Membrane Proteins

Question 1

What does the phospholipid bilayer act as to ions and most uncharged polar molecules? Why is this?

Answer 1

barrier due to its hydrophobic interactions

Question 2

What do transmembrane proteins do?

Answer 2

act as channels or transporters controlling the concentration of ions and other molecules within the cell

Question 3

To perform specialised functions, what do different cell types and different cell compartments have?

Answer 3

different channel and transporter proteins

Question 4

What kind of process is the passage of molecules through channel proteins?

Answer 4

passive

Question 5

Water can pass across the plasma membrane by diffusing through the phospholipid bilayer or through water channels. What are these water channels called?

Answer 5

aquaporins

Question 6

What can some channel proteins be and what does this mean?

Answer 6

gated and change conformation to allow or prevent diffusion

Question 7

What are the 2 types of gated channels?

Answer 7

ligand gated channels

voltage gated channels

Question 8

What do gated channels respond to?

Answer 8

a stimulus which causes them to open or close

Question 9

Describe the difference between a ligand gated channel and a voltage gated channel

Answer 9

Ligand gated channels respond to a chemical stimulus e.g. neurotransmitter. Voltage gated channels respond to the relative concentration of ions

Question 10

State the function of transporter proteins?

Answer 10

change conformation to transport molecules across a membrane

Question 11

What two mechanisms do transported proteins work by?

Answer 11

Facilitated transport

Active Transport

Question 12

Give an example of facilitated transport

Answer 12

glycose symport

Question 13

Give an example of active transport

Answer 13

sodium-potassium pump

Question 14

What kind of process is facilitated transport?

Answer 14

passive

Question 15

What does the sodium-potassium pump do?

Answer 15

Transports sodium out of cells, and potassium ions in

Question 16

What is the sodium-potassium pump driven by?

Answer 16

the hydrolysis of ATP

Question 17

What are the key features of the sodium-potassium pump?

Answer 17

• it is a transmembrane carrier (transporter) protein
• it has 3 binding sites for Na+ ions
• it has 2 binding sites for K+ ions
• there is a phosphorylation site to accept Pi from ATP
• two different conformations of the protein are possible (controlled by the phosphorylation state).

Question 18

Describe the 6 stages that take place during the sodium-potassium pump.

Answer 18

1. The pump is open to the cytosol. Na+ ions bind to Na+ binding sites on the pump.
2. Na+ binding stimulates the ATPase function that phosphorylates the protein.
3. Phosphorylation causes the protein to change conformation. This makes the protein open to the extracellular environment. The Na+ binding site now have lowered affinity for Na+, so they are released.
4. The K+ binding sites have a high affinity for K+, so they are occupied . This triggers the release of the phosphate group.
5. Loss of the phosphate group causes a conformational change so the pump is open to the cytosol once again. K+ binding sites have a lower affinity for K+, so K+ ions are released. Cycle repeats