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Overview: The Fundamental Units of Life

• All organisms are made of cells• The cell is the simplest collection of matter

that can be alive

•

Cell structure is correlated to cellular function• All cells are related by their descent from earlier

cells

© 2011 Pearson Education, Inc.



Concept 6.1: Biologists use microscopes and

the tools of biochemistry to study cells• Though usually too small to be seen by the

unaided eye, cells can be complex




Microscopy

• Scientists use microscopes to visualize cells toosmall to see with the naked eye

• In a light microscope (LM), visible light ispassed through a specimen and then through

glass lenses• Lenses refract (bend) the light, so that the image

is magnified




• Three important parameters of microscopy – Magnification , the ratio of an object’s image size

to its real size

– Resolution , the measure of the clarity of the

image, or the minimum distance of twodistinguishable points

– Contrast , visible differences in parts of thesample




Figure 6.2 10 m

1 m

0.1 m

1 cm

1 mm

100 m

10 m

1 m

100 nm

10 nm

1 nm

0.1 nm Atoms

Small molecules

Lipids

Proteins

Ribosomes

Viruses

Smallest bacteria

Mitochondrion

Most bacteria

Nucleus

Most plant andanimal cells

Human egg

Frog egg

Chicken egg

Length of somenerve andmuscle cells

Human height

U n a i d

e d e y e

L i g h t m i c r o s

c o p y

E l e c t r o n m

i c r o s c o p y

Super-resolution

microscopy



Brightfield

(unstained specimen)

Brightfield

(stained specimen)

5 0 m

Confocal

Differential-interference-contrast (Nomarski)

Fluorescence

10 m

Deconvolution

Super-resolution

Scanning electronmicroscopy (SEM)

Transmission electronmicroscopy (TEM)

Cross sectionof cilium

Longitudinal sectionof cilium

Cilia

Electron Microscopy (EM)

1 m

1 0 m

5 0 m

2 m

2 m

Light Microscopy (LM)

Phase-contrast

Figure 6.3



• LMs can magnify effectively to about 1,000 timesthe size of the actual specimen

• Various techniques enhance contrast andenable cell components to be stained or labeled

• Most subcellular structures, includingorganelles (membrane-enclosedcompartments), are too small to be resolved by

an LM




• Two basic types of electron microscopes(EMs) are used to study subcellular structures

• Scanning electron microscopes (SEMs) focusa beam of electrons onto the surface of aspecimen, providing images that look 3-D

• Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen

• TEMs are used mainly to study the internalstructure of cells




• Recent advances in light microscopy – Confocal microscopy and deconvolution

microscopy provide sharper images of three-dimensional tissues and cells

– New techniques for labeling cells improveresolution




Cell Fractionation

• Cell fractionation takes cells apart andseparates the major organelles from oneanother

• Centrifuges fractionate cells into theircomponent parts

• Cell fractionation enables scientists to determinethe functions of organelles

• Biochemistry and cytology help correlate cellfunction with structure




Figure 6.4 TECHNIQUE

Homogenization

Tissuecells

Homogenate

Centrifugation

Differentialcentrifugation

Centrifuged at1,000 g

(1,000 times theforce of gravity)

for 10 min Supernatantpoured intonext tube

20,000 g 20 min

80,000 g

60 minPellet rich innuclei andcellular debris

150,000 g

3 hr

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

Pellet rich in“microsomes” (pieces of plasmamembranes andcells’ internal

membranes)

Pellet rich in

ribosomes



Figure 6.4a

TECHNIQUE

Homogenization

Tissuecells

Homogenate

Centrifugation



Differentialcentrifugation

Centrifuged at1,000 g

(1,000 times theforce of gravity)

for 10 min Supernatantpoured into

next tube

20 min

60 minPellet rich innuclei andcellular debris

3 hr

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

Pellet rich in“microsomes”

Pellet rich in

ribosomes

20,000 g

80,000 g

150,000 g

TECHNIQUE (cont.)Figure 6.4b



Concept 6.2: Eukaryotic cells have internal

membranes that compartmentalize their

functions

• The basic structural and functional unit of every

organism is one of two types of cells: prokaryoticor eukaryotic

• Only organisms of the domains Bacteria andArchaea consist of prokaryotic cells

• Protists, fungi, animals, and plants all consist ofeukaryotic cells




Comparing Prokaryotic and Eukaryotic

Cells

• Basic features of all cells

– Plasma membrane

– Semifluid substance called cytosol – Chromosomes (carry genes)

