ch 6 lecture (cell)...

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 6

A tour of the cell


Why study cells?• Cell Theory

– All organisms are made of cells

– The cell is the basic unit of organization for all organisms

• Cell = simplest collection of matter that can live

– All cells come from pre-existing cells


Why study cells?• Biological diversity & unity

– Underlying the diversity of life is a striking unity:

• DNA is the universal genetic language

• Cells are the basic units of structure & function

You are here


Why study cells?• Activities of life

– Almost everything you think of a whole organism needing to do, must be done at the cellular level:

• Reproduction

• Growth & development

• Energy utilization

• Response to environment

• Homeostasis


Why study cells?• Cell structure is correlated to cellular function

10 µm


How do we study cells?• Microscopes opened up the world of cells

– Robert Hooke (1665)

• the 1st cytologist


How do we study cells?• Microscopes

– Light microscope (LM)

– Electron microscope (EM)

• Transmission electron microscope (TEM)

• Scanning electron microscope (SEM)

Measurements1 centimeter (cm) = 102 meter (m) = 0.4 inch1 millimeter (mm) = 10–3 m1 micrometer (µm) = 10–3 mm = 10–6 m1 nanometer (nm) = 10–3 mm = 10–9 m


Light microscopes• visible light passes

through specimen

– use lenses to magnify & focus cellular structures

– 0.2 m resolution (~size of a bacterium)

– can be used to study live cells


Light microscopes– different methods for enhancing

visualization of cellular structuresTECHNIQUE RESULT

Brightfield (unstained specimen). Passes light directly through specimen. Unless cell is naturally pigmented or artificially stained, image has little contrast. [Parts (a)–(d) show a human cheek epithelial cell.]

(a)

Brightfield (stained specimen).Staining with various dyes enhances contrast, but most staining procedures require that cells be fixed (preserved).

(b)

Phase-contrast. Enhances contrast in unstained cells by amplifying variations in density within specimen; especially useful for examining living, unpigmented cells.

(c)

50 µm


Light microscopesDifferential-interference-contrast (Nomarski).Like phase-contrast microscopy, it uses optical modifications to exaggerate differences indensity, making the image appear almost 3D.

Fluorescence. Shows the locations of specific molecules in the cell by tagging the molecules with fluorescent dyes or antibodies. These fluorescent substances absorb ultraviolet radiation and emit visible light, as shown here in a cell from an artery.

Confocal. Uses lasers and special optics for “optical sectioning” of fluorescently-stained specimens. Only a single plane of focus is illuminated; out-of-focus fluorescence above and below the plane is subtracted by a computer. A sharp image results, as seen in stained nervous tissue (top), where nerve cells are green, support cells are red, and regions of overlap are yellow. A standard fluorescence micrograph (bottom) of this relatively thick tissue is blurry.

50 µm

50 µm

(d)

(e)

(f)with confocal enhancement

without confocal enhancement


Electron microscope (EM)– 1950s

– 2.0 nm resolution

– 100 times > light microscope

– reveals organelles

– for the most part, can only use on dead cells


Electron microscope (EM)• Uses a beam of electrons

– focused using electromagnets; generates computer image

– beam passes:

• through a specimen (TEM)

• onto its surface(SEM)


transmission electron microscope (TEM)• allows detailed study of the internal

ultrastructure of cells

• electron beam passes through thin section of specimen

Longitudinalsection ofcilium

Cross sectionof cilium

1 µm


scanning electron microscope (SEM)• allows detailed study of the surface of a

specimen

• sample surface covered with thin film of gold

• beam excites surface electrons

• great depth of field = image that seems 3-D

Cilia1 µm


SEM images


Isolating Organelles• Cell fractionation

– takes cells apart & separates major organelles

– separation based on variable density of organelles

• ultracentrifuge

Tissuecells

Homogenization

Homogenate1000 g(1000 times theforce of gravity)

10 min Differential centrifugationSupernatant pouredinto next tube

20,000 g20 min

Pellet rich innuclei andcellular debris

Pellet rich inmitochondria(and chloro-plasts if cellsare from a plant)

Pellet rich in“microsomes”(pieces of plasma mem-branes andcells’ internalmembranes)

