ICPP 4 - Membrane Permeability

Question 1

What is a “semi-permeable” membrane?

Answer 1

A layer through which only allowed substances can pass.

Question 2

What is the rate of passive transport dependent on?

Answer 2

Permeability and the concentration gradient. Rate of passive diffusion increases linearly with concentration gradient. When the concentration gradient is 0, the system is in equilibrium.

Question 3

Which molecules are permeable and which molecules are impermeable through a lipid bilayer?

Answer 3

1) Hydrophobic molecule are permeable - O2, CO2, N2, benzene
2) Small uncharged polar molecules, e.g.: H20, urea, glycerol
3) Large uncharged polar molecules are impermeable - e.g.: glucose and larger
4) Ions - impermeable due to charge

Question 4

What important roles do transport proteins have?

Answer 4

• Maintenance of ionic composition
• Maintenance of intracellular pH
• Regulation of cell volume
• Extrusion of waste products from metabolism and toxic substances
• Generation of ion gradients necessary for electrical excitability of nerve and muscle

Question 5

What is the mode of transport that membrane proteins apply?

Answer 5

Conformational change - “Ping-Pong transport”
- alternatively can occur through facilitated diffusion, where the channel is open is response to a stimulus (ligand-gated/voltage-gated).

Question 6

How do transporters contribute to membrane permeability?

Answer 6

They greatly increase the permeability for certain molecules, even though they can saturate, it is greatly more efficient than simple diffusion.

Question 7

What 2 things determines whether transport of a molecule is passive or active?

Answer 7

1) Concentration Gradient

2) Membrane Potential

Question 8

Where does energy for AT come from?

Answer 8

Hydrolysis of ATP (some cells spend 30-50% of their ATP on AT)

Question 9

Water has a dipole created by electron distribution, can it still move through the bilayer? If so, what is it driven by?

Answer 9

• Yes it can as it is small and uncharged, it moves by simple diffusion driven by an osmotic gradient.

Question 10

State the standard IC and EC ion values for Na, K, Ca & Cl ions.

Answer 10

EC:
Na = 145mM
K+ = 4mM
Ca = 1.5mM
Cl = 123mM

IC:
Na = 12mM
K = 140mM
Ca = 10^-7mM
Cl = 4.2mM

Question 11

What is the direction of osmosis determined by?

Answer 11

The comparison of solute concentration (tonicity), i.e.: water moves from hypo to hypertonic and vice versa

Question 12

What happens to animal cells placed in hypotonic and hypertonic solutions?

Answer 12

Hypotonic = Water moves in, cells swell and burst (cytolysis)
Hypertonic = Water moves out, cellls shrink and lyse

Question 13

What is an osmole?

What is it proportional to?

Answer 13

The measure of a solutions ability to create osmotic pressure and thus affect the movement of water, proportional to the number of osmotic particles in solution (e.g.: 1 mole of NaCl forms 2 osmolar concentration in 1L water).

Question 14

What is osmolality?

Answer 14

When the concentration of a solution is expressed is osmles per Kg of water.

Question 15

What kind of proteins are AQP’s?
How many are there?
What do they permit?

Answer 15

• Integral membrane proteins NOT ion channels
• 13 isoforms (AQP0-10)
• Permit rapid passive diffusion of water through lipid bilayer

Question 16

What does water movement through AQP’s depend on?
How many water molecules can move through per second?
What do positively charge residues in the hydrophilic pore do?

Answer 16

• Solute concentration gradient
• 3 billion (3x10^9) - in single file fashion
• Prevent movement of charged ions such as H+, so H+ gradient is not disrupted.

Question 17

Are AQP’s narrowly or widely distributed across different tissue types?

Answer 17

Widely distributed