– Ribosomes (make proteins)




• Prokaryotic cells are characterized by having – No nucleus

– DNA in an unbound region called the nucleoid

– No membrane-bound organelles – Cytoplasm bound by the plasma membrane


Fi



Fimbriae

Bacterial

chromosome

A typicalrod-shaped

bacterium

(a)

Nucleoid

Ribosomes

Plasmamembrane

Cell wall

Capsule

Flagella A thin sectionthrough the

bacterium Bacillus coagulans (TEM)

(b)

0.5 m

Figure 6.5



• Eukaryotic cells are characterized by having – DNA in a nucleus that is bounded by a

membranous nuclear envelope

– Membrane-bound organelles

– Cytoplasm in the region between the plasmamembrane and nucleus

• Eukaryotic cells are generally much larger than

prokaryotic cells




• The plasma membrane is a selective barrierthat allows sufficient passage of oxygen,nutrients, and waste to service the volume ofevery cell

• The general structure of a biological membraneis a double layer of phospholipids


Fi 6 6



Figure 6.6

Outside of cell

Inside of cell0.1 m

(a) TEM of a plasmamembrane

Hydrophilicregion

Hydrophobicregion

Hydrophilicregion

Carbohydrate side chains

ProteinsPhospholipid

(b) Structure of the plasma membrane



• Metabolic requirements set upper limits on thesize of cells

• The surface area to volume ratio of a cell iscritical

• As the surface area increases by a factor of n 2,the volume increases by a factor of n 3

• Small cells have a greater surface area relative

to volume


Figure 6 7



Surface area increases whiletotal volume remains constant

Total surface area[sum of the surface areas(height width) of all boxsides number of boxes]

Total volume[height width length

number of boxes]

Surface-to-volume(S-to-V) ratio[surface area volume]

1

5

6 150 750

1

1251251

1.26 6

Figure 6.7



A Panoramic View of the Eukaryotic Cell

• A eukaryotic cell has internal membranes thatpartition the cell into organelles

• Plant and animal cells have most of the sameorganelles


Figure 6 8a



Figure 6.8a

ENDOPLASMIC RETICULUM (ER)

RoughER

SmoothER

Nuclearenvelope

Nucleolus

Chromatin

Plasmamembrane

Ribosomes

Golgi apparatus

LysosomeMitochondrion

Peroxisome

Microvilli

Microtubules

Intermediate filamentsMicrofilaments

Centrosome

CYTOSKELETON:

Flagellum NUCLEUS

Figure 6 8b



Figure 6.8b

Animal Cells

Cell

Nucleus

Nucleolus

Human cells from liningof uterus (colorized TEM)

Yeast cells budding(colorized SEM)

1 0 m Fungal Cells

5 m

Parentcell

Buds

1 m

Cell wall

Vacuole

Nucleus

Mitochondrion

A single yeast cell(colorized TEM)

Figure 6 8c



NUCLEUS

Nuclearenvelope

Nucleolus

Chromatin

Golgiapparatus

Mitochondrion

Peroxisome

Plasma membrane

Cell wall

Wall of adjacent cell

Plasmodesmata

Chloroplast

Microtubules

Intermediatefilaments

Microfilaments

CYTOSKELETON

Central vacuole

Ribosomes

Smoothendoplasmicreticulum

Roughendoplasmic

reticulum

Figure 6.8c

Figure 6 8d



Figure 6.8d

Plant Cells

Cells from duckweed(colorized TEM)

Cell

5 m

Cell wall

Chloroplast

Nucleus

Nucleolus

8 m

Protistan Cells

1 m

Chlamydomonas (colorized SEM)

Chlamydomonas (colorized TEM)

Flagella

Nucleus

Nucleolus

Vacuole

Chloroplast

Cell wall

Mitochondrion



Concept 6.3: The eukaryotic cell’s genetic

instructions are housed in the nucleus and

carried out by the ribosomes

• The nucleus contains most of the DNA in a

eukaryotic cell• Ribosomes use the information from the DNA to

make proteins




The Nucleus: Information Central

• The nucleus contains most of the cell’s genesand is usually the most conspicuous organelle

