Study Exercise Biology 17 - 20

8/10/2019 Study Exercise Biology 17 - 20

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Study exercise Biology 17 20

1. Describe the organization of the membranes of a chloroplast.How does this organization differ from that of mitochondria

CHLOROPHYLL

greenpigment found in the chloroplasts of plants, algae, and cyanobacteria

is an extremely important biomolecule, critical inphotosynthesis,which allows plants to

absorb energyfrom light

absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed

by the red portion. However, it is a poor absorber of green and near-green portions of the

spectrum, hence the green color of chlorophyll-containing tissues in plants.

broken down into three major compartments

o stromathe fluid space inside of the chloroplast where chloroplast DN,

en!ymes, and free ribosomes are found

o "hylakoid

flat disks that function in light absorbtion and photosynthesis.

arranged in stacks called granum #plural$ grana%, are connected by

lamella, and have an interior space known as the lumen

o

inner and outer membranes.SIMILARII!S "IH MIOCHO#$RIA

re ovoid or elliptical in shape

re surrounded by an outer membrane and an inner membrane.

o &n the mitochondrion, the term crist%eis applied to the folds of the inner

membrane.

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https://www.boundless.com/biology/definition/pigment/https://www.boundless.com/biology/definition/photosynthesis/https://www.boundless.com/biology/definition/energy/https://www.boundless.com/biology/definition/pigment/https://www.boundless.com/biology/definition/photosynthesis/https://www.boundless.com/biology/definition/energy/


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Have membranes surrounded by fluid.

o &n the mitochondrion, the fluid &%trixis inside the inner mitochondrial

membrane, and fluid in the i'ter&e&(r%'e s)%ceis between the inner and outer

mitochondrial membranes.

o &n the chloroplast, the stro&%surrounding the grana is a fluid, and fluid also

occupies the lu&e'of each thylakoid.

both contain an a(ueous matrix containing en!ymes and coen!ymes, concerned with

dehydrogenations, electron transport and ") exchange, but these en!ymes and

coen!ymes are used in different ways in chloroplasts and mitochondria.

both contain DN and *N, which are involved with the synthesis of the membrane and

en!yme proteins, when the organelles replicate during cell division.

both contain + type ribosomes #*N% which may be free in the matrix or attached to

membranes.

$I**!R!#C!S "IH MIOCHO#$RIA

hloroplast /itochondria

chloroplasts are shaped like minute biconvex /itochondria are usually rod-shaped, about

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lenses, 1-' mm in diameter and 0-2 mm thick

'-0 mm long and .2-.+ mm wide. # few

species may have spherical or spiral shaped

mitochondria%.

chloroplasts contain many double membranes

called lamellae. "hese form disc like structures

called thylakoids which are piled on top of eachother, making the structures known as grana. 3ranaare joined to each other by intergranal lamellae.

"he thylakoids contain many small and large

particles which can only be seen under the highest

powers of the electron microscope. "hese are the

(uantosomes and house the photosystem systems

of pigments

"he innermost membranes of mitochondria

are called christae and are extensions of the

inner membrane of the mitochondrialenvelope. "he christae and inner membrane

are covered with thousands of small spherical

bodies called oxysomes which are attached to

the membranes by short stalks #oxysome 4

stalk 5 stalked particle%.

chloroplasts may contains temporary stores of

starch and lipids

mitochondria do not contains temporary

stores of starch and lipids

chloroplasts contain the photosynthetic pigments,

chlorophyll a, chlorophyll b, b-carotene andsometimes xanthophyll. "hese are situated in the

(uantosomes of the thylakoids and make up the

photosystems

/itochondria do not contain photosyntheticpigments.

chloroplasts are concerned with the process of

photosynthesis only operates in light

mitochondria are concerned with aerobic

respirationoperate all the

time, whether light or dark.

chloroplasts absorb carbon dioxide, for use inphotosynthesis during light periods, and release

oxygen

mitochondria continually absorb oxygen for

respiration and release carbon dioxide

it also absorb pyruvic acid, the final product

of glycolysis, which occursin the cytoplasm of cells.

the light dependent stage of photosynthesis #light

reaction% occurs in the (uantosomes of the

thylakoids. "he small and large (uantosomes

are thought to house the pigment systems of

photosystem & and photosystem && respectively.yclic photophosphorylation involves only

photosystem & but non-cyclic photophosphorylation

involves both photosystems & and &&.. "he light

independent stage of photosynthesis #dark reaction

or alvin pathway% occurs in the stroma of the

chloroplast and uses ") and ND)H, generatedby the light reaction, to fix carbon dioxide onto the

acceptor, ribulose bisphosphate. "his results in the

synthesis of sugars.

