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A Tour of the CellFriday Sept 16, 2005

BCOR 011

Lecture 8

10 µm 2

Common features of all cells

Plasma Membrane– defines inside from outside

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Plasma membrane– Functions as a selective barrier– Specific portals for selective transport of

materials in and out of cell

Carbohydrate side chain

Figure 6.8 A, B

Outside of cell

Inside of cell

Hydrophilicregion

Hydrophobicregion

Hydrophilicregion

(b) Structure of the plasma membrane

Phospholipid ProteinsTEM of a plasmamembrane. Theplasma membrane,here in a red bloodcell, appears as apair of dark bandsseparated by alight band.

(a)

0.1 µm

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Common features of all cells

Plasma Membrane– defines inside from outside

DNA “chromosomes”- Genetic material – hereditary instructions

Ribosomes– “factories” to synthesize proteins

Cytosol- Semifluid “inside” of the cell

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– Carry out protein synthesisER

CytosolFree ribosomes

Membrane Boundribosomes

Largesubunit

Smallsubunit

TEM showing ER and ribosomes

Diagram of a ribosome

0.5 µm

Figure 6.11

Ribosome – RNA & Protein Complex

ProteinsTo be exported

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Two Broad Two Broad Classes of CellsClasses of Cells

ProProkaryoteskaryotes EuEukaryoteskaryotesPro = before Eu = true

karyon = nucleus

DO NOT HAVEA NUCLEUS

NO internal membranes

HAVEA NUCLEUS

membrane-bound organelles

bacteria, cyanobacteriaarchaebacteria Plants, Animals,

Fungi, protists

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Bacterial Cell(Prokaryotic)

Nointernal

membranes

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(b) A thin section through thebacterium Bacillus coagulans(TEM)

Pili: attachment structures onthe surface of some prokaryotes

Nucleoid: region where thecell’s DNA is located (notenclosed by a membrane)

Ribosomes: organelles thatsynthesize proteins

Plasma membrane: membraneenclosing the cytoplasm

Cell wall: rigid structure outsidethe plasma membrane

Capsule: jelly-like outer coatingof many prokaryotes

Flagella: locomotionorganelles ofsome bacteria

(a) A typicalrod-shaped bacterium

0.5 µmBacterial

chromosome

Figure 6.6 A, B

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Bacterial cell(Prokaryotic

On the samesize scale:

Animal Cell (Eukaryotic) 10

Relative SizesRelative Sizes

“Typical” ~ 1-2 µMBacterium

“Typical” ~ 5 to 20 µM diameterAnimal Cell

“Typical” ~ 5 to 50 µM diameterPlant Cell

µM = micrometer or micron =10-6 meter

11Animal Cell (Eukaryotic)

Internalmembrane-bound

organelles

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Why Internal Membranes?Why Internal Membranes?

CompartmentalizationCompartmentalization(Division of Labor)(Division of Labor)

I’m I’m sleepingsleeping

I’m watching

TV

I’m cookingdinner

I’m playing

mysax

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Rough ER Smooth ER

Centrosome

CYTOSKELETON

Microfilaments

Microtubules

Peroxisome

Lysosome

Golgi apparatus

Ribosomes

In animal cells but not plant cells:LysosomesCentriolesFlagella (in some plant sperm)

NUCLEUS

Intermediate filaments

ENDOPLASMIC RETICULUM (ER)

Mitochondrion

Plasma membrane

Figure 6.9

endoplasmic reticulumnucleus

mitochondrion lysosome

Golgi apparatus

cytosol

ribosomes

Animal CellAnimal Cell

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Nucleus: Information storagedouble membrane

“Nuclear Envelope”

nucleolus

DNA housed, copied, read

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The NUCLEUSThe NUCLEUS

DoubleDoublemembranemembrane

DNADNARNARNAproteinproteinlipid (membrane)lipid (membrane)

NuclearNuclearporespores

NucleolusNucleolus

EuchromatinEuchromatinHeterochromatinHeterochromatin

NuclearNuclearLaminaLamina

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nuclear envelope

Figure 6.10

Nucleus

NucleusNucleolus

Chromatin

Nuclear envelope:Inner membraneOuter membrane

Nuclear pores

Rough ER

Porecomplex

Surface of nuclear envelope.

Pore complexes (TEM). Nuclear lamina (TEM).

