Chapter 6.

A Tour of the Cell

Why do we study cells?

Cell Theory1. All organisms are made up of cells2. The cell is the basic living unit of organization for all

organisms3. All cells come from pre-existing cells

Biological diversity & unity• Underlying the diversity of life is a striking unity

– DNA is universal genetic language

– Cells are the basic units of structure & function

• lowest level of structure capable of performing all activities of life

Activities of life• Most everything you think of a whole organism needing to do, must

be done at the cellular level…– reproduction– growth & development– energy utilization– response to the

environment– homeostasis

Tour of the Cell

Cell characteristicsAll cells:– surrounded by a plasma membrane– have cytosol

• semi-fluid substance within the membrane• cytoplasm = cytosol + organelles

– contain chromosomes which have genes in the form of DNA– have ribosomes

• tiny “organelles” that make proteins using instructions contained in genes

Chromosomes

Types of cells

Prokaryotic cell•DNA in nucleoid region, without a membrane separating it from rest of cell

Eukaryotic cell•chromosomes in nucleus, membrane-enclosed organelle

Prokaryotic vs. eukaryotic cells Location of chromosomes

The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the eukaryotic cell.

Eukaryotic cells• Eukaryotic cells are more complex than prokaryotic cells

– within cytoplasm is a variety of membrane-bounded organelles– specialized structures

in form & function• Eukaryotic cells are

generally bigger than prokaryotic cells

Limits to cell sizeLower limit–smallest bacteria, mycoplasmas

• 0.1 to 1.0 micron (µm = micrometer)–most bacteria

• 1-10 microns

Upper limit –eukaryotic cells

• 10-100 microns• Giraffe nerve cells, ostrich egg

are the largest cells• micron = micrometer = 1/1,000,000 meter• diameter of human hair = ~20 microns

What limits cell size?• Surface to volume ratio

– as cell gets bigger its volume increases faster than its surface area• smaller objects have greater

ratio of surface area to volume

Why is a huge single-cell creature not possible?

Surface area = 96 mm2

Volume = 64 mm3

Limits to cell size• Metabolic requirements set upper limit

– in large cell, cannot move material in & out of cell fast enough to support life

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What process is this?

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How to get bigger?• Become multi-cellular (cell divides)

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But what challenges do you have to solve now?

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Cell membrane• Exchange organelle

– plasma membrane functions as a selective barrier

• allows passage of O2, nutrients & wastes

Organelles & Internal membranes• Eukaryotic cell

– internal membranes• partition cell into compartments• create different local environments• compartmentalize functions• membranes for different compartments are specialized for their

function– different structures for specific functions– unique combination of lipids & proteins

Mitochondrion

Chloroplast Golgi apparatusEndoplasmic Reticulum

Cell Organelles

• Nucleus• Chromatin & Chromosomes

• Nucleolus• Nuclear Envelope• Ribosomes• Endoplasmic Reticulum (smooth & rough)• Golgi Apparatus• Lysosome• Food vacuoles• Contractile vacuoles• Central Vacuole• Mitochondria• Chloroplasts

• Peroxisome• Cytoskeleton• Centrosomes• Centrioles• Cilia & Flagella• Cell wall• Microfilaments• Microtubules• Plasmodesmata

Nucleus & Nuclear structure• Enclosed by nuclear envelope (double

membraned) w/ nucleopores• Internally lined with nuclear lamina (network of

protein) for support & shape• Nuclear matrix fibers internally• Chromatin (uncoiled DNA & protein) then

become chromosomes• Nucleolus (no membrane) produces rRNA &

then ribosomes

RibosomesFound in Eukaryotic and Prokaryotic cellsMade of rRNA and proteinCarry out Protein SynthesisHuman pancreas cell has a few millionNot membrane boundFree or bound to Rough Endoplasmic ReticulumFree produce enzymes for glycolysis of glucoseBound to Endoplasmic Reticulum for protein synthesis

Endomembrane system• Series of membranes for specific tasks• Endoplasmic reticulum (ER) – network of membranes & sacs called

cisternae. Continuous w/ nuclear envelope– Smooth – synthesis of lipids, metabolism of carbohydrates,

detoxification of drugs & poisons & storage of Calcium ions– Rough – studded w/ ribosomes for protein synthesis.– Transport vesicles – sacs containing proteins that break free from RER

to be transported to another cell

Golgi ApparatusShipping & receiving center Products of ER are modified, packaged & shipped to other

parts of the cell or to other cellsFlattened cisternaeVesicles on ends for transport bud offCis side for receiving and trans for shipping

