1 Lipids, proteins + membrane structure

Question 1

Describe the main functions of biological membranes

Answer 1

1. signal transducer: stimulus –> A.P.
2. communication: between cells + environment
3. recognition e.g. cell signalling
4. selective permeability to certain ions

Question 2

Describe the structure of phospholipid

Answer 2

1. range of polar heads: AA, amines, choline, sugars
2. fatty acid chain: C16-C18 most common, if double bond present in cis (kink), reduce phospholipid packing together so closely
3. predominant lipid e.g. phosphatidylcholine

Question 3

Describe the structure of sphingolipids

Answer 3

(like phospholipid but based on GLYCEROL instead of phosphate polar head group)
there are 2 groups of sphingolipids: sphingomyelin + glycolipids

Question 4

Describe the structure of the sphingomyelin

Answer 4

the only phospholipid not based on glycerol

phosphocholine group joins the fatty acid chains together instead of glycerol being the backbone

Question 5

Describe the structure of glycolipids

Answer 5

sugar containing lipid
replaces phosphocholine in sphingomyelin with a sugar group
2 types: gangliosides (head group is oligosaccharide 3-10 sugars) + cerebrosides (head group has 1 sugar monomer)

Question 6

Describe the structure of cholesterol

Answer 6

45% of total membrane lipid

1. prevents extremes of CRYSTALLISING or fluid (e.g. at extreme temps)
2. intercalation of rigid planar conjugated ring structure reduces phospholipid PACKING –> increases fluidity
3. conversely, rigid conjugated ring structure reduces phospholipid ALIPHATIC tail MOBILITY, reducing fluidity

Question 7

What are the 4 ways of lipid mobility in the bilayer?

Answer 7

of the fatty acid tail

1. fast lateral diffusion
2. fast axial rotation
3. flip flop: not energetically favourable (hydrophilic has to go through hydrophobic core) NOT in proteins (as it would affect direction of travel of ions)
4. intra-chain (tail moves)

Question 8

What is the function of membrane proteins?

Answer 8

ion channels
transporters
pumps e.g. SERCA, PMCA: use ATP directly to go against gradient
energy transducer: influx of ions for AP
enzymes e.g. ATPase
receptor e.g. GPCR in PNS

Question 9

where do membrane proteins tend to aggregate?

Answer 9

where cholesterol is as it is more stable and rigid where the cholesterol is

Question 10

What are the restrains on mobility of membrane proteins?

Answer 10

1. lipid-mediated effect: to poor cholesterol regions (less stable) or fluid phase
2. membrane protein associations: neighbouring proteins stick together
3. associated with extra-membranous proteins e.g. actin + spectrin, band 3 (cytoskeleton)

Question 11

What are the main functions of biological membranes?

Answer 11

1. recognition e.g. cell signalling
2. selective permeability to certain ions
3. signal transducer: stimulus –> AP
4. communication: between other cells or with environment
5. control of enclosed chemical environment (maintain ion concentration)

Question 12

Describe the composition of biological membranes

Answer 12

40% lipids
60% proteins
1-10% carbohydrates

20% water if not dry

Question 13

What does the biological membrane form because of its composition?

Answer 13

amphipathic bilayer, phospholipid head, fatty a tail (hydrophobic)
spontaneously form micelles + bilayer (phospholipid + glycolipids)

Question 14

What are the forces within the bilayer?

Answer 14

van de Waals forces between hydrophobic tails
co-operative structure of bilayer: held together by non-covalent forces between hydrophilic head + water (hydrogen + electrostatic forces)