Close-up ofnuclearenvelope

Ribosome

1 µm

1 µm

0.25 µm

Nuclear lamina

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NucleolusNucleolusSite of Site of Ribosome Ribosome Subunit Subunit Assembly Assembly

Note:Note:No membraneNo membrane

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EuchromatinEuchromatin regionregionSite of Site of mRNAmRNA synthesissynthesis

ExpressionExpressionOf Of InformationalInformationalRNAsRNAs

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Endoplasmic reticulum (ER)

1 :m

Rough ER Smooth ER

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Endoplasmic reticulum (ER)[Reticulum – network] Continuous network of flattened sacstubules, vesicles, throughout eukaryotic cytoplasm

Smooth ER

– Synthesizes membrane lipids– Synthesizes steroids– Stores calcium– Detoxifies poison

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Example: detoxification in smooth ER

Benzo(a)pyrenecharred meat, cigarette smoke

Oxidations –more soluble

Some metabolitesare more toxic

Chronic use ofbarbiturates, alcohol-SER proliferation,resistance

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Rough ER –ribosomes attached to cytoplasmic face

• Large flattened sheets• Synthesizes secreted proteins, membrane proteins

exported

• Protein modification; initial steps of carbohydrate addition

- glycoproteins

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Rough ERRough ER

Slips proteins Slips proteins Through ER Through ER membranemembrane

GlycosylationGlycosylationAdds Adds oligosaccharidesoligosaccharidesadded as protein being made

24Plasma membrane expandsby fusion of vesicles; proteinsare secreted from cell

Transport vesicle carriesproteins to plasma membrane for secretion

Lysosome availablefor fusion with anothervesicle for digestion

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Nuclear envelope isconnected to rough ER, which is also continuous

with smooth ER

Nucleus

Rough ER

Smooth ERcis Golgi

trans Golgi

Membranes and proteinsproduced by the ER flow in

the form of transport vesiclesto the Golgi

Golgi pinches off transport Vesicles and other vesicles

that give rise to lysosomes and Vacuoles

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3

2

Plasmamembrane

Figure 6.16

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Golgi Apparatus: protein secretionProcessing, packaging

and sorting center

CisGolgiClose

To RER

TransGolgi

Far sideAway From RER

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Functions of the Functions of the GolgiGolgi ApparatusApparatus

cis Golgi- processing center“near”

trans Golgi- sorting centersorting center“far”

Present wrapping Service –

modifies proteins

Fed Ex CentralFed Ex CentralSorts for delivery

To specific compartments

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Functions of the Functions of the GolgiGolgi ApparatusApparatus

•Trimming of Oligosaccharide side chains onglycosylated proteins

•Addition of new Oligosaccharide residues toexisting side chains of glycosylated proteins

••PhosphorylationPhosphorylation of specific sugar residuesof specific sugar residues onoligosaccharide side chains of

glycosylated proteins

“molecular zip codes”“molecular zip codes”

•“Maturation” Cleavages of specific proteinse.g., insulin

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Molecular tags route proteins to proper destination

P added in cis Golgi

Proteins with M-6-P tag bind receptorin trans Golgi

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Lysosomes: “Recycling Center”

sacs of digestive enzymes

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(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome containsactive hydrolyticenzymes

Food vacuole fuses with lysosome

Hydrolyticenzymes digestfood particles

Digestion

Food vacuole

Plasma membrane

Lysosome

Digestiveenzymes

Lysosome

Nucleus

EndocytosisAnd

Phagocytosis

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EXTRACELLULARFLUID

PseudopodiumCYTOPLASM

“Food” or other particle

Foodvacuole

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium viaphagocytosis (TEM).

PINOCYTOSIS

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM).

0.5 µm

In pinocytosis, the cell “gulps” droplets of extracellular fluid into tinyvesicles. It is not the fluiditself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.

Plasmamembrane

Vesicle

In phagocytosis, a cellengulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

Figure 7.20

PHAGOCYTOSIS

320.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coatedpit

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

Plasmamembrane

Coatprotein

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptorproteins are usually already clustered

in regions of the membrane called coated pits, which are lined on their cytoplasmicside by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

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• Autophagy

Figure 6.14 B

(b) Autophagy: lysosome breaking down damaged organelle

Lysosome containingtwo damaged organelles 1 µ m

Mitochondrionfragment

Peroxisomefragment

Lysosome fuses withvesicle containingdamaged organelle

Hydrolytic enzymesdigest organellecomponents

Vesicle containingdamaged mitochondrion

Digestion

Lysosome

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Vesicles move thru the endomembrane system

exocytosis

endocytosis

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Mitochondria: Powerhouses of the cell

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Mitochondria Mitochondria singular = singular = mitochondrionmitochondrion

•powerhouse of the animal cellproduces ~ 90% of ATP

•Carries out oxidative reactions

•Believed Derived from prokaryotic ancestor- DNA- ribosomes- double membrane – inner and outer

*define two functional spaces

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Mitochondria are enclosed by two membranes– A smooth outer membrane– An inner membrane folded into cristae