"he en!ymes of the 6ink *eaction and 7rebs

cycle, for metaboli!ing pyruvic acid, #and the

en!ymes for the b-oxidation of fatty acids%,are present in the matrix of the mitochondria.

"he bases of the stalked particles house the

coen!ymes of the respiratory chain, including

the electron transport chain. "he spherical

heads of the stalked particles contain the

en!ymes, such as ")ase, which link the

respiratory chain to oxidativephosphorylation, which occurs in the

spherical heads. ") is thus generated in the

spherical heads.

the coen!yme used for hydrogen transfer in the

process of photosynthesis, in the chloroplasts, is

ND) #nicotinamide adenine dinucleotide

phosphate%

"he initial coen!yme used for hydrogen

transfer in the process of respiration, in the

mitochondria, is ND #nicotinamide adenine

dinucleotide%.

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2. In what ways is photosynthesis the reverse of respiration

P+otosy't+esis Res)ir%tio'

Productio' o, AP- 8es

8es9 theoretical yield is 2: ")

molecules per glucose but actualyield is only about 2-20.

Re%ct%'ts-;lectron "ransport hain

#oxidative phosphorylation%.

"+%t )oers AP

sy't+%se-

H4 gradient across thylakoid

membrane into stroma. High

H4 concentrationin the

thylakoid lumen

H4 gradient across the inner

mitochondria membrane into matrix.

High H4 concentration in the

intermembrane space

Products-; H'0 lectron transport chain

electrochemical gradient created

energy that the protons use to flow

passively synthesi!ing atp

Occurs i' +ic+

org%'elle/-hloroplasts /itochondria 3lycolysis #cytoplasm%

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*i'%l electro' rece)tor- ND)4 #forms ND)H %


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. In general terms! how do the light-independent reactionsdiffer from the light " dependent reactions# $hat are theprimary products of the two types of reactions

refers to the use of light energy fromphotosynthesisto ultimately provide the energy to

convert D) to "),thus replenishing the universal energy currency in living things.

&n the simplest systems in )ro%ryotes3 )+otosy't+esis is used 4ust ,or t+e )roductio' o,

e'ergy3and not for the building of any biological molecules. &n these systems there is a

process called cyclic )+oto)+os)+oryl%tio' %cco&)lis+es t+e A$P to AP )rocess ,or

i&&edi%te e'ergy ,or t+ese cells5

&n the process called 'o'cyclic )+oto)+os)+oryl%tio'3 % )l%'tmust accomplish the

s)litti'g o, %ter3 t+e co'6ersio' o, A$P to AP3 %'d t+e )ro6isio' o, t+e reduced

coe'y&e #A$PH to power the synthesis of energy storage molecules.

#O#CYCLIC PHOOPHOSPHORYLAIO#

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consists of two sets of pigments to excite.

)', or photosystem ' )- +

)0, or photosystem 0 )- ;:.

Process

>nergy enters the system when )0 becomes excited by light.

>lectrons are shed by the excited )0 #oxidation%, which grabs electrons from

water, producing a molecule of oxygen gas for every two waters split.

"he electron then travels from the excited reaction center of ) 0 to

plasto(uinone #E%, to the b;-f complex, to plastocyanine #pc% and finally to the

reaction center of ) '

"his electron transport system generates a proton motive force that is used to

produce ") Bhen photosystem & absorb a photon of light, it ejects a high-energy electron.

"he energy from this light absorption is used to generate reducing powder in the

form of ND)H

"he ejected electron is replaced by an electron from photosystem &&

CYCLIC PHOOPHOSPHORYLAIO#

Process

6ight of the photons is captured by the antenna complexand transferred to the

)hotosystem & reaction center, which contributes two high energy electrons to the

primary electron receptor. >lectrons are passed to ferrodoxin #Ad%, an iron containing protein which acts as

an electron carrier.

second electron carrier plasto(uinone #)(% carries the electrons to a complex of

two cytochromes.

&n the process, energy is provided to produce a proton gradient across the

membrane which can be used for the D) to ") conversion.

