Membranes

Question 1

What is the major component of membranes and what is there structure?

Answer 1

Phospholipids (amphipathic) -
Hydrophillic phosphate group
Glycerol
2 hydrophobic fatty acid tails

Question 2

What is a glycolipid and what are the 2 examples?

Answer 2

Phospholipids whose phosphate group has been replaced by a sugar group
Cerebrosides: monosaccharide group
Ganglioside: Oligosaccaharide group

Question 3

Define mobility

Define fluidity

Answer 3

Ability of a single phospholipid to move

Ability of the membrane to move

Question 4

What kind of mobility do phospholipids have?

Answer 4

Flexion (lowest energy)
Rotation
Lateral diffusion
Flip-flop (highest energy)

Question 5

Describe the features of the two types of fatty acid tails

Answer 5

Saturated:
Only C-C bonds (no kinks)
Increased packing and decreased fluidity

Unsaturated:
Some C=C double bonds (cis-kinked)
Increased fluidity and decreased packing

Question 6

Describe the structure of cholesterol and their specific functions

Answer 6

Very small Hydrophillic OH group: forms H-bonds with C=O on fatty acids
Rigid planar steroid ring: restricts movement
Flexible Hydrophobic hydrocarbon chain: allows movement

Question 7

What is the function of cholesterol in the membrane?

Answer 7

Prevents membrane becoming semi-crystalline
Keeps the membrane at constant fluidity:
High temperatures - restricts movement
Low temperatures - restricts packing

Question 8

What are the two types of membrane proteins and how are they bonded to the membrane?

Answer 8

Peripheral (hydrophillic) - forms van der waals forces and H-bonds with phosphate groups
Integral (hydrophobic) - bound to the middle of the membrane by the hydrophobic effect

Question 9

How can membrane proteins be removed from the membrane?

Answer 9

Peripheral - changes in pH, ionic concentrations

Integral - other non-polar organic solvents

Question 10

What kind of mobility do membrane proteins have?

Answer 10

Flexion
Rotation
Lateral diffusion
(No flip-flop)

Question 11

What restrictions do membrane proteins have on their mobility?

Answer 11

Proteins separate out in the bilayer

Associations with extramembrous proteins and other membrane proteins

Question 12

What is the structure of the erythrocytes cytoskeleton?

Answer 12

Forms a membrane skeleton:
Protein spectrin is a heterotetramer of alpha and beta chains which attaches to the membrane to produce the biconcave shape
Ankyrin (band 4.9) and band 4.1 attach spectrin to the glycoproteins band 3 and glycophorin A respectively

Question 13

What are to haemolytic anaemias caused by defective spectrin?

Answer 13

Spherocytosis:
Lack of spectrin, erythrocytes round up and lyse

Elliptocytosis:
Abnormal spectrin (cannot form heterotetramers)

Question 14

How are proteins destined for secretion transported to the ER?

Answer 14

SRP (Signal recognition particle) binds to partially synthesised protein (at the signal peptide sequence) and transports it to a translocation channel
Protein is released from the ribosome into the ER lumen
Signal peptide sequence is cleaved by signal peptidase

Question 15

How are Membrane proteins transported into the ER and how is orientation determined?

Answer 15

Contain a stop transfer sequence (hydrophobic region of the peptide) that prevents full translocation of the protein
Depending on the location of the charges on the protein and whether or not it is cleaved will determine the orientation of the protein

Question 16

How are multiple-transmembrane proteins inserted into the membrane?

Answer 16

Simultaneously