Mitochondrion

Intermembrane space

Outermembrane

Freeribosomesin the mitochondrialmatrix

MitochondrialDNA

Innermembrane

Cristae

Matrix100 µmFigure 6.17 38

Cell – organelles = CytosolGelImportant chemical reactionscytoskeleton - eukaryotes

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• The cytoskeleton– Is a network of fibers extending throughout

the cytoplasm– Structural Support– Movement of Materials and Organelles

Figure 6.20

Microtubule

0.25 µm MicrofilamentsFigure 6.2040

There are three types

of fibers that make

up the cytoskeleton

Table 6.1

Microtubules Microfilaments IntermediateFilaments

Tubulin25 µM dia

Actin7 µM dia various

8-15 µM dia

Cell shapeOrganelle movtChromosome

separationFlagellar movt

Cell shapeCell cleavageCytoplasmic

streaming Muscle contract

Nuclear laminaTension bearing

elementsAnchors

Motors:DyneinKinesis

Motors:Myosin

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– Movement of Vesicles along Microtubules

VesicleATP

Receptor formotor protein

Motor protein(ATP powered)

Microtubuleof cytoskeleton

(a) Motor proteins that attach to receptors on organelles can “walk”the organelles along microtubules or, in some cases, microfilaments.

Microtubule Vesicles 0.25 µm

(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, two vesicles can be seen moving along a microtubule. (A separate part of the experiment provided the evidence that they were in fact moving.)Figure 6.21 A, B 42

Motor MAPs transport vesicles

MTOC

Dyneininbound

outboundkinesin

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– Contains a pair of centrioles

Centrosome

Microtubule

Centrioles0.25 µm

Longitudinal sectionof one centriole

Microtubules Cross sectionof the other centrioleFigure 6.22

“microtubule-organizing center”

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• The ECM Is made up of glycoproteins

Collagen

Fibronectin

Plasmamembrane

EXTRACELLULAR FLUID

Micro-filaments

CYTOPLASM

Integrins

Polysaccharidemolecule

Carbo-hydrates

Proteoglycanmolecule

Coreprotein

Integrin

Figure 6.29

A proteoglycancomplex

• Animal cells– Lack cell walls– Are covered by an elaborate matrix, the ECM

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• Functions of the ECM include– Cell-Cell adhesion– Cell-Cell recognition– Regulation of cellular processes

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plant cellplant cell

CYTOSKELETON

Ribosomes (small brown dots)

Central vacuole/Tonoplast

MicrofilamentsIntermediate filaments

Microtubules

Rough endoplasmic reticulum

Smooth endoplasmicreticulum

NUCLEUS

Chloroplast

PlasmodesmataWall of adjacent cell

Cell wall

Golgi apparatus

PeroxisomePlasma membrane

Mitochondrion

Figure 6.9

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Plant Central vacuoles - Tonoplasts– Are found in plant cells– Hold reserves of important organic

compounds and water– Regulates Turgor

Central vacuole

Cytosol

Tonoplast

Centralvacuole

Nucleus

Cell wall

Chloroplast

5 µm

Figure 6.15 48

Chloroplast

ChloroplastDNA

Ribosomes

Stroma

Inner and outermembranes

Thylakoid

1 µm

Granum

Figure 6.18

In plant cells, chloroplastscapture energy from the sun

Photosynthesis

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ChloroplastsChloroplasts

-Contain DNA-Contain bacterial-like ribosomes-Believed derived from prokaryotic ancestor

cyanobacterium = blue-green alga

-Double membrane organelledefines three functional spaces

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StromaThylakoid

Space

Thylakoid Membrane

Intermembrane Space(transports things in and out of the chloroplast, but not central

to photosynthesis itself

Inner Inner ChlorplastChlorplastMembraneMembrane

OuterChlorplastMembrane

3 Central Players

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Cell Walls of Plants

• The cell wall– Is an extracellular structure of plant

cells that distinguishes them from animal cells

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• Plant cell walls– Are made of cellulose fibers embedded in

other polysaccharides and protein– May have multiple layers Central

vacuoleof cell

Plasmamembrane

Secondarycell wall

Primarycell wall

Middlelamella

1 µm

Centralvacuoleof cell

Central vacuole

Cytosol

Plasma membrane

Plant cell walls

PlasmodesmataFigure 6.28 54

• Plasmodesmata– Are channels that perforate plant cell walls

Interiorof cell

Interiorof cell

0.5 µm Plasmodesmata Plasma membranes

Cell walls

Figure 6.30

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Summary

Features of all cells

Features of Prokaryotes

Organelles of Animal Cells

Organelles of Plant Cells