Basic Cell Structure

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Into the CellInto the CellInto the Cell The Cell TheoryThe Cell Theory““Cell DoctrineCell Doctrine””

1. All organisms are constructed of oneor more cells.

2. The cell is the basic unit of life.¸ Minimum level of complexity that

exhibits all characteristics of life.

3. All cells derive from previous cells.

What does a cell need?

n Selective isolation from environment(plasma membrane)

n Energy (ATP)n Instructions (DNA)n Machinery to carry out instructions and

regulate processes (proteins)n Compartmentalization of incompatible or

specialized activities (organelles)

Isolated activity compartmentsIsolated activity compartments

•• Cell membraneCell membrane••PlasmalemmaPlasmalemma

•• Organelle membranesOrganelle membranes

Cell & Organelle Membranes:More than just a phospholipid bilayer

n Human nerve cell:> 1 meter

n Frog egg: 2 mmn Average animal cell:

50 µmn Human red blood cell:

~8 umn Bacteria: 1–2 µmn Organelle: 1 µmn Virus: 50 nm

Cell Size Varieswith Function

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Why Must CellsBe Small?

Smaller cells have more total surface arean Increased surface area makes it easier for gasses,

nutrients, etc. to enter a cell

‹ Same volume fi

v Volume controlledby nucleus.

Why Must CellsBe Small?

Upper limits on cellsize are set by

diffusion distance.Molecules must

diffuse through cell.

So, Why Can’t CellsBe Even Smaller?vThey must be big

enough to hold alltheir parts!

vMacromoleculescannot be shrunk.

Early views of cellsEarly views of cells

n Plasma membrane

n “Nucleus” ( “center”)

– filled with “chromatin”(“colored stuff”)

n “Cytoplasm” (“cell fluid”)

unstained

human cheek cell

stained

50µm

Modern views of cellsModern views of cells

5 µm

n Better microscopes andstains– Electron microscope.

n “Cytoplasm” and“chromatin” much morecomplicated, structured,and dynamic thanpreviously appreciated.

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Two major types of cellsTwo major types of cellsEUKARYOTIC CELL

Membrane

Cytoplasm

Organelles

Nucleus (contains DNA)1 µm

PROKARYOTIC CELL

DNA (no nucleus)

Membrane

Contrastingeukaryotic andprokaryotic cells insize and complexity

I. Prokaryotic — Bacteria• No membranous organelles

II. Eukaryotic — Protists• Single celled or simple colonial

III. Eukaryotic / multicellular— Plant• Organelles present, including chloroplasts• Cellulose cell wall around plasma membrane

IV. Eukaryotic / multicellular—Fungi• No chloroplasts• Chitinous cell wall

V. Eukaryotic / multicellular— Animal• No chloroplasts nor cell wall• Varied cell morphology

Cell-TypeSystematics

Five-Kingdom Model

Prokaryotic Cell Structure

nn Single-celledSingle-celledorganismsorganisms

nn No nucleusNo nucleusnor othernor othermembranousmembranousorganellesorganelles

nn No cytoskeletonNo cytoskeletonnn Most withMost with

cell wallcell walloutside plasmaoutside plasmamembranemembrane

nn Some with capsuleSome with capsuleoutside cell walloutside cell wall

v The plasmamembrane isthe onlymembrane.

Photosynthetic prokaryote

Prokaryotes lack a trueendomembrane system

Eukaryotic Cell Structure—organelles & cytoskeleton

Cytoplasmn Fills cell; contains

– cytosol• fluids and more

• gel & sol state

– membrane-bound organelles• concentrate specific

enzymatic activities

• isolate incompatible reactionsor toxic products

– cytoskeleton• maintain and alter cell shape

• hold and move organelles, etc.

Basic Cell Structure

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Organelles & Cytosol

n Cytosol: Dense, semisolid aqueous gel containing atremendous variety of solutes and macromolecular machines