• The nuclear envelope encloses the nucleus,separating it from the cytoplasm

• The nuclear membrane is a double membrane;each membrane consists of a lipid bilayer


Figure 6.9



Nucleus

Rough ER

Nucleolus

Chromatin

Nuclear envelope:

Inner membrane

Outer membrane

Nuclear pore

Ribosome

Porecomplex

Close-up

of nuclearenvelope

Surface of nuclearenvelope

Pore complexes (TEM)

0 . 2

5 m

1 m

Nuclear lamina (TEM)

Chromatin

1 m

gu e 6 9

Figure 6.9a



Nucleus

Rough ER

Nucleolus

Chromatin

Nuclear envelope:

Inner membrane

Outer membrane

Nuclear pore

Chromatin

Ribosome

Porecomplex

Close-upof nuclear

envelope

g



• Pores regulate the entry and exit of moleculesfrom the nucleus

• The shape of the nucleus is maintained by thenuclear lamina, which is composed of protein




• In the nucleus, DNA is organized into discreteunits called chromosomes

• Each chromosome is composed of a single DNAmolecule associated with proteins

• The DNA and proteins of chromosomes aretogether called chromatin

• Chromatin condenses to form discrete

chromosomes as a cell prepares to divide • The nucleolus is located within the nucleus and

is the site of ribosomal RNA (rRNA) synthesis




Ribosomes: Protein Factories

• Ribosomes are particles made of ribosomalRNA and protein

• Ribosomes carry out protein synthesis in twolocations

– In the cytosol (free ribosomes)

– On the outside of the endoplasmic reticulum orthe nuclear envelope (bound ribosomes)


Figure 6.10



0.25 m

Free ribosomes in cytosol

Endoplasmic reticulum (ER)

Ribosomes bound to ER

Largesubunit

Smallsubunit

Diagram of a ribosomeTEM showing ER andribosomes

Figure 6.10a



0.25 m

Free ribosomes in cytosol

Endoplasmic reticulum (ER)

Ribosomes bound to ER

TEM showing ER andribosomes



Concept 6.4: The endomembrane system

regulates protein traffic and performs

metabolic functions in the cell

• Components of the endomembrane system

– Nuclear envelope – Endoplasmic reticulum

– Golgi apparatus

– Lysosomes

– Vacuoles – Plasma membrane

• These components are either continuous orconnected via transfer by vesicles




The Endoplasmic Reticulum: Biosynthetic

Factory

• The endoplasmic reticulum (ER) accounts formore than half of the total membrane in manyeukaryotic cells

• The ER membrane is continuous with thenuclear envelope

• There are two distinct regions of ER

– Smooth ER, which lacks ribosomes

– Rough ER, surface is studded with ribosomes


Figure 6.11 Smooth ERNuclear



Rough ER

ER lumen

CisternaeRibosomes

Smooth ERTransport vesicle

Transitional ER

Rough ER200 nm

Nuclearenvelope

Figure 6.11a



Smooth ER

Rough ER

CisternaeRibosomes

Transport vesicle

Transitional ER

Nuclearenvelope

ER lumen

Figure 6.11b



Smooth ER Rough ER 200 nm



Functions of Smooth ER

• The smooth ER – Synthesizes lipids

– Metabolizes carbohydrates

– Detoxifies drugs and poisons

– Stores calcium ions




Functions of Rough ER

• The rough ER – Has bound ribosomes, which secrete

glycoproteins (proteins covalently bonded tocarbohydrates)

– Distributes transport vesicles, proteinssurrounded by membranes

– Is a membrane factory for the cell




• The Golgi apparatus consists of flattenedmembranous sacs called cisternae

•

Functions of the Golgi apparatus – Modifies products of the ER

– Manufactures certain macromolecules

– Sorts and packages materials into transport

vesicles

The Golgi Apparatus: Shipping and

Receiving Center


Figure 6.12



cis face

(“receiving” side of Golgi apparatus)

trans face(“shipping” side of Golgi apparatus)

0.1 m

TEM of Golgi apparatus

Cisternae

Figure 6.12a



TEM of Golgi apparatus

0.1 m



Lysosomes: Digestive Compartments

•A lysosome is a membranous sac ofhydrolytic enzymes that can digestmacromolecules

