Cell Membrane Structure and Function

1. Controls movement of substances in and out of the cell

2. Semi-permeable: Barrier to water-soluble substances, can't pass through the hydrophobic core.

-Allows passage of lipid-soluble substances

3. Cell signalling

4. Cell Recognition

5. Cell-to-cell adhesion

6. Site for enzymes to catalyze reactions

7. Anchoring for the cytoskeleton

8. Form H bonds with water for stability

-Fluid: phospholipids and protein molecules are able to move about and diffuse sideways within its monolayer-Mosaic: proteins interspersed/scattered within membrane

1. Phospholipids

2. Cholesterol

3. Proteins

4. Glycolipids (carb+lipids)

5. Glycoproteins (carb+proteins)

a) Hydrophilic head

-phosphate group

-charged, polar

-forms H bonds with water

Role: Stabilize membrane

b) Hydrophobic tails

-fatty acid residues

-hydrocarbon chains are insoluble and non-polar

-repels water

-points inwards facing each other.

-In water, phospholipids self-assemble into bi-layer structure or micelle(if phospholipids are too few)

Roles: -Barrier to water-soluble substances.

-Allow passage to lipid soluble substances.

-Only lipid-soluble, small, uncharged molecules

can diffuse through the phospholipid bilayer

-ALSO: fatty acids help maintain fluidity of

membrane.

1. Temperature: Higher temp, higher KE, more fluidly

2. Ratio of unsaturated to saturated fatty acids: High unsat : sat ratio,

more fluidly

-unsat FA has C=C which cause kinks

-Phospholipids more loosely arranged

Lower unsat : sat ratio, less fluidly, no kinks, more area for

phospholipids to interact.

3. Length of phospholipid tails: The LONGER the tails, the LESS fluid,

More surface area for interaction between tails

4. Cholesterol

HIGH TEMP

-increase sat. fatty acids

-make tails longer

-add cholesterol

LOW TEMP

-increase unsat. fatty acids

-tails shorter

-add cholesterol

hydrophilic head (OH group)

hydrophobic tail

fit between the phospholipid molecules, only in eukaryotes

Roles

1. Regulates fluidity of membrane

2. Stabilises the membrane especially the hydrophobic layer

3. Block passage of very small ions through membrane

a) At lower temperatures, less fluid

-Cholesterol increases fluidity by preventing close packing of

phospholipid tails

b) At higher temperatures, more fluid

Cholesterol decreases fluidity by reducing the mobility

of phospholipids

Extrinsic/peripheral proteins: inner or outer surface of the membrane

Intrinsic/integral proteins:

-Extend into hydrophobic core

-mobile or fixed

-some are transmembrane proteins( span across membrane)

-Hydrophobic and hydrophilic regions

Hydrophobic: interacts with hydrophobic core

Hydrophilic: extends into aqueous external environment inside/outside the cell.

Both regions help intrinsic proteins to be stable.

Transport proteins: Passage for ions/polar/charged/larger molecules through membrane