Plasma Membrane(Edited)

7/31/2019 Plasma Membrane(Edited)

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Membranes: Their Structure,

Function and Chemistry


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The Functions of Membranes

1. Membranes define boundaries andserve as permeability barriersa. Plasma(cell) membraneb. Intracellular membranes

2. Membranes are sites of specificfunctions

3. Membranes regulate the transport of solutes

4. Membranes detect and transmitelectrical and chemical signals

5. Membranes mediate cell-to-cellcommunication


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Models of Membrane Structurea) Lipid Nature of membrane

b) Lipid monolayer

c) Lipid bilayer

d) Lipid bilayer plus proteinlamellae

e) Unit membrane

f) Fluid-Mosaic model

g) Membrane proteinstructure

alpha helix

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Membranes function properly only in the fluid state- T then fluidity; T then fluidity also

The effects of fatty acid composition onmembrane fluidity- depends on the length of fa present and

degree of unsaturation of their side chainse.g. Membranes w/ Oleate (unsaturatedfa) are more fluid than stearate (saturatedfa)

The effects of sterols on membrane fluidity-cholesterol has the paradoxical effect of decreasing membrane fluidity at high Tand increasing at low T (in animal CMs)


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Most organisms can regulate membrane fluidity-whether prokaryote or eukaryote by 1oly changing

the lipid composition of the membranese.g poikilotherms (bacteria, fungi, protists, plants &

cold -blooded animals that can not regulate their own temperature)

-membranes would gel upon cooling if they hadno way to compensate for the decrease in T

-at high T, their bilipid layers become so fluid thatthey no longer serve as an effective permeability

barrier e.g. Cold-blooded animals (paralyzed by T >45 oC)possible reasons: nerve CMs become so leaky

to ions thus ion gradients cant be maintained and overall nervous function is disabled


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e.g. homeotherm or warm - blooded organism-effects on humans during chilly days, fingers and toes

get so cold that the membranes of sensory

nerve endings cease to function, resulting intemporary numbness

How to regulate or compensate T changes?- by changing lipid composition of their membranes

thru Homeoviscous adaptation ( in poikilotherms)-the main effect of this regulation is to keep

the viscosity of the membrane approximatelythe same despite the changes in T

Example:1. Micrococcus (transferred from high T to

low T results to an increase in theproportion of 16-C rather than 18-C fa

in the PM thus minimizing effect of thelow T.


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*shorter fa chains decrease the melting T of a membrane

2. E. Coli (alteration in the extent of unsaturation of membrane fa rather than in length)

-low T triggers synthesis of desaturase E thatintroduces double bonds into the HC chains of fa.

- HVA also occurs in yeasts in plants (membrane fluiditydepend on the increased solubility of oxygen in the cyto-plasm at lower T)

Oxygen- substrate of desaturase ETherefore: more Oxygen available at low T, more

unsaturated fa synthesized at rapid rate andmembrane fluidity increases

Amphibians and reptiles adapt to lower T by increasingproportion of unsaturated fa in their membrane aswell as cholesterol


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Mammals or animals entering hibernation, the body Tdrops substantially but adapts to this changeby incorporating a greater proportion of unsaturated fa into membrane phospholipidsas its body T falls.


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MEMBRANE PROTEINS-divided into 3 general classes:(based on the nature of their attachmentwith the membrane)

a. integral membrane proteins-embedded directly

within the lipid bilayer(by the affinity of hydrophobic segmentson the protein for the hydrophobic interior of the lipid bilayer)

b. peripheral membrane proteins- not inserted into thelipid bilayer but are associated with the membrane indirectly,

generally by interactions with integral proteins(hydrophilic,located on the surface of the membrane where they are linkednoncovalently to the polar head groups of phospholipids and/or tothe hydrophilic parts of other membrane proteins)

c. Lipid-anchored proteins- though not a part of the original fluid mosaic model but are now included

as a third class of membrane lipids.-essentially hydrophilic proteins and reside on

membrane surfaces but they are covalently boundto lipid molecules that are embedded within bilayer


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a. Integral monotopic proteins- appear to be embedded on only one of the bilayer

b. Singlepass proteins- transmembrane proteins that span the bilayer once

c. Multipass proteins- span the bilayer multiple times

- may consist of either a1. single polypeptides2. several associated polypeptides (Multisubunit

proteins)

d. Peripheral membrane proteins- too hydrophilic to penetrate into the membrane- attached to the membrane by electrostatic and H-bondsthat link them to adjacent membranes proteins or tophospholipid headgroups