• Lysosomal enzymes can hydrolyze proteins,

fats, polysaccharides, and nucleic acids

• Lysosomal enzymes work best in the acidicenvironment inside the lysosome





Animation: Lysosome FormationRight-click slide / select “Play”



•

Some types of cell can engulf another cell byphagocytosis; this forms a food vacuole

• A lysosome fuses with the food vacuole anddigests the molecules

• Lysosomes also use enzymes to recycle thecell’s own organelles and macromolecules, aprocess called autophagy


Figure 6.13



Nucleus

Lysosome

1 m

Digestiveenzymes

Digestion

Food vacuole

LysosomePlasma membrane

(a) Phagocytosis

Vesicle containingtwo damagedorganelles

1 m

Mitochondrionfragment

Peroxisomefragment

(b) Autophagy

Peroxisome

VesicleMitochondrion

Lysosome

Digestion

Figure 6.13a

Nucleus1 m



Nucleus

Lysosome

Digestiveenzymes

Digestion

Food vacuole

LysosomePlasma membrane

(a) Phagocytosis

Figure 6.13aa



Nucleus

Lysosome

1 m

Figure 6.13bVesicle containing

d d



two damagedorganelles

1 m

Mitochondrionfragment

Peroxisomefragment

Peroxisome

VesicleMitochondrion

Lysosome

Digestion

(b) Autophagy



Vacuoles: Diverse Maintenance

Compartments

• A plant cell or fungal cell may have one orseveral vacuoles, derived from endoplasmicreticulum and Golgi apparatus




•

Food vacuoles are formed by phagocytosis• Contractile vacuoles, found in many freshwater

protists, pump excess water out of cells

• Central vacuoles, found in many mature plantcells, hold organic compounds and water


Figure 6.14



Central vacuole

Cytosol

Nucleus

Cell wall

Chloroplast

Centralvacuole

5 m

Figure 6.14a



Cytosol

Nucleus

Cell wall

Chloroplast

Centralvacuole

5 m



The Endomembrane System: A Review •

The endomembrane system is a complex anddynamic player in the cell’s compartmental

organization


Figure 6.15-3



Smooth ER

Nucleus

Rough ER

Plasmamembrane

cis Golgi

trans Golgi



Concept 6.5: Mitochondria and chloroplasts

change energy from one form to another

• Mitochondria are the sites of cellular respiration,a metabolic process that uses oxygen togenerate ATP

• Chloroplasts, found in plants and algae, are thesites of photosynthesis

• Peroxisomes are oxidative organelles




• Mitochondria and chloroplasts have similarities

with bacteria

– Enveloped by a double membrane – Contain free ribosomes and circular DNA

molecules

– Grow and reproduce somewhat independently

in cells


The Evolutionary Origins of Mitochondria

and Chloroplasts



• The Endosymbiont theory – An early ancestor of eukaryotic cells engulfed

a nonphotosynthetic prokaryotic cell, whichformed an endosymbiont relationship with its

host – The host cell and endosymbiont merged into

a single organism, a eukaryotic cell with amitochondrion

– At least one of these cells may have taken upa photosynthetic prokaryote, becoming theancestor of cells that contain chloroplasts


NucleusEndoplasmicreticulum

Figure 6.16



reticulum

Nuclearenvelope

Ancestor ofeukaryotic cells(host cell)

Engulfing of oxygen-using nonphotosyntheticprokaryote, which

becomes a mitochondrion

Mitochondrion

Nonphotosyntheticeukaryote

Mitochondrion

At leastone cell

Photosynthetic eukaryote

Engulfing ofphotosyntheticprokaryote

Chloroplast



Mitochondria: Chemical Energy Conversion

•

Mitochondria are in nearly all eukaryotic cells• They have a smooth outer membrane and an

inner membrane folded into cristae

• The inner membrane creates two compartments:intermembrane space and mitochondrial matrix

• Some metabolic steps of cellular respiration arecatalyzed in the mitochondrial matrix

• Cristae present a large surface area for enzymesthat synthesize ATP


Figure 6.17



Intermembrane space

Outer

membrane

DNA

Innermembrane

Cristae

Matrix

Freeribosomesin themitochondrialmatrix

(a) Diagram and TEM of mitochondrion (b) Network of mitochondria in a protist

cell (LM)