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"he electrons are returned by plastocyanin #)c% to the )+ pigment in the

reaction center to complete the cycle.

%. How does the proton gradient lead to the formation of &'(#

C+e&ios&osis

is the process of using )roton movement to join D) and )i

accomplished by en!ymes called ") synthases or ")ases as protons pass through

this en!yme D) and )i are joined to make "). "he movement of the )rotons through

this en!yme provides the >nergy needed to make ").

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). Describe the basic plan of the *alvin cycle! indicating thereactions that re+uire energy input. $hy is it described as acycle#

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"he alvin cycle is a series of reactions that results in conversion of carbon dioxide

into the organic molecules needed to build new cells.


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o a biochemical barrier

o a medium for$

extracellular communication that is assisted by /s

the stable positioning of cells in tissues through cell matrix adhesion

the repositioning of cells by cell migration during cell development and

wound repair

!CM )ro6ides-

o tensile strength for tendons

o compressive strength for cartilage

o hydraulic protection for many types of cells

o elasticity to the walls of blood vessels

A$$IIO#AL #O!S 8lycoc%lyx 9cell co%t: &edi%tes cellcell ; cellsu(str%tu& i'ter%ctio'sxperiment$ digest >/ surrounding cultured cartilage or mammary

gland epithelial cells with en!ymes J= get decrease in secretory @

synthetic activities of cells

dd back >/ materials into culture J= restores differentiated state @

cells produce usual products

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?. /ay consist of ill-defined, amorphous associations of proteins @

polysaccharides #like loose connective tissue% or may be in the form of a

distinct structure

!CM t%es di6erse ,or&s i' di,,ere't tissues ; org%'is&s3 (ut is co&)osed

o, si&il%r )rotei's

/ost proteins in cells are compact @ globular9 those of extracellular space

are extended @ fibrous

mong their diverse functions, >/ proteins serve as scaffolds, girders,

mortar @ wire

lterations in amino acid se(uence of extracellular proteins can lead to

serious disorders

!CM 6ery )ro&i'e't i' co''ecti6e tissues 9c%rtil%ge3 (o'es3 te'do's3 cor'e%l

stro&%:

&n connective tissue, cells occupy a small fraction of tissue volume

>/, not cells, gives tissues their identifiable properties$ bone matrix

hardness, cartilage matrix toughness @ flexibility, tendon matrix tensile

strength, corneal stroma matrix transparency

Co&)o'e'ts o, t+e !CM &e&(ers o, % s&%ll 'u&(er o, &olecul%r ,%&ilies

ollagens G one of most important @ ubi(uitous >/ molecules9 fibrous

glycoprotein family Aunctions only as part of >/ @ only found there

)roteoglycans - protein-polysaccharide complex

Aibronectin, 6aminin, >/ )roteins

'0


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"he >xtracellular pace$ ?asement /embrane as an >xample of >/ B%se&e't &e&(r%'e 9(%s%l l%&i'%:< % co'ti'uous =>0 200 '& t+ic

s+eet< o'e o, (est de,i'ed ex%&)les< ,ou'd i' t+e ,olloi'g )l%ces-

&t surrounds muscle @ fat cells

&t underlies basal surface of epithelial tissues #skin epidermis, digestive

@ respiratory tract linings%

&t underlies the inner endothelial lining of blood vessels

*u'ctio's o, (%se&e't &e&(r%'e

)rovides mechanical support for the attached cells

3enerates signals that maintain cell survival

/aintains epithelial cell polarity

erves as a substratum for cell migration @ determines cell migration

path

eparates adjacent tissues within an organ #compartmentali!ation%

cts as barrier to passage of macromolecules @ errant cancer cells -

prevents passage of proteins out of blood as it flows through porous-

walled body capillaries #kidney G good example%

7idney glomerulus - blood filtered under high pressure through

double-layered basal lamina separating glomerular capillaries

from kidney tubule wall

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?asal lamina around glomeruli may thicken abnormally in