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Aquaporin

T r a n s p o r t

P r o c e s s e s

W i t h i n a c o m p o s i

t e E u k a r y o

t i c c e

l l


14/24Important Transport Processes of the Erythrocyte


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2 Categories of Transport1. Non- carrier mediated does not require carrier proteins(simple diffusion)2. Carrier-mediated requires specific carrier proteins

a. facilitated diffusion (uniport)b. active transport

Fig. 4 Basic mechanisms by which solute molecules move across membranes


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Examples

Factor More permeable Less Permeable Permeability

Ratio*1. Size : bilayer more permeableto smaller molecules

H2O (water) H 2N-CO-NH 2 (urea)

10 2:1

2. Polarity: bilayer more permeableto nonpolar molecules

CH 3-CH 2-CH 2-OH(Propanol)

HO-CH 2-CHOH-CH 2-OH (glycerol)

10 3:1

3. Ionic: bilayer highlyimpermeable toions

O 2 (oxygen) OH - (Hydroxideion)

10 9:1

*Ratio of diffusion rate for the permeable solute to the less permeable solute

Table 2. Factors Governing Diffusion Across Lipid Bilayers


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Channel proteins-facilitate diffusion by forming hydrophilic trans-membrane channels

3 Kinds:1. Ion channels- transmembrane proteins that allow rapid

passage of specific ions (remarkably selective)- single channel can conduct almost a million

ions per second!-most ion channels are gated (opened and closed

by conformational changes in the protein regulating theflow of ions thru the channel)

3 gated channels :1. Voltage-gated = open and close in response

to changes in membrane potential2. Ligand-gated = triggered by the binding of

specific substances to the channel protein 3. Mechanosensitive = respond to mechanical

forces that act on membrane


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2. Porins- transmembrane proteins that allow rapid passage of varioussolutes-pores found in the outer membranes of mitochondria,

chloroplasts and bacteria-larger & much less specific-formed by multipass transmembrane proteins-made of closed cylindrical sheet called barrel

-inside pore (water-filled) is lined by polar chains while outsidethat of nonpolar side chains-pore allows passage of various hydrophilic solutes with sizedepending on the pore size of the particular porin

3. Aquaporins (AQPs)-transmembrane that allow rapid passage of water


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Active Transport-ATP-powered pumps that transport ions andvarious small molecules against their concngradient


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Three Major functions in cells and organelles:

It makes possible the uptake of essential nutrients fromthe environment or surrounding fluid, even when thetheir concns in the environment are much lower thaninside the cell

it allows various substances (secretory prodts and wastematls) to be removed from the cell or organelle, evenwhen the concn outside is > than the inside

it enables the cell to maintain constant, nonequilibriumintracellular concentrations of specific inorganic ionssuch a K +, Na +, Ca + and H +


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2 types:-based on the energy source

1. Direct active transport-also called primary active transport -accumulation of solute molecules or ions on one side

of the membrane coupled directly to an exergo-nic reaction particularly hydrolysis of ATP.

-transport proteins driven directly by ATP hydrolysisare called ATPases or ATPase pumps.

2. Indirect active transport

-also called secondary active transport -depends on the cotransport of two solutes withthe movement of 1 solute down its gradientdriving the movement of the other solute up itsgradient.


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Direct Active Transport Depends on Four Types of Transport ATPases

-transport ATPases or pumps are responsible for mostdirect active transport in both prokaryotic and eukaryo-tic cells.

1. P-type2. V-type3. F-type4. ABC-type


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Solutes Transported Kind ofMembrane

Kind of Organisms Function of ATPase

P-type ATPases (P for

phosphorylation ) Na + and K + Plasma membrane Animals Keeps [Na +] low and

[K+] high within cell;maintains membranepotential

H+

Plasma membrane Plants, fungi Pumps protons out of cell; generatesmembrane potential

Ca 2+ Plasma membrane Eukaryotes Pumps Ca 2+ out of cell;keeps [Ca2+] lowin cytosol

V-type ATPases (V forvesicle

H+ Lysosomes;secre-tory vesicles

Animals Keep pH in organellelow, which activateshydrolytic enzymes

H+

Vacuolar membrane Plants, fungi Keeps pH in vacuolelow which activates E

Table 2.1 Main Types of Transport ATPases (Pumps)

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Plasma Membrane(Edited)