Membrane structure and permeability

Question 1

In what ways can a protein interact with a lipid bilayer?

Answer 1

Integral (inserted deep in membrane)

• extensive hydrophobic interactions
• requires detergents or organic solvents (compete for non-polar interactions)

Peripheral (associated with the surface)

• electrostatic or hydrogen bond interactions
• removed by changes in pH or ionic strength

Question 2

What is the influence of cholesterol on the bilayer?

Answer 2

• increases mechanical rigidity
• decreases flexibility of surrounding phospholipid chains
• low temp: prevents cracking, maintains fluidity
• high temp: maintains stability of mobile lipids

Question 3

How does protein movement differ from lipid movement in the bilayer?

Answer 3

Lipid: vibrational, axial rotation, lateral diffusion, flip-flop
Protein: vibrational, axial rotation, lateral diffusion ONLY

Question 4

Briefly explain how proteins are inserted into the membrane

Answer 4

• ribosomal synthesis of mRNA by ER
• signal recognition protein binds to ribosome
• SRP binds to docking protein
• synthesis through signal sequence receptor
• stop transfer signal: transmembrane
• signal peptidase: N terminus on inside

Question 5

What are the functions of a biological membrane?

Answer 5

• highly selective permeable barrier
• communication within and between cells
• recognition of signalling molecules, antigens etc.
  *

Question 6

Explain passive diffusion

Answer 6

• the movement of non-polar molecules down their concentration gradient across a hydrophobic domain of a lipid bilayer

Question 7

Explain active transport

Answer 7

• movement against unfavourable chemical or electrical gradients
• requires energy, usually ATP

Question 8

List 3 cotransport systems

Answer 8

• Na - glucose transport system of the small intestine and kidney
• Na-Ca exchange - inward flow of sodium
• Na-H exchange - inward flow of sodium

Question 9

What is a uniport?

Answer 9

the movement of one species from one side of the membrane to the other

Question 10

What is an antiport?

Answer 10

when the movement of one molecule across the membrane in one particular direction, is associated to the movement of a different species in the opposite direction

Question 11

What is a symport?

Answer 11

when the movement of one molecule across the membrane depends on the simultaneous movement of a second species in the same direction

Question 12

which pump forms an essential Na-K gradient?

Answer 12

• Na-K-ATPase

* drives many secondary active transport processes

Question 13

how is intracellular calcium controlled?

Answer 13

PMCA - high affinity, low capacity
SERCA - high affinity, low capacity
NCX - low affinity, high capacity
Ca mitochondrial uniports

Question 14

which pump moves most of the Ca2+ out of the cell?

Answer 14

NCX - low affinity, but high capacity

Question 15

How is cell volume regulated?

Answer 15

• get rid of ions if cell swells
• take on ions if cell shrinks
• water follows
• variety of ion pumps employed

Question 16

What are the most important pumps for maintaining Na, K and Ca2+

Answer 16

• Na/K-ATPase maintains low Na, high K

* SERCA/ PMCA keep Ca2+ low

Question 17

what are the consequences of plasma proteins leaking into interstitial fluid?

Answer 17

• increased interstitial colloid osmotic pressure

* oedema