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Cell Membranes
Structure and function depends on constituents: lipids, proteins and carbs
...

Freeze-etching

reveals

proteins

embedded

in

the

lipid

bilayer

of

cellular

membranes
...

Proteins and lipids are independent of each other
...

ü Hydrophilic; the ‘head’ is energetically charged and associates with polar
water molecules
ü Hydrophobic; long, nonpolar fatty acid ‘tails’ and associates with other
nonpolar materials
...
This
helps membranes fuse during vesicle formation and
phagocytosis
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Membranes contain cholesterol (imp to membrane integrity)
...
The fatty acids in the hydrophobic interior makes the membrane
fluid, which permits some molecules to move within the membrane
...
Short fatty
acid chains increase fluid, unsaturated fatty acid increase fluid
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Proteins:
ü Hydrophilic regions: stretches of amino acids with hydrophilic R groups give
certain regions a polar character
...
(interact with fatty acids in lipid bilayer –
away from water)
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There are two types of membrane proteins:
1
...
Their hydrophilic ends protrude into aqueous environment
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They move materials through the membrane
(transport proteins), receive chemical signals from the cell’s external
environment (receptors), catalyzing reactions (enzymes) and inner surface:
attach to the cytoskeleton to maintain shape and stability of the cell
...
Peripheral membrane proteins: lack hydrophobic regions and are not
embedded in the bilayer
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Carbohydrates: located on the outer surface of the membrane and serves as
recognition sites for other cells
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They serve as recognition signals for
interactions between cells
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g
...

ü Glycoproteins: covalently bound to a protein (oliglusaccharide chains) –
enable a cell to be recognised by other cells and proteins
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ü Active transport: require input of chemical energy
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The rate depends on:
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The size of the molecule

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Temperature of the solution

-

Concentration gradient in the system

In simple diffusion, small molecules pass through the lipid bilayer of the
membrane
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Charged and polar molecules do not pass readily because cells exist in water, polar
substances from hydrogen bonds with water, preventing them from entering
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As a cell increases in volume, its SA also increases
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Osmosis
It is a passive process, no metabolic energy
...

Free water molecules will diffuse from a region of its higher concentration (low conc
of solutes) to a region of its lower conc (higher conc of solutes)
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Channel proteins: have polar/charged substance and water on the inside and
nonpolar amino acids on the outside
...

Water can enter by hydrating ions as they pass through ion channels or through
water channels (aquaporisis)
Carrier proteins: actual binding of the transported substance to a membrane
protein
...
5

a point is reached at
which further increase

2

in the conc gradient
rate of
uptake

1
...
5

rate

Simple

1

(saturated)
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concentration

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Active transport
Movement of a substance across a membrane against a concentration, gradient –
active transport requires energy
...
Its energy is supplied
by an ion conc gradient established by primary active transport
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e
...
Na⁺- K⁺ pump (integral membrane glycoprotein) breaks down ATP into ADP and
Pand uses energy released to bring two K⁺ ions into the cell and exports the three
Na⁺ ions
...
E
...
once Na-K pump establishes a conc gradient of Na⁺ ions, the passive
diffusion of some Na⁺ ions back into the cell can provide energy for 2° active ions of
glucose into the cell
...


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Endocytosis: membrane folds around the material and forms a pocket
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-

Pinocytosis: small, the same as phagocytosis but it traps fluids
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g
...
Cytoplasm coated by clathrin
...
It loses clathrin coat
and fuses with lysosome (digested and released in cytoplasm)

Exocytosis: materials in vesicles are secreted from a cell
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Materials released
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Secretes indigestible materials

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