0.1 m

MitochondrialDNA

Nuclear DNA

Mitochondria

10 m

Figure 6.17a



Intermembrane space

Outer

DNA

Innermembrane

Cristae

Matrix

Freeribosomesin themitochondrial

matrix

(a) Diagram and TEM of mitochondrion

0.1 m

membrane



Chloroplasts: Capture of Light Energy

•

Chloroplasts contain the green pigmentchlorophyll, as well as enzymes and othermolecules that function in photosynthesis

• Chloroplasts are found in leaves and other

green organs of plants and in algae




•

Chloroplast structure includes – Thylakoids, membranous sacs, stacked to

form a granum – Stroma, the internal fluid

• The chloroplast is one of a group of plantorganelles, called plastids


Figure 6.18



RibosomesStroma

Inner and outer

membranes

Granum

1 mIntermembrane spaceThylakoid

(a) Diagram and TEM of chloroplast (b) Chloroplasts in an algal cell

Chloroplasts(red)

50 m

DNA

Figure 6.18a



RibosomesStroma

Inner and outermembranes

Granum

1 mIntermembrane spaceThylakoid

(a) Diagram and TEM of chloroplast

DNA

Figure 6.18b



(b) Chloroplasts in an algal cell

Chloroplasts(red)

50 m



Peroxisomes: Oxidation

•

Peroxisomes are specialized metaboliccompartments bounded by a single membrane

• Peroxisomes produce hydrogen peroxide andconvert it to water

• Peroxisomes perform reactions with manydifferent functions

• How peroxisomes are related to other organelles

is still unknown


Figure 6.19



Chloroplast

PeroxisomeMitochondrion

1 m



Concept 6.6: The cytoskeleton is a network

of fibers that organizes structures and

activities in the cell

• The cytoskeleton is a network of fibersextending throughout the cytoplasm

• It organizes the cell’s structures and activities,

anchoring many organelles

• It is composed of three types of molecular

structures – Microtubules – Microfilaments – Intermediate filaments


Figure 6.20



1 0 m



Roles of the Cytoskeleton:

Support and Motility

• The cytoskeleton helps to support the cell andmaintain its shape

• It interacts with motor proteins to produce

motility• Inside the cell, vesicles can travel along

“monorails” provided by the cytoskeleton

• Recent evidence suggests that the cytoskeletonmay help regulate biochemical activities


Figure 6.21

Vesicle



ATPVesicle

(a)

Motor protein(ATP powered)

Microtubuleof cytoskeleton

Receptor formotor protein

0.25 mVesiclesMicrotubule

(b)

Figure 6.21a



0.25 mVesiclesMicrotubule

(b)

C f h C k l



Components of the Cytoskeleton

•

Three main types of fibers make up thecytoskeleton

– Microtubules are the thickest of the threecomponents of the cytoskeleton

– Microfilaments , also called actin filaments, arethe thinnest components

– Intermediate filaments are fibers withdiameters in a middle range


Table 6.1



Column of tubulin dimers

Tubulin dimer

25 nm

Actin subunit

7 nm

Keratin proteins

812 nm

Fibrous subunit (keratinscoiled together)

10 m 10 m 5 m

Table 6.1a



Tubulin dimer

25 nm

Column of tubulin dimers

10 m

Table 6.1b



10 m

Actin subunit

7 nm

Table 6.1c



5 m

Keratin proteins

Fibrous subunit (keratinscoiled together)

812 nm

Mi t b l



Microtubules

•

Microtubules are hollow rods about 25 nm indiameter and about 200 nm to 25 microns long

• Functions of microtubules

– Shaping the cell

– Guiding movement of organelles

– Separating chromosomes during cell division




Centrosomes and Centrioles • In many cells, microtubules grow out from a

centrosome near the nucleus

• The centrosome is a “microtubule-organizingcenter”

• In animal cells, the centrosome has a pair ofcentrioles, each with nine triplets of

microtubules arranged in a ring


Figure 6.22



Centrosome

Longitudinalsection ofone centriole

Centrioles

Microtubule

0.25 m

Microtubules Cross section

of the other centriole

Figure 6.22a



Longitudinal

section ofone centriole

0.25 m

Microtubules Cross sectionof the other centriole



Cilia and Flagella • Microtubules control the beating of cilia and

flagella, locomotor appendages of some cells

• Cilia and flagella differ in their beating patterns


Direction of swimmingFigure 6.23



(b) Motion of cilia

Direction of organism’s movement

Power stroke Recovery stroke

(a) Motion of flagella5 m

15 m



•

Cilia and flagella share a common structure – A core of microtubules sheathed by the plasma

membrane

– A basal body that anchors the cilium or

flagellum – A motor protein called dynein, which drives the

bending movements of a cilium or flagellum





Animation: Cilia and FlagellaRight-click slide / select “Play”