long-term diabetics J= kidney failure

omponents of the >xtracellular /atrix$ ollagens

Co&)rise % ,i(rous glyco)rotei' ,%&ily< )rese't o'ly i' !CMs< ,ou'd

t+roug+out %'i&%l i'gdo&

#oted ,or +ig+ te'sile stre'gt+ 9resist%'ce to )ulli'g ,orces:< it

is esti&%ted t+%t % 1 && di% coll%ge' ,i(er c%' sus)e'd % 10 g

?22 l(@ eig+t it+out (re%i'g

It is t+e si'gle &ost %(u'd%'t )rotei' i' +u&%' (ody

9co'stitutes 2> o, %ll )rotei':< re,lectsides)re%d

occurre'ce o, extr%cellul%r &%teri%ls

Coll%ge' &olecules )ro6ide t+e i'solu(le ,r%&eor t+%t

deter&i'es &%'y !CM &ec+%'ic%l )ro)erties

M%de &ostly (y ,i(ro(l%sts 9,ou'd i' 6%rious co''ecti6e

tissue ty)es:3 s&oot+ &uscle ; e)it+eli%l cells

=0 distinct types identified9 each restricted to

particular sites in body9 0 or more can be present

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together in same >/9 get functional complexity by

mixing several types in same fiber #heterotypic%

Heterotypic fibers are biological e(uivalent of metal

alloys

Different structural @ mechanical properties result from

different mixtures of collagens in fibers

M%'y di,,ere'ces %&o'g coll%ge' ,%&ily &e&(ers3 (ut %ll

s+%re 2 i&)ort%'t structur%l ,e%tures-

All coll%ge' &olecules %re tri&ers co'sisti'g o,

)oly)e)tide ?D@ c+%i's &%y (e ide'tic%l or 2 or di,,ere't

c+%i's

Alo'g %t le%st )%rt o, le'gt+3 t+e c+%i's i'd %rou'd e%c+

ot+er< ,or& u'i.ue3 rodlie tri)le +elix

So&e %re ,i(rill%r coll%ge's 9I3 II3 II: %sse&(le i'to rigid3

c%(lelie ,i(rils< t+e' i'to t+icer ,i(ers 6isi(le i' lig+t

&icrosco)e

&ndividual collagen molecules of a fibril are not in

register but are staggered K'L1 length relative to their

neighbors

"he staggered arrangement adds to the mechanical

strength of the complex @ causes banding patterns

characteristic of collagen fibers

*i(rils %re stre'gt+e'ed ,urt+er (y co6%le't crossli's

(etee' lysi'e ; +ydroxylysi'e residues o' %d4%ce't

coll%ge' &olecules i, disru)ted e%e'ed

ross-linking process continues through life

/ay contribute to decreased skin elasticity @ increased

brittleness of bones among elderly

Coll%ge' )ro6ides i'solu(le ,r%&eor t+%t deter&i'es

&%'y o, t+e !CM &ec+%'ic%l )ro)erties ; $

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"ype & collagen mutations - osteogenesis imperfecta,

potentially lethal condition characteri!ed by extremely fragile

bones, thin skin @ weak tendons

"ype && collagen mutations - alter cartilage properties9 causes

dwarfism @ skeletal deformities

number of collagen gene mutations - cause various distinct

but related collagen matrix structure defects #>hler-Danlos

syndromes% G one causes hyperextendable joints @ highly

extensible skin

"ype &I collagen gene mutations - lport syndrome, an

inherited kidney disease in which the glomerular basement

membrane is disrupted

omponents of the >xtracellular /atrix$ )roteoglycans

B%se&e't &e&(r%'es ; ot+er !CMs co't%i' l%rge %&ou'ts o, disti'cti6e

ty)e o, )rotei')olys%cc+%ride co&)lex c%lled %proteoglycan

onsist of core protein to which glycosaminoglycan #33% chains are

covalently attached

33s - repeating disaccharides #0 different sugars9 --?--?-%9 very

acidic due to both carboxyl @ sulfate groups on their component sugar

rings

>/ proteoglycans may assemble into gigantic complexes by linking

core proteins to hyaluronic acid #a nonsulfated 33%9 can occupy very

large volumes #e(uivalent to that of bacterial cell%

$ue to sul,%ted 8A8 'eg%ti6e c+%rges3 )roteoglyc%'s (i'd l%rge 'u&(ers

o, c%tio's3 +ic+3 i' tur'3 %ttr%ct lots o, H2O "hey form porous hydrated gel9 that fills extracellular space @ acts

like packing material to resist crushing #compression% forces

"his complements adjacent collagens, which resist pulling forces @

provide scaffold for proteoglycans

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"ogether they give cartilage @ other >/s strength @ resistance to

