Mechanisms of Solute and Water Transport

Question 1

What are the triggers the transition from open (conducting) to closed (non-conducting) states for gated channels?

Answer 1

1. Depolarization of the cell membrane (voltage-gated)
2. Binding of hormone
3. Binding of ligand
4. Tension on the cell membrane (stretch activated)

Question 2

Can carriers also transition from open to closed randomly?

Answer 2

Yes, due to thermal energy

Question 3

What is facilitated diffusion?

Answer 3

Movement of a (usually impermeable) solute by a carrier channel across the lipid bilayer

Question 4

Describe the kinetics of facilitated diffusion via the carrier-mediated system compared to simple diffusion.

Answer 4

see pic

Question 5

What are P-class ATPases?

Answer 5

Active transporters that function via alternating phosphorylation/dephosphorylation steps.

Question 6

What are 3 major P-class active transporters? What is their nickname?

Answer 6

1. Na/K ATP ase
2. Ca2+ ATPase
3. H+ ATPase

Nicknamed primary active transporters because they are directly linked to the cell’s metabolic energy

Question 7

How many ions does a Na/K ATPase move per molecule of ATP hydrolyzed?

Answer 7

3 Na+ out of the cell, 2 K+ in.

Question 8

What is the function of the Na/K ATPase?

Answer 8

Maintain low Na+ and high K+ levels in the cytoplasm (i.e. membrane potential)

Question 9

How does the Ca2+ ATPase function?

Answer 9

Transports 1 (PMCA, PM) or 2 (SERCA, ER) Ca+ ions in exchange for H+ ions.

Question 10

What is the function of the Ca2+ ATPase?

Answer 10

It maintains very low free calcium concentration in the cytoplasm.

Question 11

How does the H+ (and H+K+) ATPase work?

Answer 11

Secretes protons and/or exchanges H+ for K+.

Question 12

What are ABC transporters?

Answer 12

ATP-binding cassette transporters that bind and hydrolyze ATP (or just bind!) to actively transport their substrates. Have 6 membrane-spanning segments and 2 extracellular binding domains.

Question 13

What are 2 common ABC transporters?

Answer 13

1. MDR1

2. CFTR

Question 14

What are the 2 classes of coupled transporters?

Answer 14

Symport/Co-transport

Exchanger/antiport

Question 15

What are secondary active transporters and what is a major example?

Answer 15

Co-transporters/Antiporters that do not directly use ATP but rely on the exchange of a solute whose concentration is maintained by ATP. Major example = Na+/glucose cotransporter

Question 16

What is the vant Hoff expression and what does it describe?

Answer 16

Describes the osmotic pressure of a solution:

pi = RTCs

Question 17

How do you calculate osmotic pressure when a membrane is permeable to a solute?

Answer 17

Pi effective = sigma RTCs

sigma is a reflection coefficient

Question 18

What does a reflection coefficient represent?

Answer 18

The fraction of solute molecules colliding with the membrane that are reflected (do not cross it).

Thus for impermeable solutes, sigma = 1

Question 19

What effect would a very permeable solutes (sigma

Answer 19

Osmotic pressure would be nearly zero

Question 20

What is oncotic pressure?

Answer 20

The colloid osmotic pressure in the plasma as a result of proteins.

Question 21

Where in the body do you find the lowest reflection coefficients?

Answer 21

Discontinuous capillaries such as the liver.

Question 22

Where in the body do you find the highest reflection coefficients?

Answer 22

Tight continuous capillaries such as the brain.

Question 23

Describe aquaporins.

Answer 23

Tetramer channel with a high selectivity for water.

Question 24

Can you reach a maximum level of carrier flux?

Answer 24

Yes! When all facilitators are saturated. It is based on the substrates’ affinity for the carrier

Question 25

The osmotic pressure created by a permeable solute is ______.

Answer 25

Lower