Membrane Transporters

Question 1

1. Describe the difference between primary and secondary active transport.

Answer 1

Primary active transporters derive energy directly from ATP splitting (they are ATPases). Ex: Na/K pump, H pump, Ca pump. Some of them pump protons and Calcium into organelles (endosomes, vesicles, etc.) to keep them out of the cytoplasm. F1-ATPase at mitochondria pumps H out to synthesize ATP.

Secondary active transport = mechanism most substances use = do not directly use ATP (though it eventually comes to rely on this via reliance on Na/K pump). Energy to direct work of pumping comes from a second source, like downhill leak of Na into the cell. These carriers capture energy released by inward leak of Na and use it to pump. Ex: AA/Na co-transporter, Na/Ca exchanger, Na/H exchanger. These don’t always have to go in one direction, can reverse based on conditions.
These are dependent on [Na]o  if you ruin this, transport won’t work. That’s why ultimately, secondary transport mechanisms depend on the Na/K pump and ATP. If you block the pump, cells fill up with Na, and the gradient is reduced.

Question 2

2. Define cotransport and exchange transport.

Answer 2

Cotransport = when secondary active transporters move solute species in same direction (like AA/Na transport  both go into cell). Antiport or exchange works such that solutes are moving in opposite directions (ex: Na/Ca exchange or Na/H exchange).
Sometimes these transporters can change direction and tap the bigger leak to drive the smaller pump. (Ex Na/Ca pump in the heart; reverses direction when the heart beats).

Question 3

3. Identify physical forces that can determine the gating properties of ion channel.

Answer 3

Electric field (voltage gated, require depolarization), mechanical (require membrane stretching, like cochlear hair cells), chemical (synaptic receptors), and temperature (cutaneous temp receptors). Some may require 1 or 2 of these forces to open, others may require none (like aquaporins).

Voltage gated, ligand, mechanical

Question 4

4. Determine if a pump for a particular ion must exist in a cell at rest (steady state), given an ion’s concentration inside and out, the membrane potential, and knowledge that the membrane is permeable to the ion in question; and, if a pump must exist, determine which direction it pumps the ion.

Answer 4

Compare []i and []o to figure out which way the ion moves. Charge of ion vs. membrane potential – can determine which way ion moves. If these are the same direction, there must be a pump moving ions against the gradient. If these are different directions, compare the E to the MP.
If E > Vm  ion wants to make inside of cell more positive. If ion is +, it wants to go into cell. If ion is -, it wants to leave.
If E is < Vm, it wants to make the inside more negative. If ion is +, it wants to leave cell. If ion is -, it wants to enter cell. So they’re pumped in the opposite direction of where it wants to move.
If E = Vm, ion is distributed at its electrochemical equilibrium.

Question 5

5. Describe how cells concentrate glucose inside, even though the glucose transporter cannot pump glucose against its concentration gradient.

Answer 5

Glucose levels regulated by insulin. Insulin causes vesicles containing glucose transporter to fuse with plasma membrane. Glucose transporter transports glucose in either direction, in or out of the cell, and uses no energy to do so. When glucose molecule is transported into the cell, it is phosphorylated to G-6-P  can’t fit on transporter and gets trapped inside.