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Lipid-1b

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Fluid-Mosaic ModelJonathan Singer and Garth

Nicolson, 1972

Fluid: Lipid components are in

constant motion, capable of

lateral diffusion

Mosaic: Proteins embedded in

lipid bilayer

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Fluid-Mosaic Properties

Freedom of motion of membrane components, especially

lipid molecules and some proteins

Chain length, degree of unsaturation of fatty acids and

amount of cholesterol present affect the membrane fluidity

Some organisms compensate for temperature changes

through lipid composition alteration, known as

homeoviscous adaptation

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Cell Fusion

(a) Tag proteins of membranes

of two cells, human and

mouse, with fluorescent

antibodies

Induce cell fusion through

membrane encapsulated

Sendai virus

(b) Photomicrograph shows

separate colors

(c) After about 40 minutes, we

see mixed colors, indicating

that the proteins haveintermingled

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Membrane Protein

Membrane proteins may

move laterally; some may be

restricted if they are

anchored to the cytoskeleton

Two broad categories:

Integral proteins

Peripheral proteins

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Integral (or intrinsic) membrane proteins:

Possesses hydrophobic regions, with affinity for the interior

of the lipid bilayer

The hydrophilic regions extend outward to the aqueous

environment

Some proteins, known as transmembrane proteins, span the

membrane. The number of segments and orientation vary

according to function Proteins not easily removed

Peripheral (or extrinsic) proteins:

No discrete hydrophobic sequences

Do not penetrate into lipid bilayer

Weak interaction with membrane and easily removed

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Bacteriorhodopsin - Transmembrane helicesAn all helical membrane

protein

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Hydropathy Plot

Indicates which region is

hydrophilic and hydrophobic

I 4.5 G -0.4V 4.2 T -0.7

L 3.8 S -0.8

F 2.8 W -0.9

C 2.5 Y -1.3M 1.9 P -1.6

A 1.8 H -3.2

E -3.5

Q -3.5

D -3.5

N -3.5

K -3.9R -4.5

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Outer membrane protein F - barrel

An all beta membrane

protein

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Erythrocyte

No organelles and minimum

function; well characterized

Its biconcave shape facilitates

O2 diffusion to hemoglobin Membrane contains several

proteins

The blood groups (A, B, AB,

O) are marked by presence ofA, B antigens (components of

erythrocyte surface

sphingoglycolipids), both

antigens or no antigenrespectively

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Erythrocyte Membrane Proteins

Erythrocyte as a case study