0.1 m Outer microtubuledoublet

Dynein proteins

Plasma membraneFigure 6.24



Microtubules

Plasmamembrane

Basal body

Longitudinal sectionof motile cilium

(a)

0.5 m 0.1 m

(b) Cross section ofmotile cilium

Dynein proteins

Centralmicrotubule

Radialspoke

Cross-linkingproteins betweenouter doublets

Triplet

(c) Cross section ofbasal body

Figure 6.24a



Microtubules

Plasmamembrane

Basal body

Longitudinal sectionof motile cilium

0.5 m

(a)

Figure 6.24b



0.1 m


Outer microtubuledoublet

Dynein proteins

Centralmicrotubule

Radialspoke


Plasma membrane

Figure 6.24ba



0.1 m


Outer microtubuledoublet

Dynein proteins

Centralmicrotubule

Radialspoke


Figure 6.24c



0.1 m

Triplet

(c) Cross section of

basal body

Figure 6.24ca



0.1 m

Triplet

(c) Cross section of

basal body



•

How dynein “walking” moves flagella and cilia − Dynein arms alternately grab, move, and release

the outer microtubules

– Protein cross-links limit sliding

– Forces exerted by dynein arms cause doublets tocurve, bending the cilium or flagellum


Figure 6.25 Microtubuledoublets ATP



Dynein protein

(a) Effect of unrestrained dynein movement

Cross-linking proteinsbetween outer doublets

ATP

Anchoragein cell

(b) Effect of cross-linking proteins

(c) Wavelike motion

1

2

3

MicrotubuleATP

Figure 6.25a



doublets

Dynein protein

ATP

(a) Effect of unrestrained dynein movement

Figure 6.25b



Cross-linking proteins

between outer doublets

ATP

Anchoragein cell

(b) Effect of cross-linking proteins (c) Wavelike motion

31

2

Microfilaments (Actin Filaments)



Microfilaments (Actin Filaments)

•

Microfilaments are solid rods about 7 nm indiameter, built as a twisted double chain of actin

subunits

• The structural role of microfilaments is to bear

tension, resisting pulling forces within the cell• They form a 3-D network called the cortex just

inside the plasma membrane to help support thecell’s shape

• Bundles of microfilaments make up the core ofmicrovilli of intestinal cells


Figure 6.26

Microvillus



Plasma membrane

Microfilaments (actinfilaments)

Intermediate filaments

0.25 m



•

Microfilaments that function in cellular motilitycontain the protein myosin in addition to actin

• In muscle cells, thousands of actin filaments arearranged parallel to one another

• Thicker filaments composed of myosininterdigitate with the thinner actin fibers


Figure 6.27

Muscle cell

0.5 m



Actinfilament

Myosin

Myosin

filament

head

(a) Myosin motors in muscle cell contraction

0.5 m

100 m

Cortex (outer cytoplasm):gel with actin network

Inner cytoplasm: solwith actin subunits

(b) Amoeboid movement

Extendingpseudopodium

30 m(c) Cytoplasmic streaming in plant cells

Chloroplast

Figure 6.27a



Muscle cell

Actin

filamentMyosin

Myosin

filament

(a) Myosin motors in muscle cell contraction

0.5 m

head

Figure 6.27b



100 m

Cortex (outer cytoplasm):

gel with actin network

Inner cytoplasm: solwith actin subunits

(b) Amoeboid movement

Extendingpseudopodium



•

Localized contraction brought about by actin andmyosin also drives amoeboid movement

• Pseudopodia (cellular extensions) extend andcontract through the reversible assembly and

contraction of actin subunits into microfilaments




•

Cytoplasmic streaming is a circular flow ofcytoplasm within cells

• This streaming speeds distribution of materialswithin the cell

• In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming


Intermediate Filaments



Intermediate Filaments

• Intermediate filaments range in diameter from8 –12 nanometers, larger than microfilaments butsmaller than microtubules