Pellet rich inribosomes

150,000 g3 hr

80,000 g60 min


ultracentifuge• spins up to 130,000 rpm

– force > 1 million X gravity (1,000,000g)


microcentrifuge

Figure 6.5

• used in biotechnology research

– study cells at protein & DNA level


General cell characteristics• All cells:

– are surrounded by 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


Types of cells• Prokaryotic vs. eukaryotic cells

– location of chromosomes

Prokaryotic cell• DNA in nucleoid

region, without a membrane separating it fromrest of cell

Eukaryotic cell• Chromosomes in

nucleus, membrane-enclosed organelle


Prokaryotic cells

(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


Eukaryotic Cells• Have extensive and elaborately arranged

internal membranes, which form organelles

• Generally larger than prokaryotic cells


Eukaryotic cells – animal cell

Rough ER Smooth ER

Centrosome

CYTOSKELETON

Microfilaments

Microtubules

Microvilli

Peroxisome

Lysosome

Golgi apparatus

Ribosomes

Not in animal cells:ChloroplastsCentral vacuole & tonoplastCell wallPlasmodesmata

Nucleolus

Chromatin

NUCLEUS

Flagelium

Intermediate filaments

ENDOPLASMIC RETICULUM (ER)

Mitochondrion

Nuclear envelope

Plasma membrane


Eukaryotic cells – plant cell

CYTOSKELETON

Ribosomes (small brwon dots)

Central vacuole

MicrofilamentsIntermediate filaments

Microtubules

Rough endoplasmic reticulum Smooth

endoplasmic reticulum

ChromatinNUCLEUS

Nuclear envelope

Nucleolus

Chloroplast

PlasmodesmataWall of adjacent cell

Cell wall

Golgi apparatus

Peroxisome

Tonoplast

Centrosome

Plasma membrane

Mitochondrion

Not in plant cells:LysosomesCentriolesFlagella (in some plant sperm)


Limits to cell size• The logistics of carrying out cellular

metabolism sets limits on the size of cells

• Lower size limit: need enough DNA, enzymes, & “equipment” to maintain homeostasis & reproduce

– Smallest bacterial cells are 0.1 – 1.0 m (most bacteria are 1-10 m)

• Upper size limit: surface area to volume ratio

– Largest eukaryotic cells are generally 1-100 m


Size limits??• Thiomargarita namibiensis ("Sulfur pearl of

Namibia")

– gram-negative coccus Proteobacterium

– found in ocean sediments

– largest bacterium ever discovered

– up to 750 µm (0.75 mm); easily visible to the naked eye.


Limits to cell size• As a cell gets bigger, its volume increases

faster than its surface area

– Smaller objects have a greater ratio of surface area to volume

– facilitates the exchange of materials into & out of the cell

Surface area increases whiletotal volume remains constant

5

11

Total surface area (height width number of sides number of boxes)

Total volume (height width length number of boxes)

Surface-to-volume ratio (surface area volume)

6

1

6:1

150

125

~1:1

750

125

6:1


Limits to cell size• A large cell cannot move material in & out

fast enough to support life

• How to get bigger?? Become multicellular!


Cell Membrane

Carbohydrate side chain

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

• Exchange organelle

– aka plasma membrane

– Functions as a selective barrier

– Allows sufficient passage of O2, nutrients & waste


Nucleus – Genetic Library of the Cell• about 5 m in diameter

• contains most of the genes in the eukaryotic cell

• Where else are genes found in the cell?

mitochondria&

chloroplasts


Nucleus structureNucleus

NucleusNucleolus

Chromatin

Nuclear envelope:Inner membrane

Outer membrane

Nuclear pore

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 envelope• double membrane (separated by 20-40 nm

space)

• encloses the nucleus, separating its contents from the cytoplasm


Nuclear envelope• Perforated by pores (~100 nm diam.)

• Pores lined with pore complex (protein structure) that regulates entry & exit of large molecules & particles

• Examples?

