lecture 8

Question 1

Arrange the following from most permeable to least permable to a membrane with no proteins: Polar uncharged molecules, charged atoms, and hydrophobic molecules.

Answer 1

• Most permeable: hydrophobic molecules. (O2 and CO2)
• polar, uncharged molecules (like glucose) very low permeability, usually needs a transporter
• charged molecules: zero permeability (Na+ Cl- etc)

Question 2

What type of transport do transporters mediate? And channels?

Answer 2

• Transporters: Active or passive transport (!)
• Channels: ONLY passive transport

Question 3

What is the difference between primary active transport and secondary active transport?

Answer 3

• Primary transport makes use of ATP hydrolysis to transport things against their concentration gradient.
• Secondary transporters use the co-transport of a second substrate to drive the transport of the other agains its concentration gradient. The energy to do this is obtained from the electrochemical gradient!!!

Question 4

Why is the delta G0 of a passive transporter zero?

Answer 4

Because at equilibrium, the ratio of in/out is equal, meaning ln(keq) = ln(1), and ln(1) is zero. Therefore, the delta G0 of a passive transporter at equilibrium is zero.

Question 5

Which energy sources can active transporters use?

Answer 5

1. Electrochemical gradient from another solute (coupled transport)
2. ATP hydrolysis
3. Light

Question 6

What are the four types of ATP-driven pumps?

Answer 6

1. P-type pump
2. ABC-transporter
3. V-type proton pump
4. F-type ATP synthase

Question 7

How does a P-type pump work? (rough outline)

Answer 7

The P-type pump uses an aspartic acid residue to phosphorylate te solute. In muscle cells, it pumps calicium into the sarcoplasmic reticulum in exhange for protons. The phosphate is released when the pump returns to its original state.

Question 8

What do V-type proton pumps do?

Answer 8

They pump protons into organelles like lysosomes and vacuoles, to make the environment of these organelles more acidic so that the enzymes in them can function.

Question 9

What does F-type ATP synthase do?

Answer 9

It pumps protons from the intermembrane space into the mitochondrial matrix to synthesize ATP.

Question 10

What is the difference in functioning of ABC transporters in eukaryotes vs bacteria?

Answer 10

In eukaryotes, ABC transporters exclusively transport things out of the cell (like toxins, or peptide hormones).
In bacteria, however, they can play a role in either secretion or uptake.

Question 11

What are two ways in which ABC transporters can have negative effects?

Answer 11

1. They can prevent drugs and pharmaceuticals from reaching their targets (especially in the brain, since they’re important in maintaining the blood-brain barrier)
2. They can be upregulated in their expression, making cancer cells resistant to cytotoxic compounds.

Question 12

Where are porins located?

Answer 12

In the outer membrane of mitochondria, and in gram-negative bacteria.

Question 13

How do aquaporins facilitate the transport of water, while preventing the transport of protons?

Answer 13

Typically, protons like to ‘hop’ across the hydrogen-bonds, from one molecule to another. Aquaporins have two Asn residues that arrange a water molecule so that is has complete binding and has no other coordination space for a proton to move.

Question 14

How can ion channels be gated?

Answer 14

1. Voltage-gated channels (key in neurotransmission)
2. Ligand-gated (extracellular ligand)
   2.5. Ligand-gated (intracellular ligand)
3. Mechanically gated (controlled by osmopressure)