Golgi Apparatus song

Lysosomes• Membrane bound sacs containing hydrolytic enzymes• Animal cells only• Acidic for optimal enzyme activity• Autodigestion (autolysis) when lysosomes break open (cleans up dead cells)

or autophagy (cleans up non-functioning organelles)• Fuse with food vacuoles for digestion• Tay-Sach’s disease – inheritable – lysosomes do not make enzymes

Vacuoles• Food vacuoles formed by phagocytosis• Contractile vacuoles – like sump pumps to control osmotic

pressure in freshwater protists• Central vacuole in plants enclosed by tonoplast – holds reserve

organic compounds, K and Chloride, stores metabolic byproducts, may contain defensive toxins, stores water reserve to maintain structure (turgor pressure)

MITOCHONDRIA & Chloroplasts• Both involved in energy conversion

• Mitochondria – site of cellular respiration– 1 – 10 um long (same size as bacteria!) Also can move– Double membrane– Inner membrane folds in – Cristae (> surface area)– Electron Transport Chain of respiration occurs here– Mitochondrial matrix – inner portion – Krebs Cycle– Highest in human muscles and liver cells– Contains own DNA so can be replicated as needed

PlastidsAmyloplasts – store starchChromoplasts – store flower & fruit pigmentsChloroplasts – contain chlorophyll & enzymes

involved in photosynthesis 2 – 5 um with DNA (same as bacteria!!) At least double membraned Granum – interconnected membrane stacks of individual

thylakoids with fluid called stroma

Peroxisomes• Single membrane• Contain enzymes to transfer H to O2 to produce H2O2 for

fatty acid breakdown for fuel or for the liver to detoxify alcohol

• Also contains catalase & superoxide dismutase to break down hydrogen peroxide

Cytoskeleton• Network of fibers throughout the cytoplasm• Provides support (especially in animal cells)• Cell Motility – motor proteins • Cilia & flagella motion

– Microfilaments and muscle contractions– Moves vesicles around cells– Streaming of cytoplasm - cyclosis

Components of Cytoskeleton• Microtubules - thickest• Microfilaments – thinnest• Intermediate filaments – middle range diameter

Microtubules• Hollow rods (~25nm in diameter) & 200nm – 25um in length• Wall is made of protein tubulin• Shape & support the cell• Tracks for motor proteins to travel along• Form centrioles (9+3 organization)in animal cells for cell division

Cilia and Flagella• Specialized collection of microtubules

for movement of cilia & flagella• Cilia & Flagella

– Cilia are found in large #’s & are small - .25um in diameter to 2 – 20un in length

• Move like oars w/ power & recovery strokes

– Flagella usually single to few and longer – 10 – 200um

• Undulating motion or as a propeller– Animation

Structure of Cilia & Flagella• 9 + 2 microtubule organization• Proteins cross link pairs to central

pair• Anchored by basal body • Dynein – protein connecting one

pair onto next pair.– Responsible for bending

movements of cilia/flagella

Microfilaments: Actin fibers

• Solid ~ 7nm rods across• Actin – globular proteins• Twisted double chain of actin subunits• Form structural networks throughout cytoplasm• 3D network inside cell membrane• Make up microvilli in intestines• Along with myosin, makes up

muscles

Muscle movement• Myosin acts as motor protein along actin filaments• Myosin “walks” along actin• Also involved in amoeboid motion and formation of

pseudopods and cytoplasmic streaming in plant cells

Intermediate Filaments• 8 – 12nm diameter• More permanent structures in cytoskeleton• Reinforce shape of cell and “fix” organelle positions

Extracellular Components & Connections

• Cell Wall in plant, fungi, bacteria and some protist cells aka, every cell but animals & some protist– Protection– Structure– Prevents excess uptake of water– Supports plant against gravity– Range in size from 0.1um to several nm– Composition varies from species to species

Structure of plant Cell Wall• Primary cell wall – first to be produced, flexible & thin• Middle Lamella of pectin – thin sticky polysaccharide to hold

layers together (glue)• Secondary cell wall forms when plant is mature & stops

growing. More rigid• Plasmodesmata perforate cell walls and form channels

Animal Cell Extracellular Matrix (ECM)• ECM made up of glycoproteins including collagen – strong fibers

outside cell• Collagen accounts for ½ total body proteins• Collagen fibers embedded in Proteoglycan molecules• Fibronectin (purple rods) attached EMC to integrins that bind to

plasma membrane• Involved in communications between cells

Fibronectin

Intracellular connections• Plasmodesmata - Connects

plant cells to other plant cells.

• Tight junctions (form seals like a tongue and groove joint)

• Desmosomes (act like rivets)

• Gap junctions (act like channels) found in epithelial cells