RNA,ribosomal subunits


Nuclear contents• Within nucleus, DNA organized into fibrous

material, chromatin

– Appears as diffuse mass in non-dividing cell

• When cell prepares to divide, chromatin fibers coil up as distinct structures, chromosomes


Nuclear envelope• Except at pores, nuclear side of envelope is

lined with nuclear lamina

– netlike array of protein filaments (intermediate filaments; lamin polymers) embedded in inner membrane

– maintains nuclear shape

– mutations in lamins & lamin-binding proteins cause a wide range of human diseases, collectively termed laminopathies including muscular dystrophy & accelerated aging


Nucleolus • prominent in non-dividing cells

– number of nucleoli depends on species

• composed of densely staining granules & fibers

• synthesis of ribosomal RNA

• assembly with imported proteins to form ribosomal subunits (exit through pores)


Ribosomes• Function

– protein synthesis

– cells that make many proteins have more ribosomes (and nucleoli!)• Ex. pancreas (digestive enzymes; insulin)

• Structure

– contain ribosomal RNA (rRNA) & protein

• rRNA (not protein) catalyzes peptide bond formation in protein synthesis!

– composed of 2 subunits


ribosomes

Ribosomes Cytosol

Free ribosomes

Bound ribosomes

Largesubunit

Smallsubunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 µm


Types of ribosomes• Free ribosomes

– suspended in cytosol

– synthesize proteins that function within cytosol

• Bound ribosomes

– attached to outside of endoplasmic reticulum or nuclear envelope

– synthesize proteins for export or for membranes

• Structurally identical! Can switch roles as needed


Eukaryotic vs. prokaryotic ribosomes• Prokaryotes & eukaryotes have different

ribosomes

– different size subunits

– different proteins

– evolutionarysignificance?


Endomembrane system• Includes many different structures:

– Endoplasmic reticulum, Golgi apparatus, Lysosomes, Vacuoles, Vesicles

• Related either thru direct contact or bytransfer of vesicles

• Plays key role in synthesis (& hydrolysis) of macromolecules in cell (biosynthesis)

• Various “players” modify macromolecules for various functions


Plasma membrane expandsby fusion of vesicles; proteinsare secreted from cell

Transport vesicle carriesproteins to plasma membrane for secretion

Lysosome can fuse with Another vesicle for digestion

Nuclear envelope isconnected to rough & smooth ER

Nucleus

Rough ER

Smooth ER cis Golgi

trans Golgi

Membranes & proteinsMade by ER flow to Golgi in transport vesicles

Golgi pinches off transport Vesicles & other vesiclesthat give rise to lysosomes & Vacuoles

1

3

2

Plasmamembrane

Endomembrane system• Relationships among organelles of the

endomembrane system


Endoplasmic Reticulum: Biosynthetic Factory• Function

– Makes membranes

– Performs many biosynthesis functions

• Structure

– Membrane connected to nuclear envelope & extends throughout cell

– Accounts for ~50% of total membrane in many eukaryotic cells

• Rough ER = w/ bound ribosomes

• Smooth ER = w/o bound ribosomes


Endoplasmic reticulum

Smooth ER

Rough ER

ER lumenCisternae

RibosomesTransport vesicleSmooth ER

Transitional ER

Rough ER 200 µm

Nuclearenvelope

• Consists of membranous tubules & sacs called cisternae

• ER separates internal compartment from cytosol (cisternal space)


Endoplasmic reticulum• Functions of smooth ER

– Synthesizes lipids

• Oils, phospholipids, steroids

– STF: ovaries & testes are rich in smooth ER . . . Why??

– Metabolizes carbohydrates

• Hydrolysis of glycogen in liver cells

– 1st produces glucose phosphate which can’t cross membrane; ER enzymes remove PO4 so it can enter bloodstream


Endoplasmic reticulum• Functions of smooth ER (cont’d)

– Detoxifies drugs & poisons

• Usually by adding OH–; makes toxin more soluble

– Cell responds to increased drug use by making more smooth ER = tolerance

– Stores calcium ions for muscle contraction

• Pumps Ca2+ from cytosol to cisternal space

– When Ca 2+ rushes back out, muscle contracts


Endoplasmic reticulum• Functions of rough ER

– Membrane factory

• Grows in place by adding proteins & phospholipids (assemble by enzymes in ER)

– as ER membrane expands, it buds off & transfers to other cell parts that need it

• Membrane bound proteins synthesized directly into membrane

– anchored by hydrophobic portions


Endoplasmic reticulum

• Functions of rough ER (contd.)

