Exam 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes

Most eukaryotic ABC transporters are involved in exporting small molecules from the cytosol. In this subset of the ABC transporters, does the small molecule bind better to the ATP-bound (B) or the ATP-free (F) transporter? Write down your answer as B or F. Hint: The ATP-bound transporter "faces outward."

Dopamine is a neurotransmitter involved in the reward pathways in the brain, and its decreased activity has been associated with diseases such as Parkinson's disease and attention-deficit/hyperactivity disorder (ADHD). Which of the following drugs is NOT a likely candidate to treat these diseases?

A certain neuron in a mouse brain is firing about 300 times per second, whereas almost all of its neighboring neurons are firing at much lower rates of only a few times per second. This implies that ...

Aquaporin has a pair of key asparagine residues located on the wall almost halfway through its pore. These residues simultaneously bind to the oxygen atom of a passing water molecule, rendering it unavailable for hydrogen-bonding. As a result, …

The inactivation rate of voltage-gated Na⁺ and K⁺ channels can regulate the firing frequency of neurons. Which of the following combinations results in the highest firing frequency?

After release into the synaptic cleft, the neurotransmitter dopamine is actively taken up by the cells via specific dopamine transporters. The drug cocaine interferes with this process and is therefore called a reuptake inhibitor. The inhibition of the transporter at a certain cocaine concentration is qualitatively represented in the following graph. Based on this graph, which of the following describes the effect of cocaine on the kinetics of dopamine reuptake by the transporter?

Indicate true (T) and false (F) statements below regarding ion channels in cellular membranes. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTT. ( ) With patch-clamp recordings, it is possible to study ion transport through a single molecule of a channel protein. ( ) Cells that are not electrically excitable often lack gated ion channels in their plasma membrane. ( ) The aggregate current crossing a patch of plasma membrane represents the degree to which each individual channel is open at that time. ( ) Patch-clamp recordings have shown that any voltage-gated ion channel opens as soon as the membrane potential is altered.

An ion channel …

A neuron's repetitive firing rate is limited by an absolute refractory period, during which a new action potential cannot be generated. Which event is chiefly responsible for this limit?

This family of ATPases is structurally related to the turbine-like pumps that acidify lysosomes and vesicles; however, they usually function in reverse, generating ATP from ADP and Pᵢ using proton gradients across membranes. What are they called?

A potassium channel conducts K⁺ ions several orders of magnitude better than Na⁺ ions, because …

In contrast to transporters, the channel proteins in cellular membranes …

Voltage-gated cation channels in the plasma membrane of a neuron open when the membrane potential passes a threshold value that is about 20 mV above the resting potential. This corresponds to a change of about 4 × 10⁶ newtons per coulomb in the magnitude of the electric field across the membrane compared to the resting state. The tetrameric channels have a voltage-sensing helix S4 in each of their four subunits. Most S4 helices contain seven residues with positively charged side chains. The charge carried by each such residue is about 1.6 × 10-¹⁹ coulombs. By reaching threshold depolarization, what are the magnitude and the direction of the collective force that the four S4 helices in the channel experience as a result of the change in the electric field? The force that particles of charge q in an electric field E experience is calculated as F = q.E.

Once expressed in a neuron in the mouse brain and subjected to a flash of light, how does channelrhodopsin affect the membrane potential?

Neuromuscular transmission involves the sequential activation of several ion channels. Sort the following events to reflect the sequence leading to muscle contraction. Your answer would be a five-digit number composed of numbers 1 to 5 only, e.g. 12543. (A) Opening of voltage-gated Na? channels in the muscle cell plasma membrane. (B) Opening of Ca²⁺ release channels in the sarcoplasmic reticulum membrane. (C) Opening of ionotropic acetylcholine receptors on the muscle cell plasma membrane. (D) Opening of voltage-gated Ca²⁺channels in the T tubules. (E) Opening of voltage-gated Ca²⁺channels in the motor neuron.

Which of the following is correct regarding the electrical properties of cellular membranes?

The Nernst equilibrium potential for an ion that is 10 times more concentrated in the cytosol compared to the extracellular fluid is about -60 mV. How much would the potential be if the extracellular concentration decreases 100-fold with no change in the intracellular concentration?

Which of the following is a pump that hydrolyzes two ATP molecules per transport cycle?

Indicate whether each of the following descriptions matches an ABC transporter (A), a P-type pump (P), or a V-type pump (V). Your answer would be a five-letter string composed of letters A, P, and V only, e.g. PPVVV. ( ) The pumps in this family are phosphorylated at a key Asp residue in each transport cycle. ( ) This family is the largest among membrane transport proteins and includes some channels as well as pumps. ( ) The pumps in this family are responsible for the acidification of synaptic vesicles. ( ) The sodium-potassium pump is a member of this family. ( ) The multidrug resistance protein is a member of this family.

Many amino acids in our diet are absorbed via the transcellular transport pathway by the intestinal epithelial cells. This process requires ATP hydrolysis by …