and active transports of materialsoccur through these nuclear pore complexes. Active importand export of proteins and RNAs involves built-in signals thattarget cargo to the correct compartment.

The endomembrane system is an extensive, intercon-nected system of membranes and membrane-bound com-partments that can extend from the nucleus to the plasmamembrane. Two principal organelles in the endomem-brane system are the endoplasmic reticulum (ER) and theGolgi apparatus. The ER is the site of synthesis for a widearray of proteins and lipids. Most ER products areshipped to the Golgi apparatus, which serves as a process-ing and dispatching station. In many proteins the majorprocessing step is glycosylation, or the addition of carbo-hydrate groups.

The movement of materials through the endomembranesystem is highly organized and takes place inside membrane-bound transport organelles called vesicles. Prior to productsleaving the endomembrane system, they are sorted with mo-lecular zip codes that direct them to vesicles headed for theirfinal destination. The vesicles contain proteins that interactwith receptor proteins on the surface of a target organelle orthe plasma membrane, and allow the contents to be delivered.

Web Tutorial 7.1 Transport into the Nucleus

Web Tutorial 7.2 A Pulse-Chase Experiment

■ Cells are dynamic. Thousands of chemical reactions occureach second within cells; molecules constantly enter and exitacross the plasma membrane; cell products are shippedalong protein fibers; and elements of the cell’s internal skele-ton grow and shrink.

The cell is a membrane-bound structure with a highly orga-nized, dynamic interior. Inside the cell, thousands of differ-ent chemical reactions take place at incredible speeds. Theproducts of these chemical reactions allow the cell to acquireresources from the environment, synthesize additional mole-cules, dispose of wastes, and reproduce.

The cytoskeleton is an extensive system of fibers thatserves as a structural support for eukaryotic cells. Elementsof the cytoskeleton also provide the machinery for movingvesicles inside cells and for moving the cell as a wholethrough the beating of flagella or cilia, or cell crawling.Both cell motility and the movement of vesicles inside cellsdepend on motor proteins, which can convert chemical en-ergy stored in ATP into movement. Within the cell, move-ment of transport vesicles occurs as the motor proteinkinesin “walks” along microtubule tracks. Cilia and flagel-la bend as the motor protein dynein “walks” along micro-tubule tracks. The bending motion allows these structuresto beat back and forth, enabling cells to swim or generatewater currents.

The data reviewed in this chapter provide a view of thecell as a dynamic reaction vessel that synthesizes and shipsan array of products in a highly regulated manner. Howdoes all this activity inside the cell relate to what is going onoutside? This is the issue taken up in Chapter 8.


Group Discussion Problems

1. In addition to delivering cellular products to specific organelles,eukaryotic cells can take up material from the outside and trans-port it to specific organelles. For example, specialized cells of thehuman immune system ingest bacteria and viruses and then deliverthem to lysosomes for degradation. Suggest a hypothesis for howthis material is tagged and directed to lysosomes. How would youtest this hypothesis?

2. The leading hypothesis to explain the origin of the nuclear envelopeis that a deep infolding of the plasma membrane occurred in an an-cient prokaryote. Draw a diagram that illustrates this infolding hy-

Answers to Multiple-Choice Questions: 1. a; 2. c; 3. c; 4. b; 5. a; 6. d

www.prenhall.com/freeman is your resource for the following:Web Tutorials; Online Quizzes and other Online Study Guidematerials; Answers to Conceptual Review Questions; Solutions toGroup Discussion Problems; Answers to Figure Caption Questionsand Exercises; and Additional Readings and Research.

Conceptual Review1. Compare and contrast the structure of a generalized plant cell, ani-

mal cell, and prokaryotic cell. Which features are common to allcells? Which are specific to certain lineages?

2. Draw a diagram that traces the movement of a secreted protein fromits site of synthesis to the outside of a eukaryotic cell. Identify all ofthe organelles that the protein passes through, and indicate the di-rection of movement.

3. Describe how a motor protein such as kinesin can move a trans-port vesicle down a microtubule track. Include all necessary stepsand components.

4. Describe the logic of a pulse-chase experiment. How was this ap-proach used to document the pattern of protein transport throughthe endomembrane system?

5. Briefly describe how researchers use centrifugation to isolate partic-ular cell components for further study.

6. Compare and contrast the structure and function of actin filaments,intermediate filaments, and microtubules. Why is it misleading torefer to the cytoskeleton as “scaffolding”?

pothesis. Does your model explain the existence of the structure’sinner and outer membranes? Explain.

3. Propose a function for cells that contain (a) a large number of lyso-somes, (b) a particularly extensive cell wall, and (c) many peroxi-somes.

4. Suggest a hypothesis or a series of hypotheses to explain why bacte-ria, archaea, algae, and plants have cell walls. Suppose that mutantindividuals from each group lacked a cell wall. How could you usethese individuals to test your idea(s)?

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