– Produces proteins to be distributed by transport vesicles

• mainly secretory proteins (for export)

– mostly glycoproteins (= proteins that are covalently bonded to carbohydrates)

• carbohydrate (oligosaccharides) attached to protein in ER membrane

• growing polypeptide chain threads into cisternal space (through pore) where it folds into native shape


Golgi Apparatus : receiving, processing, & shipping

• first reported by Camillo Golgi (1843-1926)

– at a meeting of the Medical Society of Pavia on 19 April 1898

– he named it the 'internal reticular apparatus'


Golgiapparatus

TEM of Golgi apparatus

cis face(“receiving” side)

Vesicles movefrom ER to GolgiVesicles also

transport certainproteins back to ER

Vesicles coalesce toform new cis Golgi cisternae

Cisternalmaturation:

move from cis-to-trans

Vesicles bud & carry proteins to other locations or to plasma membrane for secretion

Vesicles transport someproteins backward to newerGolgi cisternae

Cisternae

trans face(“shipping”)

0.1 0 µm16

5

2

3

4

Golgi apparatus


Golgi Apparatus

• Structure

– consists of flattened membranous sacs called cisternae

• look like stack of pita bread

• usually 3-8 sacs per set

– can be as few as 1 in animal cells

– can be hundreds in some plant cells

• specialized secretory cells have more sets


Golgi Apparatus

• Structure (contd.)

– Usually near nucleus

– 2 sides = 2 functions (polarity!)

• Cis = “receiving” = receives material by fusing with vesicles from ER

• Trans = “shipping” = buds off vesicles that travel to other sites (transport)


• Functions

– Finishes, sorts, & ships cell products

– Extensive in cells specialized for secretion

– Which cells have a lot of Golgi?

Golgi Apparatus

Secretory cells(pancreatic,

intestinal goblet, salivary)


Golgi apparatus• Functions (contd.)

– modification of rough ER products

• done by changing sugar monomers

• allows wide variety of oligosaccharides

– manufacture of certain macromolecules

• pectins & non-cellulose polysaccharides

– sorting macromolecules & targeting for proper destination

• done by adding molecular ID tags (ex. phosphate groups)


Lysosomes: Digestive Compartments• membranous sacs of hydrolytic enzymes

– can digest all kinds of macromolecules

• examples?

– enzymes & membrane of lysosomes are synthesized by rough ER & transferred to Golgi

PolysaccharidesPolypeptides/proteins

Lipids/fatty acidsNucleic acids


lysosomes• fuse with vacuoles containing ingested food of

cell parts to be recycled

• polymers are digested into monomers

– pass to cytosol to become nutrients of cell


Lysomes • Help carry out intracellular digestion of

engulfed particles. What is this process called?

(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome containsactive hydrolyticenzymes

Food vacuole fuses with lysosome

Hydrolyticenzymes digestfood particles

Digestion

Food vacuole

Plasma membraneLysosome

Digestiveenzymes

Lysosome

Nucleus

Phagocytosis

• what kinds of cells do this?

Protists (ameba)Macrophages (WBCs)


lysosomes• Also use hydrolytic enzymes to recycle cell’s

own organic material . . . called?

(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

Autophagy

A human liver cell recycles half of its macromolecules

each week!


Lysosomal enzymes• Work best at pH 5 (cytosol pH ~7.3)

– organelle creates custom pH

• How?

• Why?

Proteins in lysosomal membrane pump H+ ions from cytosol into lysosome

• Enzymes are proteins• Proteins are sensitive to pH – affects structure• Digestive enzymes won’t work well if they leak

into cytosol – helps prevent autodigestion


When things go wrong . . . • If lysosomal enzymes don’t function,

– biomolecules collect & accumulate in lysosomes

• Lysosomes grow larger & larger, eventually disrupting cell & organ function

• “Lysosomal storage diseases” are usually fatal

• Ex. Tay-Sachs disease– Lipids build up in

brain cells

– Child dies before age 5


Sometimes, it’s supposed to work that way . . • Apoptosis = programmed cell death