deformation #wiggle your ears%

"he >/ of bone is also made of collagen @ proteoglycans but it is

hardened by impregnation with calcium phosphate salts

omponents of the >xtracellular /atrix$ Aibronectin, 6aminin @ /

)roteins

/atrix implies a structure made up of a network of interacting components9

this is apt for >/

&t contains a number of proteins, in addition to collagen @

proteoglycans that interact with one another in highly specific ways

/any >/ proteins occur in families #more than ' form9 each formed

by alternate m*N splicing%

Different family members made in different tissues @ at

different times during development

Different protein forms may have different properties

#characteristics may not apply to all forms%

*i(ro'ecti' 9,i(rous: o'e o, (est studied !CM )rotei's< +%s ,e%tures

co&&o' to &ost ot+er &%trix co&)o'e'ts ; !CM )rotei's< co'sists o,

li'e%r %rr%y o, disti'ct (uildi'g (locs 9% &odul%r co'structio':

>ach fibronectin polypeptide is constructed from a se(uence of K2

independently folding An modules of 2 distinct types #An&, An&& @ An&&&%

An modules were first found in fibronectin, but they are part of many other

proteins.

Aound in proteins from blood clotting factors to membrane receptors @ other

>/ proteins >ach of the two polypeptide chains making up fibronectin contains$

?inding sites for other >/ components #collagen, proteoglycans,

etc.%9 link these molecules into stable, interconnected networks

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?inding sites for cell surface receptors #form stable >/-cell

attachments%9 endothelial cell will adopt shape unlike it does in body

when it spreads over a s(uare surface coated with fibronectin

*i(ro'ecti' ; ot+er !CM )rotei's %re i&)ort%'t +e' tissues %re

i'6ol6ed i' dy'%&ic %cti6ities 9e&(ryo'ic de6elo)&e't:

Development involves waves of cell migration over pathways

containing >/ proteins9 different cells follow different routes from

one part of embryo to another

/igrating cells guided by proteins like fibronectin contained in

landscape over which they pass

Neural crest cells follow fibronectin pathways from early nervous

system throughout embryo9 antibodies to fibronectin bind @ block

recognition sites on fibronectin J= inhibit cell movements

L%&i'i'also has specific domains G family of extracellular glycoproteins9

consist of 2 different polypeptide chains linked by disulfide bonds9 organi!ed

into a cross with 2 short arms @ ' long arm

>xtracellularly, it greatly influences cellMs potential for migration,

growth @ differentiation

3uides embryonic axon tips as grow outward from central

nervous system to distant targets

ritical role in primordial germ cell #)3% migration G )3s

follow laminin paths from yolk sac #outside embryo% through

blood @ embryonic tissues to developing gonad, become eggs

or sperm

During migration, )3s traverse surfaces particularly

rich in laminin

ertain cells migrate over laminin-containing matrix that they

secrete #keratinocytes G skin cells%

&solate keratinocytes from mice genetically engineered

to lack genes for this type of laminin

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lso binds tightly to other laminins, proteoglycans,

basal lamina components, cell surface receptors

?asal lamina type &I collagens @ laminin may form

separate, but interconnected, networks

"hese interwoven networks give basal lamina

both strength @ flexibility

?asal lamina with this structure are not restricted

to vertebrates9 seen throughout animal kingdom

/. ist two characteristic that distinguish hemidesmosomesfrom local adhesions

&nteractions of ells with Noncellular ubstrates$ Aocal dhesions @ Hemidesmosomes

Aocal contacts anchor cells to their substratum G it is much easier to study cell adhesion

to a surface in vitro #in a culture dish% than with an extracellular matrix inside an animal

/uch of our knowledge of cell-matrix interactions is derived from studies of

cells adhering to various substrates in vitro

teps in cell adhesion to culture dish

ell initially has rounded morphology like most animal cells suspended

in a(ueous medium

s cell contacts substratum, it sends out projections that make

increasingly stable attachments

/ @ cytoskeleton

#actin%9 cytoskeleton attachment seems necessary for cell adhesion

ctin filaments along with myosin molecules are part of cellMs contractile

machinery, which can create or respond to mechanical forces

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Aocal adhesions form in cells grown in vitro, but similar types of adhesive contacts are

found in certain tissues, like muscle @ tendon

&n body, the tightest attachment between a cell @ >/ is at epithelial cell basal surface

where they are anchored to underlying basement membrane by speciali!ed adhesive

structure #hemidesmosome%

"hey contain dense pla(ue on membrane inner surface with filaments coursing outward

