Membrane Composition

Question 0

What is the composition of phospholipids?

Answer 0

Glycerol bound to fatty acid chains (ester bonds) and to a head group by a phosphate molecule

Head group determines type of phospholipid e.g. choline, inositol, amines, sugar

note: cis double bond in fatty acid chain introduces kink - reduces phospholipid packing

Question 1

List some general and specific functions of biological membranes.

Answer 1

GENERAL:

• continuous, highly selective permeable barrier
• control of enclosed chemical environment
• communication
• recognition of signalling molecules, adhesion proteins, and immune surveillance
• signal generation in response to stimuli

SPECIFIC:

• interaction with adjacent cells
• absorption & secretion
• changing shape for transport
• synapses: electrical signal conduction

Question 2

What is the composition of sphingomyelin?

Answer 2

Phosphocholine bound to fatty acid (but NO GLYCEROL)

Often found in myelin sheaths

Question 3

What is the composition of a glycolipid, and what two types are there?

Answer 3

Replace sphingomyelin phosphocholine with sugar

CEREBROSIDE = sugar monomer head group

GANGLIOSIDE = oligosaccharide head group

Question 4

Describe the range of motion of phospholipids.

Answer 4

LATERAL DIFFUSION

ROTATION

FLEXION (vibration)

FLIP-FLOP (rare - high energy conformation needed)

Question 5

Describe how cholesterol affects phospholipid packing.

Answer 5

OH forms hydrogen bonds with fatty acids -> reduced phospholipid chain motion -> reduced fluidity

Forces phospholipid molecules further apart -> reduces phospholipid packing -> increased fluidity

Question 6

How does cholesterol make membranes more stable?

Answer 6

Eliminates endothermic phase transition of phospholipid bilayers, therefore less energy required to be fluid.

note: formation of crystalline islands -> fractures -> leakage

Question 7

Describe the range of motion of proteins in phospholipid bilayers.

Answer 7

LATERAL DIFFUSION
CONFORMATIONAL CHANGE
ROTATION

note: NO FLIP-FLOP (proteins too large/hydrophobic)

Question 8

What is the evidence that there are proteins in membranes (generally)?

Answer 8

Functional:

• facilitated diffusion
• ion gradients
• specificity of cell responses

Biochemical:

• membrane fractionation
• freeze fracture

Question 9

How is the movement of membrane proteins restricted?

Answer 9

Aggregation of adjacent membrane proteins

Tethering of membrane proteins (intracellularly or extracellularly)

Interaction with membrane proteins of other cells

How fluid the surrounding lipid parts of the membrane are

Question 10

What is the functional difference between integral and peripheral membrane proteins, and how can they each be separated from the membrane?

Answer 10

INTEGRAL:
Interact with hydrophobic domains of lipid bilayer.
Only removed with agents that compete for these interactions e.g. detergents, organic solvents

PERIPHERAL:
Form electrostatic interactions/hydrogen bonds with phosphate heads
Removed by changes in pH/ionic strength

Question 11

Describe the components of the cytoskeleton.

Answer 11

Spectrin (alpha and beta subunits in antiparallel arrangement)

Ankyrin between membrane proteins and spectrin

Actin cross links & adducin present

Band 3, glycophorin (band 7) in membrane; band 4.1 by actin

Question 12

What two disorders can result from reduction/defect of spectrin?

Answer 12

Hereditary spherocytosis:

• spectrin depleted by ~40%-50%
• erythrocytes round up
• less resistant to lysis - cleared by spleen (anaemia)

Hereditary elliptocytosis:

• defect in spectrin molecule - unable to form heterotetramers
• fragile elliptoid cells

Question 13

How might a membrane protein interact with the hydrophobic domain of the membrane bilayer?

Answer 13

• transmembrane sequence: ~20-22aa with hydrophobic R regions
• lipid-linked proteins: insertion of hydrophobic lipid moieties e.g. palmitoylation, mystriolation
• insertion of amphipathic alpha-helix from cytoplasmic side