Cellular Organellesn Nucleus

– Nuclear envelope

– Nucleolus• Nucleus of nucleus• Transcribes rRNA for

ribosomes

– Chromatin/chromosomes

– Nuclear lamina• Protein scaffolding

underlying theenvelope andsupporting thechromatin

n Double-membranewith pores

n Contains DNA ofchromosomes

n Controls cell– structure– function

n Blueprints for new cells

Nucleus

Nuclearenvelopesurface

Porecomplexes

Nuclearlamina

Nuclear envelope: 2 membranes

Nuclear envelope: 2 membranes Cellular Organelles

n Endoplasmic Reticulum

Smooth E.R.– New membrane synthesis– Lipid synthesis &

processing– Steroid synthesis– Lipophilic detoxification

– Intra-cellular Ca++ store

– Usually tubular appearance

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Cellular Organelles

n EndoplasmicReticulum

Rough E.R.– Ribosome attachment sites– Synthesis of membrane proteins– Synthesis of proteins for

secretion or intra-organellestorage

– Flattened beaded stacks;Membrane often continuouswith nuclear envelope

Cellular Organelles

n Ribosomes– Free and RER-bound

Cellular Organellesn Golgi Complex

– Further processing and trafficking of proteins from RER

– Receives vesicles from ER; buds off vesicles to plasma membranefor export

Transporting proteins in vesicles

RER to Golgi

Progression of membrane & proteinsCellular Organelles

n Vesicles– Shuttle

betweenorganellesand plasmamembrane

Basic Cell Structure

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Cellular Organelles

n Lysosome– Contains

hydrolyticenzymes

– Digest food,bacteria, oldcell parts

– Apoptosis(cellularself-destruct)?

Lysosomes are digestive compartments

Lysosomes

Lysosomes are digestive compartments

Large storage vessels

Vacuoles

150

µm

Central vacuolein a plant cell

Fat vacuolesin adipose cells

Paramecium lives infresh water— takes on waterby osmosis.

Pumps out waterwith the contractilevacuole.

Contractile vacuole Endomembrane Systemn Continuous exchange of membrane and

membranous contents

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wNuclear envelope

wEndoplasmic reticulum

wGolgi apparatus

w Lysosomes

wVacuoles & vesicles

Endomembrane System Peroxisomesn May form from ER or autonomouslyn Both produces and removes hydrogen peroxide

– Detoxifies organic compounds, e.g. ethanol– Destroy bacteria

n ß-oxidation offatty acids

n Phospholipidsynthesis— esp. myelin

ChloroplastPeroxisome

Mitochondrion

1 µmFigure 6.19

Cellular Organelles: bioenergetics

n Mitochondria– Double-membrane

– Aerobic respiration• Powerhouse of cell

• Converts chemicalenergy fromcatabolism into ATP

– Have some of ownDNA to maintainactivity whennucleus isunavailable

Cellular Organelles: bioenergetics

n Chloroplast– Triple-membrane– Photosynthesis

• uses sunlight to construct organic molecules (CO2Æ sugar)

– Also have some of their own DNA– Plants

& someprotists

Cytoskeleton

Organizingthe

organelles

protein framework thatextends throughout the

cytoplasm

Cytoskeleton

Basic Cell Structure

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Cell Support Structuresn Cytoskeleton

∅MyosinDynein

(and others)

motorprotein

Keratin

(and others)

ActinTubulin(dimer)

structuralprotein

intermediatefilaments

micro-filaments

micro-tubules

Cytoskeletal structures

Cytoskeleton

Tensegrity:A balance of

tension &compression

Cytoskeleton

Cytoskeleton Cellular Structure:Centrioles

Microtubule organizing centers

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The nucleus & the cytoskeleton

n Intermediate filaments penetrate envelope toconnect with nuclear lamina

n Microtubule organizing center adjacent to nucleus

Vesicle transport alongmicrotubule “rails”

ATP energy isused to pullvesicles alongmicrotubules.

Cell Mobility Structures

n Flagella

n Cilia

Eukaryotic versions

flagella & cilia

HOW DO CILIA AND FLAGELLA MOVE ?HOW DO CILIACILIA AND FLAGELLAFLAGELLA MOVE ?

n Dynein arms causemicrotubules to bend

Fig. 4.19

Molecularmotors:structure ofcilia &flagella

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Myosin cross-bridges pullactin filaments along

Crowded networkof cross-linkedmicrofilaments(red) underlyingthe plasmamembrane (blue) .

“cytoplasmic cortex”

Cytoskeletal support ofcell membrane

Cytoplasmic Streaming

n Motor proteins pullcytoplasmicmacromolecularcomplexes andorganelles along themicrofilamentmeshwork

Cytoskeleton and pseudopodia

n Selective growthof themicrofilamentmeshworkproduces a bulge(pseudopodium)in cell membranefor amoeboidmovement orphagocytosis.

The Cell: A Living Unit greater than the sum of Its parts

n Cells rely on the integration of structures and organellesin order to function

5 µ

m

Figure 6.32

Many Variations in Theme