Composition and properties of biological membranes

Question 1

The appearance of biological membranes differs depending on location and function

Answer 1

Plasma membrane - provides cell boundary and prevents movement of materials into and out of the cell
Organelle membranes - divide cytoplasm into compartments

Question 2

Fundamental properties of membranes

Answer 2

Barrier - Flexible, self-repairing, continuous
Selectively permeable - Only certain molecules can pass into and out of cells

Question 3

Membrane composition

Answer 3

Components of all membranes basically similar
All built from:
- Lipids
- Proteins
- Carbohydrates
Oligosaccharide chains covalently linked to protein or lipids
Made up of sugar residues linked covalently in branched chains

Question 4

Phospholipids move about in the bilayer

Answer 4

Phospholipids can rotate or exchange in the lateral plane of the membrane – but normally move very slowly from one leaflet to the other (flip-flop)

Question 5

Fluidity of lipid bilayer depends on its composition

Answer 5

Number of double bonds
Number of C atoms in fatty acid chains of phospholipid
Greater the number of double bonds and shorter the acyl chains, the less tightly packed the molecules and greater the fluidity

Question 6

Phospholipids

Answer 6

Amphipathic
Polar headgroup (hydrophilic)
Hydrophobic tail
Chains can be anywhere between 12 and 22 carbons long
All membranes are fluid at the temperature of the organism – so plant and cold-blooded animal membranes, which exist at ambient temperature, have more double bonds and/or shorter acyl chains than membranes of warm blooded animals

Question 7

Mayor membrane phospholipids

Answer 7

need to sumaries drawing

Question 8

Strucutre of cholesterol

Answer 8

Has a small polar head group
Has an amphipathic nature

Question 9

Cholesterol makes membranes less permeable

Answer 9

Packs between phospholipids and makes membrane less deformable at the surface
Doesn’t make the overall membrane more rigid
At high concentrations it stops the membrane from becoming crystalline (like butter)

Bacteria cell membranes have a simple phospholipid composition

Question 10

Hydrophilic and hydrophobic molecules behave differently in aqueous solution

Answer 10

In aqueous solutions lipids form micelles (single layers generally spheres) or bilayers (to layers generally straight chains)
Lipid bilayers presfer to form sealed compartments more energetically favourable

Question 11

Structural organisation of biological molecules

Answer 11

Orientation of protein in cell membrane will always stay the same as is important for function
Phospholipids move faster than proteins in the bilayer by about 100 times

FRAP allows us to tell how fast if at all a protein moves

Integral membrane proteins directly insert in the membrane by a hydrophobic domain

Question 12

Peripheral membrane proteins

Answer 12

Associate with integral membrane proteins or directly bind lipids
Ca-ovalently bound lipids which insert into the membrane
Small G proteins have a fatty acid modificatin which allows them to cycle on and off membranes: Active on membranes, inactive in the cytosol
Ras, mutated in many cancers, is a good example of this

Question 13

Proteins are floating in a sea of lipids

Answer 13

Membrane lipids are not homogenous
Can form clusters or microdomains
Differences in lipid composition visualised on a vesicle generated in vitro

Question 14

Cholesterol and sphingolipids can form microdoamins called rafts

Answer 14

The membrane is slightly thicker in microdomains, tight junctions prevent movement between apical and basolateral membranes

Question 15

Summary

Answer 15

The properties of membranes are determined by their protein and lipid composition
The lipid bilayer is fluid. Proteins and phospholipids diffuse in the plane of the membrane
Lipids provide the overall structure of the membrane and proteins perform specific functions
Integral or peripheral membrane proteins