• They support cell shape and fix organelles inplace

• Intermediate filaments are more permanentcytoskeleton fixtures than the other two classes


Concept 6.7: Extracellular components and



Concept 6.7: Extracellular components and

connections between cells help coordinate

cellular activities

• Most cells synthesize and secrete materials thatare external to the plasma membrane

• These extracellular structures include

– Cell walls of plants

– The extracellular matrix (ECM) of animal cells

– Intercellular junctions


Cell Walls of Plants



Cell Walls of Plants

•

The cell wall is an extracellular structure thatdistinguishes plant cells from animal cells

• Prokaryotes, fungi, and some protists also havecell walls

• The cell wall protects the plant cell, maintains itsshape, and prevents excessive uptake of water

• Plant cell walls are made of cellulose fibers

embedded in other polysaccharides and protein




•

Plant cell walls may have multiple layers – Primary cell wall: relatively thin and flexible

– Middle lamella: thin layer between primary wallsof adjacent cells

– Secondary cell wall (in some cells): addedbetween the plasma membrane and the primarycell wall

• Plasmodesmata are channels between adjacent

plant cells


Secondary

Figure 6.28



ycell wall

Primary

cell wallMiddlelamella

Central vacuole

Cytosol

Plasma membrane

Plant cell walls

Plasmodesmata

1 m

Figure 6.29



RESULTS 10 m

Distribution of cellulosesynthase over time

Distribution ofmicrotubules

over time

The Extracellular Matrix (ECM) of Animal



( )

Cells

• Animal cells lack cell walls but are covered by anelaborate extracellular matrix (ECM)

• The ECM is made up of glycoproteins such as

collagen, proteoglycans, and fibronectin

• ECM proteins bind to receptor proteins in theplasma membrane called integrins


Figure 6.30



EXTRACELLULAR FLUIDCollagen

Fibronectin

Plasmamembrane

Micro-

filaments

CYTOPLASM

Integrins

Proteoglycancomplex

Polysaccharidemolecule

Carbo-hydrates

Coreprotein

Proteoglycanmolecule

Proteoglycan complex

Figure 6.30a

EXTRACELLULAR FLUIDCollagen



EXTRACELLULAR FLUIDg

Fibronectin

Plasmamembrane

Micro-filaments

CYTOPLASM

Integrins

Proteoglycan

complex

Figure 6.30b

Polysaccharidemolecule



Carbohydrates

Coreprotein

Proteoglycanmolecule

Proteoglycan complex



• Functions of the ECM

– Support

– Adhesion

– Movement

– Regulation


Cell Junctions



•

Neighboring cells in tissues, organs, or organsystems often adhere, interact, andcommunicate through direct physical contact

• Intercellular junctions facilitate this contact

• There are several types of intercellular junctions – Plasmodesmata

– Tight junctions

– Desmosomes – Gap junctions




Figure 6.31



Interiorof cell

Interiorof cell

0.5 m Plasmodesmata Plasma membranes

Cell walls

Tight Junctions, Desmosomes, and Gap



g p

Junctions in Animal Cells

• At tight junctions, membranes of neighboringcells are pressed together, preventing leakage ofextracellular fluid

• Desmosomes (anchoring junctions) fasten cellstogether into strong sheets

• Gap junctions (communicating junctions) provide

cytoplasmic channels between adjacent cells





Animation: Tight JunctionsRight-click slide / select “Play”




Animation: DesmosomesRight-click slide / select “Play”




Animation: Gap JunctionsRight-click slide / select “Play”

Figure 6.32

Tight junctions preventfluid from movingacross a layer of cells

Tight junction



Tight junction

TEM0.5 m

TEM1 m

T E M

0.1 m

ExtracellularmatrixPlasma membranes

of adjacent cells

Spacebetween cells

Ions or smallmolecules

Desmosome

Intermediatefilaments

Gapjunction

The Cell: A Living Unit Greater Than the



Sum of Its Parts

• Cells rely on the integration of structures andorganelles in order to function

• For example, a macrophage’s ability to destroy

bacteria involves the whole cell, coordinatingcomponents such as the cytoskeleton,lysosomes, and plasma membrane


Figure 6.33



5 m

06 Lecture Presentation PC

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