Cell Structure and Function

Chapter 4

Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

Biology, Ninth Edition, by Solomon, Berg, Martin. Brooks/Cole, Cengage Learning 2011.

Cells and Organisms

The cell is the smallest unit that can carry out all activities associated with life

Most prokaryotes and many protists and fungi consist of a single cell

Most plants and animals have millions of cells In multicellular organisms, cells are modified in a

variety of ways to carry out specialized functions Cells exchange materials and energy with the

environment; and convert energy to chemical energy stored in ATP

4.1 What is a Cell?

Each cell has a plasma membrane, cytoplasm, and a nucleus (in eukaryotic cells) or a nucleoid (in prokaryotic cells)

Components of Cell Membranes

Lipid bilayer

Fig. 4.4, p. 53

one layerof lipidsone layerof lipids

membraneprotein

extracellularenvironment

cytoplasm

Organization of Cells

The organization and small size of cells allow them to maintain an appropriate internal environment (homeostasis)

To maintain homeostasis, the plasma membrane acts as a selective barrier between cell contents and the environment

Most cells have internal structures (organelles) specialized to carry out metabolic activities

Each cell has genetic instructions coded in DNA, which is concentrated in a limited region of the cell

Cell Size and Shape

• Cell sizes and shapes are adapted to their functions:• Amoebas and white blood cells change shape as

they move• Sperm cells have long, whiplike tails (flagella) for

locomotion • Nerve cells have long, thin extensions that enable

them to transmit messages over great distances• Rectangular epithelial cells and stack like building

blocks to form sheetlike tissues

Surface Area–To-Volume Ratio

Fig. 4-2, p. 77

Key Concepts: WHAT ALL CELLS HAVE IN COMMON

Each cell has a plasma membrane, a boundary between its interior and the outside environment

The interior consists of cytoplasm and an innermost region of DNA

4.2 Cell Theory: A Unifying Concept

• Cell theory is a unifying concept:1. All organisms consist of one or more cells

2. The cell is the smallest unit that retains the capacity for life

3. A cell arises from the growth and division of another cell

• Evidence that all living cells have a common origin is provided by basic similarities in their structures and molecules of which they are made

Relative Sizes

Fig. 4.6, p. 54

Microscopes

Different microscopes use light or electrons to reveal details of cell shapes or structures

Fig. 4.7, p. 55light source (in base)

Ocular lens enlargesprimary image formedby objective lenses.

Objective lenses (those closestto specimen) form the primaryimage. Most compound lightmicroscopes have several.

stage supportsmicroscope slide

Condenser lenses focuslight rays through specimen.

illuminator

path of light rays (bottom to top) to eye

prism thatdirects rays toocular lens

Fig. 4-4a, p. 80

Light microscope

Light beam

Ocular lens

Objective lens

Specimen

Condenser lens

Light source

(a) A phase contrast light microscope can be used to view stained or living cells, but at relatively low resolution.

Five Different Views

Key Concepts: MICROSCOPES

Microscopic analysis supports three generalizations of the cell theory:• Each organism consists of one or more cells and

their products• A cell has a capacity for independent life• Each new cell is descended from a living cell

4.3 Membrane Structure and Function

Each cell membrane is a boundary (lipid bilayer) that controls the flow of substances across it

Fluid mosaic model• Membrane is composed of phospholipids, sterols,

proteins, and other components• Phospholipids drift within the bilayer

Membrane Proteins

Many proteins are embedded in or attached to cell membrane surfaces• Receptors (insulin receptors), transporters (active

and passive), recognition adhesion proteins, and enzymes. **Table 4.1 in text**

Plasma (outer) membrane also incorporates recognition proteins (self vs. non-self),

Common Membrane ProteinsPassive Active

Membrane Structure Studies

Fig. 4.10, p. 57

proteins fromboth cellsin fused

membrane

human cell mouse cell

fusion intohybrid cell

Key Concepts: COMPONENTS OF CELL MEMBRANES

All cell membranes are mostly a lipid bilayer (two layers of lipids) and a variety of proteins

The proteins have diverse tasks, including control over which water-soluble substances cross the membrane at any given time

Prokaryotic Cells

4.4 Introducing Prokaryotic Cells

Bacteria and archaeans• The simplest cells• The groups with greatest metabolic diversity

Biofilms • Shared living arrangements of prokaryotes

Prokaryote Structure

Cell wall• Surrounds plasma membrane

Flagella• Used for motion

Pili• Protein filaments used for attachment• “Sex” pilus transfers genetic material

