Fundamentals of Biochemistry

Donald Voet, Judith G. Voet, Charlotte W. Pratt

Chapter 10

Membrane Transport - all with Video Answers

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Chapter Questions

Problem 1

Calculate the free energy change for glucose entry into cells when the extracellular concentration is $5 \mathrm{mM}$ and the intracellular concentration is $3 \mathrm{mM}$

Aadit Sharma

Aadit Sharma

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Problem 2

(a) Calculate the chemical potential difference when intracellular $\left[\mathrm{Na}^{+}\right]=10 \mathrm{mM}$ and extracellular $\left[\mathrm{Na}^{+}\right]=150 \mathrm{mM}$ at $37^{\circ} \mathrm{C}$. (b) What
would the electrochemical potential be if the membrane potential were $-60 \mathrm{mV}$ (inside negative)?

Sana Riaz

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Problem 3

For the problem in Sample Calculation $10-1,$ calculate $\Delta G$ at $37^{\circ} \mathrm{C}$ when the membrane potential is (a) $-50 \mathrm{mV}$ (cytosol negative) and (b) $+150 \mathrm{mV}$. In which case is $\mathrm{Ca}^{2+}$ movement in the indicated direction thermodynamically favorable?

Sana Riaz

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Problem 4

Calculate the free energy required to move 1 mol of $\mathrm{K}^{+}$ ions from the outside of the cell (where $\left.\left[\mathrm{K}^{+}\right]=4 \mathrm{mM}\right)$ to the inside (where $\left[\mathrm{K}^{+}\right]=$ $140 \mathrm{mM}$ ) when the membrane potential is $-70 \mathrm{mV}$ and the temperature is $37^{\circ} \mathrm{C}$

Aadit Sharma

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Problem 5

Indicate whether the following compounds are likely to cross a membrane by nonmediated or mediated transport: (a) ethanol,
(b) glycine, (c) cholesterol, (d) ATP.

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Problem 6

Rank the rate of transmembrane diffusion of the following compounds:

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Problem 7

The smallest $\beta$ -barrel protein contains only eight $\beta$ strands. Explain why porins, which are also $\beta$ barrels, usually contain at least 16 or 18 strands.

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Problem 8

Which amino acids would you expect to be particularly abundant at the entrance of a porin that is specific for phosphate ions?

Aadit Sharma

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Problem 9

The diameter of the KcsA $\mathrm{K}^{+}$ channel is $\sim 6$ A. Why can't $\mathrm{H}_{2} \mathrm{O}$ (diameter 2.75 A) pass through this channel?

Aadit Sharma

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Problem 10

In addition to neurons, muscle cells undergo depolarization, although smaller and slower than in the neuron, as a result of the activity of the acetylcholine receptor
(a) The acetylcholine receptor is also a gated ion channel. What triggers the gate to open?
(b) The acetylcholine receptor/ion channel is specific for $\mathrm{Na}^{+}$ ions. Would $\mathrm{Na}^{+}$ ions flow in or out? Why?
(c) How would the $\mathrm{Na}^{+}$ flow through the ion channel change the membrane potential?

Sana Riaz

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Problem 11

Explain why $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ ions usually move more slowly through pumps than through channels.

Aadit Sharma

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Problem 12

Why would overexpression of an MDR transporter in a cancer cell make the cancer more difficult to treat?

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Problem 13

If the ATP supply in the cell shown in Fig. $10-22 c$ suddenly vanished, would the intracellular glucose concentration increase, decrease, or remain the same?

Aadit Sharma

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Problem 14

The bacterial $\mathrm{Na}^{+}-\mathrm{H}^{+}$ antiporter is a secondary active transport protein that excretes excess $\mathrm{Na}^{+}$ from the cell. Is the extracellular $\mathrm{pH}$ higher or lower than the intracellular pH?

Aadit Sharma

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Problem 15

A certain membrane protein allows phosphate groups to enter a eukaryotic cell. (a) Would you expect the protein to function more like KesA or more like GLUT1? (b) Phosphate ions enter the cell along with $\mathrm{H}^{+}$ ions. Describe this transport system using the terms introduced in Fig. $10-15 .$ (c) The eukaryotic intracellular pH is typically slightly lower than the extracellular pH. Does this suggest that the phosphate transporter carries out secondary active transport?

Sana Riaz

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Problem 16

$E .$ coli cells transport xylose across the cell membrane via a symport system that uses the free energy of a proton gradient. The intracellular $\mathrm{pH}$ is higher than the extracellular $\mathrm{pH}$. (a) Is xylose moving up or down its concentration gradient? Into or out of the cell? (b) Draw a diagram of the xylose transport system.