into cytoplasm

Ailaments are thicker than actin of focal adhesions @ made of keratin

#intermediate filaments%

)rimarily supportive rather than contractile9 keratin-containing filaments of

hemidesmosome are linked to >/ by membrane-spanning integrins #;O1%

?ullous pemphigoid G rare autoimmune disease #people make antibodies against

hemidesmosome pla(ue proteins, bullous pemphigoid antigens%9 demonstrates

importance of hemidesmosomes

utoimmune disorders are caused by production of antibodies #autoantibodies%

directed against oneMs own tissues9 responsible for a wide variety of conditions

)resence of autoantibodies causes lower epidermal layer to lose attachment to

underlying basement membrane @ thus to underlying connective tissue layer of

dermis

auses severe blistering of skin when fluid leaks into space under epidermis

>pidermolysis bullosa #a similar inherited blistering disease% - found in patients with

genetic alterations in any one of a number of hemidesmosomal proteins #; or O1 integrin

subunit or laminin%


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I8H FG#CIO#

also referred to as a !onula occludens

is a site where the membranes of two cells come very

close together.

often occur in a belt completely encircling the cell

material cannot pass from one side of the sheet to the

other by s(uee!ing between cells. &nstead, it must go

through a cell, and hence the cell can regulate its

passage. uch an arrangement is found in the gut, to regulate absorption of digested

nutrients.

*G#CIO#S

o "hey prevent the passage of molecules and ions through the space between cells.

o materials must actually enter the cells #by diffusionor active transport% in

order to pass through the tissue. "his pathway provides control over what

substances are allowed through.

o "hey block the movement of integral membrane proteins#red and green ovals%

between the apical and basolateral surfaces of the cell. "hus the special functions

of each surface, for example receptor-mediated endocytosisat the apical

surface and exocytosisat the basolateral surface can be preserved.

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8AP FG#CIO#

there is a channel between the membranes of contacting cells in the gap junction so that

the cyto)l%s& of the two is connected

"he basic building block of each gap junction is the connexin subunit lipid bilayers

are penetrated by protein assemblies called connexons. "wo connexons join across the

intercellular space to form a continuous a(ueous channel that links the two cells

?ecause ions can flow through them, gap junctions permit changes in membrane potential

to pass from cell to cell.

*u'ctio'

o "heaction potentialin heart #cardiac% muscle flows from cell to cell through the

heart providing the rhythmic contraction of the heartbeat.

o t some so-called electrical synapsesin the brain, gap junctions permit the

arrival of an action potential at the synaptic terminals to be transmitted across to

the postsynaptic cell without the delay needed for release of a neurotransmitter.o s the time ofbirthapproaches, gap junctions between the smooth muscle cells

of the uterus enable coordinated, powerful contractions to begin.

o everal inherited disorders of humans such as certain congenital heart defects

and certain cases of congenital deafnes have been found to be caused by mutant

genes encoding co''exi's.

A$H!R!#S FG#CIO#S A#$ *OCAL CO#ACS

02
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sometimes called !onula adherens

are found at sites of cell-cell interaction.

Aocal contacts mediate association of cells with the extracellular matrix.

?oth associate with the actin cytoseleto' and both are involved in adhesion #sticking

cells together or sticking cells to surfaces%.

Aocal contacts possess specific transmembrane receptors of the integrin family that link

the cell to the extracellular matrix on the outside of the cell and the microfilament system

on the inside. onversely, members of a family of calcium ion-dependent cell adhesion

molecules, called cadherins, mediate attachment between cells at adherens junctions.

dherens junctions and focal contacts not only tether cells together or to the extracellular

matrix, but they also tr%'sduce signals into and out of the cell, influencing a variety of

cellular behaviors including proliferation, migration, and differentiation. &n fact some

protein components of these junctions can shuttle to and from the nucleus where they are

01
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thought to play a role in regulating gene expression.

$!SMOSOM!S A#$ H!MI$!SMOSOM!S

Desmosomes #the macula adherens% and hemidesmosomes are distinguished by their

association with the keratin -based cytoskeleton

?oth are primarily involved in adhesion

"he desmosome, like the adherens junction, possesses calcium ion-dependent cell

adhesion molecules that interact with similar molecules in the adjacent cell.

integrins at the core of the hemidesmosome mediate its interaction with the extracellular

matrix.

