4. Aquaporin will not transport Na+ ions because A. Two asparagine residues in t

Question

4. Aquaporin will not transport Na+ ions because

A. Two asparagine residues in the channel prevent the passage of Na+ ions by binding the oxygen of each water molecule as it passes by

B. Na+ ions are repelled by the positive charges at the entrance to the channel

C. Na+ ions cannot make contact with the carbonyl (C=O) groups on all four sides of the channel

D. The passage of Na+ ions is energetically unfavorable because one side of the channel is hydrophobic in nature

5. The acetylcholine receptor is different from other channel proteins we have discussed, such as the K+ leak channel, because

A. The acetylcholine channel is composed of a single polypeptide rather than multiple subunits

B. The acetylcholine channel transports Na+ ions against the concentration gradient for sodium

C. The acetylcholine channel has a much smaller diameter than other channel proteins

D. The acetylcholine channel transports ions with their associated shell of water molecules

E. The acetylcholine channel is always open, and never switches to a closed conformation

6. After opening briefly along the axon of a neuron, voltage-gated ion channels switch to an inactivated conformation that slowly returns to the original closed conformation of the protein. This slow change in conformation, requiring about a millisecond, is important because

A. all of the ions that were transported across the membrane when the ion channels were open must be actively transported back to the other side of the membrane, restoring the original concentration gradient, before the ion channels can open again

B. ATP hydrolysis is necessary to drive this energetically unfavorable change in conformation

C. it prevents the action potential from reversing direction and traveling back toward the cell body of the neuron

D. the channel protein must be dephosphorylated in order to complete the transport process and return to the original nonphosphorylated conformation

Explanation / Answer

4. A. Two asparagine residues in the channel prevent the passage of Na+ ions by binding the oxygen of each water molecule as it passes by

5. C. The acetylcholine channel has a much smaller diameter than other channel proteins

6. C. it prevents the action potential from reversing direction and traveling back toward the cell body of the neuron

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