– Auto-destruct mechanism

– Some cells must die in an organized fashion, especially during development

• Ex. Space between your fingers

• Ex. Self-destruction of damaged cells before they can become cancerous


Vacuoles• Diverse maintenance compartments

– Food vacuoles

• formed by phagocytosis

– Contractile vacuoles

• Pump excess water out of protist cells


vacuoles

Central vacuole

Cytosol

Tonoplast

Centralvacuole

Nucleus

Cell wall

Chloroplast

5 µm

• Central vacuoles (in plant cells)

– Functions:

• Storage, waste disposal, protection, growth

– hold reserves of important organic compounds & water

– increase surface area (internally)


Peroxisomes: Oxidation• Peroxisomes

– Do not bud from endomembrane system

• Membrane grows in place using proteins & lipids in cytosol

– Produce hydrogen peroxide & convert it to water

– (2H2O2 2H2O + O2)

ChloroplastPeroxisome

Mitochondrion

1 µm


Mitochondria & chloroplasts• Energy conversion!

– not “creation” of energy; 1st law of thermodynamics!

• Mitochondria

– sites of cellular respiration

• Chloroplasts

– sites of photosynthesis

– found only in plants (& algae)


Mitochondria: Chemical Energy Conversion• found in nearly all eukaryotic cells


mitochondria• enclosed by two membranes

• smooth outer membrane

• inner membrane folded into cristae

– STF: what is the value of christae?

• interior filled with matrix

– contains enzymes, mitochondrial DNA, ribosomes, proteins

– Example of compartmentalization!


mitochondria

Mitochondrion

Intermembrane space

Outermembrane

Freeribosomesin the mitochondrialmatrix

MitochondrialDNA

Innermembrane

Cristae

Matrix

100 µm


Chloroplast: Capture of Light Energy• specialized member of a family of closely

related plant organelles called plastids

• contains chlorophyll


chloroplasts• found in leaves and other green organs of

plants and in algae

Chloroplast

ChloroplastDNA

RibosomesStroma

Inner and outermembranes

Thylakoid

1 µm

Granum


chloroplast• structure includes

– Thylakoids, membranous disc-shaped sacs

• Stacked to form granum (pl. = grana)

– Stroma, the internal fluid surrounding thylakoids

• Contains chloroplast DNA, ribosomes, proteins, enzymes

• Example of compartmentalization!


cytoskeleton• network of fibers extending throughout the

cytoplasm

Microtubule

0.25 µm Microfilaments


Cytoskeleton• functions

– mechanical support

– motility

– regulation


cytoskeleton– motor proteins interact with cytoskeleton to

provide motilityVesicleATP

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.)


Components of the Cytoskeleton• three main types of fibers make up the

cytoskeleton


Microtubules• Microtubules

– Hollow; resist compression

– Shape the cell

– Guide movement of organelles

– Help separate the chromosome copies in dividing cells


Centrosomes & Centrioles• centrosome

– “microtubule-organizing center”

– in animal cells,contains a pair ofcentrioles

Centrosome

Microtubule

Centrioles0.25 µm

Longitudinal sectionof one centriole

Microtubules Cross sectionof the other centriole


Cilia and Flagella• Cilia and flagella

– Contain specialized arrangements of microtubules

– Are locomotor appendages of some cells


• Flagella beating pattern

(a) Motion of flagella. A flagellumusually undulates, its snakelikemotion driving a cell in the samedirection as the axis of theflagellum. Propulsion of a humansperm cell is an example of flagellatelocomotion (LM).

1 µm

Direction of swimming


• Ciliary motion

(b) Motion of cilia. Cilia have a back-and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, afreshwater protozoan (SEM).

15 µm


Cilia & flagella• share common ultrastructure

(a)

(c)

(b)

Outer microtubuledoubletDynein arms

Centralmicrotubule

Outer doublets cross-linkingproteins inside

Radialspoke

Plasmamembrane

Microtubules

Plasmamembrane

Basal body

0.5 µm

0.1 µm

0.1 µm

Cross section of basal body

Triplet

9 + 2 doublets

9 triplets


• The protein dynein

– Is responsible for the bending movement of cilia and flagella

Microtubuledoublets ATP

Dynein arm

Powered by ATP, the dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again. If the two microtubule doublets were not attached, they would slide relative to each other.