Structure of a Prokaryotic Cell

• Bacteria and archaea are prokaryotic cells• ~1/10 the diameter of the average eukaryotic cell• DNA is located in a nuclear area, or nucleoid• Nuclear area is not enclosed by a membrane• No membrane-enclosed internal organelles• Most have cell walls outside the plasma membrane• Many have prokaryotic flagella which operate like

propellers• Some have hairlike fimbriae that increase adherence • Interior contains ribosomes and storage granules

Prokaryote Structure

Prokaryote Structure

4.5 Microbial Mobs

Biofilm formation community of microorganisms living within a shared mass of slime (polysaccharides and proteins)• Can include bacteria, algae, fungi, protists, and

archaeans • Benefit prevents being washed away by fluid currents

Key Concepts:PROKARYOTIC CELLS

Archaeans and bacteria are prokaryotic cells which have few, if any, internal membrane-enclosed compartments

In general, they are the smallest and structurally the simplest cells

Eukaryotic Cells

4.6 Introducing Eukaryotic Cells

Start with a nucleus and other organelles• Carry out specialized functions inside a cell

Fig. 4.14, p. 60

mitochondria

plasmamembrane

nucleus

Fig. 4.14, p. 60

nucleus

cell wall

plasmamembrane

centralvacuole

chloroplast

Components of Eukaryotic Cells

Lysosome Intracellular digestion

Peroxisome Inactivating toxins

Vacuole Storage

Centrioles Anchor for cytoskeleton

Structure of a Eukaryotic Cell

• Eukaryotic cells are characterized by highly organized membrane-enclosed organelles• A nucleus contains DNA• The part of the cell outside the nucleus is cytoplasm• Cytoplasm semifluid substance enclosed by a cell’s

plasma membrane

• The part of the cell within the nucleus is nucleoplasm• The fluid component of cytoplasm is cytosol• Many specialized organelles and a supporting

framework (cytoskeleton) allow a larger size than prokaryotes

• Some organelles are present only in specific cells

4.7 Components of The Nucleus

Nucleus separates DNA from cytoplasm• Chromatin (all chromosomal DNA with proteins)• Chromosomes (condensed)

Nucleolus assembles ribosomal subunits Nuclear envelope encloses nucleoplasm• The envelop is a double membrane that

constitutes the outer boundary of the nucleus• Pores, receptors, transport proteins

Nucleoplasm • Viscous fluid enclosed by the nuclear envelop

Nucleus and Nuclear Envelope

Nucleus and Nuclear Envelope

Nucleus and Nuclear Envelope

4.8 The Endomembrane System

System that includes interacting organelles between the nucleus and plasma membrane

1. Endoplasmic reticulum (ER) • An extension of the nuclear envelope• Rough ER modifies new polypeptide chains,

studded with ribosomes• Smooth ER makes lipids; other metabolic

functions

2. Golgi bodies • Further modify polypeptides and assemble lipids• Sort and package finished products into vesicles

The Endomembrane System

3. Vesicles membrane included saclike organelle that can store, transport, or degrade contents• Endocytic and exocytic: Transport or store

polypeptides and lipids• Peroxisomes: Digest fatty acids and amino acids;

break down toxins and metabolic by-products • Enzyme-filled

• Lysosomes: Intracellular digestion (animal cells)• Enzyme-filled

• Central vacuole: Storage in many plant cells • fluid-filled vesicles that produce fluid pressure

• Fluid pressure keeps plant cells plump and firm

Endomembrane System

Endomembrane System

Endomembrane System

Golgi Complex

• The Golgi complex (Golgi body or Golgi apparatus) consists of stacks of flattened membranous sacs (cisternae)

• Each Golgi stack has three areas:• Entry surface (cis face)• Exit surface (trans face)• Medial region in between

Golgi Complex and Proteins

• Transport vesicles carrying proteins from the ER move along microtubules to the cis face of the Golgi complex

• The Golgi complex processes, sorts, and modifies proteins• Carbohydrate of a glycoprotein may be modified

to route the protein to a specific organelle

• Proteins are packaged in transport vesicles in the trans face

Protein Transport Within the Cell

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are organelles specialized to facilitate conversion of energy from one form to another

Chemical energy (in food molecules such as glucose) or light energy must be converted into more convenient forms – usually the chemical energy of ATP