Aadit Sharma

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Problem 17

What happens to $\mathrm{K}^{+}$ transport by valinomycin when the membrane is cooled below its transition temperature?

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Problem 18

How long would it take 100 molecules of valinomycin to transport enough $\mathrm{K}^{+}$ to change the concentration inside an erythrocyte of volume $100 \mu \mathrm{m}^{3}$ by $10 \mathrm{mM} ?$ (Assume that the valinomycin does not also transport any $\mathrm{K}^{+}$ out of the cell, which it really does, and that the valinomycin molecules outside the cell are always saturated with $\mathrm{K}^{+}$.)

Aadit Sharma

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Problem 19

The compound shown below is the antiparasitic drug miltefosine.
(a) Is this compound a glycerophospholipid?
(b) How does miltefosine likely cross the parasite cell membrane?
(c) In what part(s) of the cell would the drug tend to accumulate? Explain.
(d) Miltefosine binds to a protein that also binds some sphingolipids and some glycerophospholipids. What feature common to all these compounds is recognized by the protein? The protein does not bind triacylglycerols.

Danielle Ashley

Danielle Ashley

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Problem 20

In eukaryotes, ribosomes (approximate mass $4 \times 10^{6} \mathrm{D}$ ) are assembled inside the nucleus, which is enclosed by a double membrane. Protein synthesis occurs in the cytosol. (a) Could a protein similar to a porin or the glucose transporter be responsible for transporting ribosomes into the cytoplasm? Explain.
(b) Would free energy be required to move a ribosome from the nucleus to the cytoplasm? Why or why not?

Joanna Quigley

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Problem 21

The rate of movement (flux) of a substance $X$ into cells was measured at different concentrations of $X$ to construct the following graph.
(a) Does this information suggest that the movement of $X$ into the cells is mediated by a protein transporter? Explain.
(b) What additional experiment could you perform to verify that a transport protein is or is not involved?

Hailey Tomashek

Hailey Tomashek

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Problem 22

Endothelial cells and pericytes in the retina of the eye have different mechanisms for glucose uptake. The figure shows the rate of glucose uptake for each type of cell in the presence of increasing amounts of sodium. What do these results reveal about the glucose transporter in each cell type?

Aadit Sharma

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Problem 23

Cells in the wall of the mammalian stomach secrete HCl at a concentration of $0.15 \mathrm{M}$. The secreted protons, which are derived from the intracellular hydration of $\mathrm{CO}_{2}$ by carbonic anhydrase, are pumped out by an $\left(\mathrm{H}^{+}-\mathrm{K}^{+}\right)$ -ATPase antiport. A $\mathrm{K}^{+}-\mathrm{Cl}^{-}$ co-transporter is also required to complete the overall transport process. (a) Calculate the pH of the secreted HCl. How does this compare to the cytosolic pH (7.4)$?$
(b) Write the reaction catalyzed by carbonic anhydrase. (c) Draw a diagram to show how the action of both transport proteins results in the secretion of HCl.

Aadit Sharma

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Problem 24

Kidney cells contain a channel that allows intracellular ammonia to exit the cells.
(a) Why did researchers originally believe that cells had no need for such a channel? (b) What is the free energy source for ammonia transport via the channel? (c) The same kidney cells also contain a proton pump that expels $\mathrm{H}^{+}$ from the cells. What is the free energy source for this pump, and how does its action prevent ammonia from moving back into the kidney cells?

Shazia Naz

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Problem 25

Most animals have neurons that respond to unfavorable environmental conditions by sending pain signals to the brain. For example, elevated $\left[\mathrm{CO}_{2}\right]$ triggers an influx of $\mathrm{Na}^{+}$ that opens voltage-gated $\mathrm{Na}^{+}$ channels to initiate an action potential. The naked mole rat (Section $8-2 D),$ which lives in crowded underground colonies, has a voltagegated $\mathrm{Na}^{+}$ channel that is inhibited by protons. Explain why this is advantageous.

VS

Vivek Singh

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Problem 26

Scorpion toxin triggers an action potential in pain-sensing neurons in animals by binding to a specific receptor. The toxin also acts to delay the inactivation of the neurons' voltage-gated $\mathrm{Na}^{+}$ channels. Explain how this helps the scorpion better avoid being eaten by an animal.

Bryan Lynn

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