(a)


Outer doubletscross-linkingproteins

Anchoragein cell

ATP

In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two ofthe nine outer doublets in Figure 6.24b are shown here.)

(b)


Localized, synchronized activation of many dynein arms probably causes a bend to begin at the base of the Cilium or flagellum and move outward toward the tip. Many successive bends, such as the ones shown here to the left and right, result in a wavelike motion. In this diagram, the two central microtubules and the cross-linking proteins are not shown.

(c)

1 3

2


Microfilaments (actin)• Solid; tension-bearing

• Network gives cortex (outer cytoplasmic layer) gel consistency

– vs. sol consistency of cytosol

• Alternate with myosin to contract

– muscle contraction

– contracting band forms cleavage furrow in cytokinesis


microfilaments– found in microvilli

0.25 µm

Microvillus

Plasma membrane

Microfilaments (actinfilaments)

Intermediate filaments


microfilaments


microfilaments• Those that function in cellular motility

– contain the protein myosin in addition to actin

Actin filament

Myosin filament

Myosin motors in muscle cell contraction. (a)

Muscle cell

Myosin arm


• Amoeboid movement

– Involves the contraction of actin and myosin filaments

Cortex (outer cytoplasm):gel with actin network

Inner cytoplasm: sol with actin subunits

Extendingpseudopodium

(b) Amoeboid movement


• Cytoplasmic streaming

– another form of locomotion created by microfilaments

Nonmovingcytoplasm (gel)

ChloroplastStreamingcytoplasm(sol)

Parallel actinfilaments Cell wall

(b) Cytoplasmic streaming in plant cells


Intermediate fibers• Solid; intermediate diameter

• Tension-bearing

• Diverse class of elements

– in keratin family of proteins

• More permanent

• Maintain overall shape & anchor some organelles


Cell Walls of Plants– extracellular structure of plant cells that

distinguishes them from animal cells

– made of cellulose fibers embedded in other polysaccharides and protein

– may have multiple layers


Plant cell walls

Central vacuoleof cell

Plasmamembrane

Secondarycell wall

Primarycell wall

Middlelamella

1 µm

Centralvacuoleof cell

Central vacuole

Cytosol

Plasma membrane

Plant cell walls

Plasmodesmata


Extracellular matrix (ECM)• Main ingredients are glycoproteins (collagen,

proteoglycans, & fibronectins)

– Fibronectins bind to integrins (receptor proteins)

Collagen

Fibronectin

Plasmamembrane

EXTRACELLULAR FLUID

Micro-filaments

CYTOPLASM

Integrins

Polysaccharidemolecule

Carbo-hydrates

Proteoglycanmolecule

Coreprotein

Integrin

A proteoglycan complex


Exctracellular matrix• Functions of the ECM include

– Support

– Adhesion

– Movement

– Regulation

• New research suggests that changes in ECM can alter internal cell structure & function

Collagen fibers

Collagen fibrils


Intercellular Junctions• Integrate cells into higher levels of

structure & function

– Organize animal & plant cells into tissues, organs, and organ systems

• Special patches of direct physical contact

– Allow neighboring cells adhere, interact, & communicate


Intercellular Junctions in plants• plasmodesmata

– channels that perforate cell walls

– plasma membrane lines channels

– cytosol flows between cells

– unifies most of plant into one living continuumInteriorof cell

Interiorof cell

0.5 µm Plasmodesmata Plasma membranes

Cell walls


Intercellular junctions in animals• integrate cells in various ways

• three types:

– Tight junctions

– Desmosomes

– Gap junctions

• structure fits function!


Intercellular junctions in animals

Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

Space between cells

Plasma membranesof adjacent cells

Extracellularmatrix

Gap junction

Tight junctions

0.1 µm

Intermediatefilaments

DesmosomeGapjunctions

•Neighboring cells fused•Form continuous belts•Prevent leakage

•Anchoring junctions•Rivets fasten cells into sheets•Reinforced w/ intermediate fibers

•Communication junctions•Provide cytoplasmic channels•Common in animal embryos

TIGHT JUNCTIONS

DESMOSOMES

GAP JUNCTIONS


Cell functions are emergent properties

• the cell is a living unit greater than the sum of its parts

• How do cell parts interact in this function?

5 µm


Any questions??

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