Mitochondria and chloroplasts have their own ribosomes and DNA molecules

4.9 Mitochondria

Mitochondria • Double membrane organelle• Break down organic compounds by aerobic

respiration (oxygen-requiring)• Produce ATP by aerobic respiration in eukaryotes• Bacteria and archaeans have no mitochondria

they make ATP in their cell wall and cytoplasm• Almost all eukaryotes have mitochondria• Have their own DNA and ribosomes. They

replicate independently from the cell

Mitochondria and Cellular Respiration

A double membrane forms two compartments within the mitochondrion: the intermembrane space and the matrix

The outer mitochondrial membrane is smooth and allows small molecules to pass through it; the inner mitochondrial membrane strictly regulates molecules that move across it

Folds in the inner membrane (cristae) extend into the matrix and increase surface area for chemical reactions

The inner membrane contains enzymes and other proteins needed to synthesize ATP

Fig. 4-21, p. 95

Outer mitochondrial membrane

Inner mitochondrial membrane

MatrixCristae

4.9 Chloroplasts

Chloroplasts• Produce sugars by photosynthesis• Organelles that contain chlorophyll, a green

pigment that traps light energy for photosynthesis

• In the cells of plants and many protists

Chloroplast Structure

Chloroplasts are disc-shaped structures with a system of folded membranes

The inner membrane encloses a fluid-filled stroma, which contains enzymes that produce carbohydrates from carbon dioxide and water, using energy trapped from sunlight

An interconnected set of flat, disclike sacs (thylakoids) arranged in stacks (grana) is suspended in the stroma

The thylakoid membrane encloses the thylakoid lumen

Chloroplast Function

In the thylakoid membrane, chlorophyll molecules absorb energy from sunlight , which excites electrons

Energy in excited electrons is used to produce ATP and other molecules that transfer chemical energy

Chloroplasts and amyloplasts are plastids• Plastids organelle that functions is photosynthesis

or storage of food materials in cells of plants

Mitochondria and Chloroplasts

Cellular Respiration and Photosynthesis

Plant Vs Animal Cells

4.10 Visual Summary: Plant Cells

Visual Summary: Animal Cells

CENTRAL VACUOLE

LYSOSOME-LIKE VESICLE

GOLGI BODY

SMOOTH ER

ROUGH ER

RIBOSOMES

NUCLEUS

CHLOROPLAST

CYTOSKELETON

MITOCHONDRION

PLASMODESMA

PLASMA MEMBRANE

CELL WALL

Fig. 4.19, p.65

nuclear envelopenucleolusDNA innucleoplasm

microtubulesmicrofilamentsintermediatefilaments(not shown)

a Typical plant cell components.

CYTOSKELETON

MITOCHONDRION

CENTRIOLES

LYSOSOME

GOLGI BODY

SMOOTH ER

ROUGH ER

RIBOSOMES

NUCLEUS

PLASMA MEMBRANE

microtubulesmicrofilamentsintermediatefilaments

nuclear envelopenucleolusDNA innucleoplasm

b Typical animal cell components. Fig. 4.19, p. 64

4.11 Cell Surface Specializations

Most prokaryotes, protists, fungi, all plant cells have a cell wall around their plasma membrane• Protects, supports, maintains cell shape• Primary and secondary cell walls• Primary wall: first cell wall of young plant cells• Secondary wall: lignin-reinforced wall that forms

inside the primary wall of a plant cell

Plasmodesmata across cell walls connect plant cells

Plant Cell Walls

Plant Cell Walls

Plant Cuticle

Protective surface secretion, limits water loss

Extracellular Matrixes (ECM)

Surrounds cells of specific tissues. Supports cells and tissues

Animal Cell Junctions

Connect cells of animals• Adhering junctions, tight junctions, gap junctions

Animal Cell Junctions

Cell junction• Structure that connects a cell to another cell or to

extracellular matrix

1. Adhering junction• Composed of adhesion proteins, anchoring cells to

each other and extracellular matrix

2. Gap junction• Forms a channel across the plasma membranes of

adjoining animal cells

3. Tight junction• Arrays of fibrous proteins, join epithelial cells and

collectively prevent fluids form leaking between them

Key Concepts: EUKARYOTIC CELLS

Cells of protists, plants, fungi, and animals are eukaryotic; they have a nucleus and other membrane-enclosed compartments

They differ in internal parts and surface specializations

4.12 The Dynamic Cytoskeleton

Cytoskeleton• Dynamic framework of protein filaments that support,

organize, and move eukaryotic cells and their internal structures

• Dense network of protein fibers• Gives cells mechanical strength, shape, ability to move• Functions in cell division and transport of materials

within the cell• Highly dynamic and constantly changing

Cytoskeleton Function

Organizes and moves cell parts

Reinforces cell shape

Interactions between motor proteins and microtubules in cilia, flagella, and pseudopods can move the whole cell