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"he hemidesmosome and, most likely, the desmosome are also sites of signal

transduction

14.Describe the components that ma5e up a plant cell wall andthe role of each in the wall6s structure and function

*u'ctio' o, cell %ll

&%i't%i'i'gdeter&i'i'g cell s+%)e#analogous to an external skeleton for every cell%

Su))ort %'d &ec+%'ic%l stre'gt+#allows plants to get tall, hold out thin leaves toobtain light%

prevents the cell membrane from bursting in a hypotonic medium #i.e.,resists %ter

)ressure%

co'trols t+e r%te %'d directio' o, cell grot+ %'d regul%tes cell 6olu&e

ultimately res)o'si(le ,or t+e )l%'t %rc+itectur%l desig' %'d co'trolli'g )l%'t

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&or)+oge'esissince the wall dictates that plants develop by cell addition #not cell

migration%

has a &et%(olic role#i.e., some of the proteins in the wall are en!ymes for transport,

secretion%

)+ysic%l (%rrier to- 9%: )%t+oge's< %'d 9(: %ter i' su(eried cells. However, the

wall is very porous and allows the free passage of small molecules, including proteins up

to ;, /B.

c%r(o+ydr%te stor%ge- the components of the wall can be reused in other metabolic

processes #especially in seeds%. "hus, in one sense the wall serves as a storage repository

for carbohydrates

sig'%li'g ,r%g&e'ts o, %ll3 c%lled oligos%cc+%ri's3 %ct %s +or&o'es5 recog'itio'

res)o'ses- for example$ #a% the wall of roots of legumes is important in the nitrogen-

fixing bacteria coloni!ing the root to form nodules9 and #b% pollen-style interactions are

mediated by wall chemistry.

eco'o&ic )roducts- cell walls are important for products such as paper, wood, fiber,

energy, shelter, and even roughage in our diet.

CH!MICAL COMPOSIIO#

15 He&icellulose

(i'd to cellulose &icro,i(ril sur,%ces3 cross

li'i'g t+e& i'to % co&)lex structur%l 'etor

o ellulose

polymer of glucosetypically

consisting of ', to ', beta-

D-glucose residues

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major component of primary and secondary wall layers.

Cellulose )oly&ers %ssoci%te t+ro

ug+ H(o'ds +e H(o'di'g o,

&%'y cellulose &olecules to e%c+ ot+er results i' t+e ,or&%tio' o,

&icro ,i(ers %'d t+e &icro ,i(ers c%' i'ter%ct to ,or& ,i(ers5

branching macromolecule contstructed of - and ;-carbon sugars

generally insoluble in pH + water, but more soluble in basic solutions

classified on the basis of their component sugars. Pylose, mannose, and galactose form

the hemicellulose backbone9 arabinose, glucuronic acid, and galactose form the side

chainsaffect chemical characteristic of the hemicellulose

abundant in primary walls but is also found in secondary walls.

*u'ctio's

o li&it t+e stretc+i'ess o, t+e cell %ll (y li'i'g %d4%ce't &icro,i(rils %'d

)re6e'ti'g t+e& ,ro& slidi'g %g%i'st e%c+ ot+er ,or u'li&ited dist%'ces

involved in controlling cell enlargement

25 Pecti'

% ,%&ily o, co&)lex )olys%cc+%rides t+%t %ll co't%i' 13li'ed D$g%l%cturo'ic

%cid5

*or& exte'si6e3 +ydr%ted gel ,illi'g s)%ce (etee' ,i(rous ele&e'ts 9%ttr%ct H2O

lie %'i&%l 8A8s:

*u'ctio'

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o Bhen plant is%tt%ced (y )%t+oge's3 )ecti' ,r%g&e'ts rele%sed ,ro& %ll

trigger de,e'si6e )l%'t cell res)o'se

o Bhen purified3 )ecti' is used co&&erci%lly to )ro6ide gellie co'siste'cy o,

4%&s ; 4ellies

5 Structur%l Protei's

trucural proteins are found in all layers of the plant cell wall but they are more abundant

in the primary wall layer

*u'ctio's

o t+ey &edi%te dy'%&ic %cti6ities

expansins facilitate cell growth9 they cause locali!ed relaxation of

cell wall, which allows the cell to elongate at that site in response to

turgor pressure generated within cell

ell wall-associated protein kinases span the plasma membrane @ are

thought to transmit signals from the cell wall to the cytoplasm

Study Exercise Biology 17 - 20

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