4.12 The Dynamic Cytoskeleton

Components of the cytoskeleton• Tubulin, Actin, and Polypeptide chain

Elements of the cytoskeleton• Microtubules• Microfilaments• Intermediate filaments (in most)

Components of the Cytoskeleton

The Cytoskeleton

1. Microtubules

• microtubules• Rigid, hollow rods about 25 nm in diameter• Function in cytoskeleton structure, movement of

chromosomes during cell division, tracks for intracellular movement, and structural components of cilia and flagella

• Consist of two forms of the protein tubulin (α-tubulin and β-tubulin) which combine to form a dimer

Organization of Microtubules

Motor Protein: Kinesin

Moves vesicles along microtubules

Kinesin is a motor protein a type of energy using protein that interacts with cytoskeleton elementsto move the cell’s parts or the whole cell

Centrosomes and Centrioles

Microtubule-organizing centers (MTOCs) microtubules to other parts of the cell; in animal cells, the main MTOC is the centrosome

The centrosome typically contains two centrioles, which are duplicated before cell division

Microtubules assemble and disassemble rapidly during cell division; tubulin subunits organize into a mitotic spindle, which helps distribute chromosomes

Centrioles

Centrioles are 9 × 3 structures consisting of nine sets of three attached microtubules arranged to form a hollow cylinder

Grow microtubules

Cilia and Flagella

Cilia and flagella help unicellular and small multicellular organisms move through a watery environment

Cells use cilia to move liquids and particles across the cell surface; flagella serve as the tails of sperm cells

Eukaryotic cilia and flagella are structurally alike (but different from bacterial flagella), with a 9 + 2 arrangement of microtubules

Cilia and Flagella (cont.)

Each cilium or flagellum is anchored in the cell by a basal body, which has a 9 × 3 structure of microtubules• Basal body develops form a centriole

Most vertebrate cells have a primary cilium on the cell surface that serves as an antenna – its receptors bind with specific molecules outside the cell or on other cells

Eukaryotic Flagella and Cilia: Dynein

Eukaryotic Flagella and Cilia: Dynein

2. Microfilaments

microfilaments (actin filaments)• Flexible, solid fibers about 7 nm in diameter• Consists of actin molecules/fibers• Provide support for cell structures• Form the cell cortex, just inside the plasma

membrane• Cell cortex reinforcing mesh of cytoskeleton

elements under a plasma membrane

Microfilament Function

• Microfilaments generate movement by rapidly assembling and disassembling

• Muscle cells have two types of specialized filaments:• Filament composed of the protein myosin• Filaments composed of the protein actin• ATP, actin, and myosin generate forces that

contract muscles

Microfilament Function (cont.)

Amoebas, human white blood cells, and cancer cells can creep along a surface by changing shape

Actin filaments push the plasma membrane outward, forming bulges (pseudopodia) that adhere to the surface

Contractions of microfilaments at the opposite end of the cell force the cytoplasm forward in the direction of locomotion

Fig. 4-28, p. 101

7 nm

(a) A microfilament consists of two intertwined strings of beadlike actin molecules.

(b) Many bundles of microfilaments (green) are evident in this fluorescent LM of fibroblasts, cells found in connective tissue.

Intermediate Filaments

• intermediate filaments • Tough, flexible fibers about 10 nm in diameter• Provide mechanical strength and help stabilize cell

shape• Only some animal groups (including vertebrates)

have intermediate filaments• Intermediate filaments include keratins in vertebrate

epithelial cells, and neurofilaments in nerve cells

• Abnormal neurofilaments are associated with the neurodegenerative disease amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease)

Fig. 4-30, p. 102

Collagen

FibronectinsExtracellular matrix

Integrin

Intermediate filament

Microtubules

Plasma membrane

Cytosol

Key Concepts: A LOOK AT THE CYTOSKELETON

Diverse protein filaments reinforce a cell’s shape and keep its parts organized

As some filaments lengthen and shorten, they move chromosomes or other structures to new locations

Animation: Animal cell junctions

Animation: Cell membranes

Animation: Common eukaryotic organelles

Animation: Cytoskeletal components

Animation: Flagella structure

Animation: How a light microscope works

Animation: Motor proteins

Animation: Nuclear envelope

Animation: Overview of cells

Animation: Plant cell walls

Animation: Structure of a chloroplast

Animation: Structure of a mitochondrion

Animation: The endomembrane system

Animation: Typical prokaryotic cell