Essential Cell Biology

Bruce Alberts, Karen Hopkin, Alexander D. Johnson

Chapter 12

Transport Across Cell Membranes - all with Video Answers

Educators

Chapter Questions

Problem 1

A simple enzyme reaction can be described by the equation $\mathrm{E}+\mathrm{S} \leftrightarrow \mathrm{ES} \leftrightarrow \mathrm{E}+\mathrm{P},$ where $\mathrm{E}$ is the enzyme, $S$ the substrate, $P$ the product, and ES the enzymesubstrate complex.
A. Write a corresponding equation describing the workings of a transporter (T) that mediates the transport of a solute (S) down its concentration gradient.
B. What does this equation tell you about the function of a transporter?
C. Why would this equation be an inappropriate choice to represent the function of a channel?

Patina Herring

Patina Herring

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Problem 2

A rise in the intracellular $\mathrm{Ca}^{2+}$ concentration causes muscle cells to contract. In addition to an ATPdriven $\mathrm{Ca}^{2+}$ pump, muscle cells that contract quickly and regularly, such as those of the heart, have an antiport that exchanges $\mathrm{Ca}^{2+}$ for extracellular $\mathrm{Na}^{+}$ across the plasma membrane. The majority of the $\mathrm{Ca}^{2+}$ ions that have entered the cell during contraction are rapidly pumped back out of the cell by this antiport, thus allowing the cell to relax. Ouabain and digitalis are used for treating patients with heart disease because they make heart muscle cells contract more strongly. Both drugs function by partially inhibiting the $\mathrm{Na}^{+}$ pump in the plasma membrane of these cells. Can you propose an explanation for the effects of the drugs in the patients? What will happen if too much of either drug is taken?

Bryan Valdivia

Numerade Educator

Problem 3

The figure above shows a recording from a patch-clamp experiment in which the electrical current passing across a patch of membrane is measured as a function of time. The membrane patch was plucked from the plasma membrane of a muscle cell by the technique shown in Figure $12-25$ and contains molecules of the acetylcholine receptor, which is a ligand-gated cation channel that is opened by the binding of acetylcholine to the extracellular face of the channel. To obtain a recording, acetylcholine was added to the solution inside the microelectrode. (A) Describe what you can learn about the channels from this recording.
(B) How would the recording differ if acetylcholine were (i) omitted or (ii) added to the solution outside the microelectrode only?

Sana Riaz

Numerade Educator

Problem 4

Using the Nernst equation and the ion concentrations given in Table $12-1$ (p. 391 ), calculate the equilibrium membrane potential of $\mathrm{K}^{+}$ and $\mathrm{Na}^{+}-$ that is, the membrane potential where there would be no net movement of the ion across the plasma membrane (assume that the concentration of intracellular $\mathrm{Na}^{+}$ is $10 \mathrm{mM}$ ). What membrane potential would you predict in a resting animal cell? Explain your answer. What would happen if a large number of $\mathrm{Na}^{+}$ channels suddenly opened, making the membrane much more permeable to $\mathrm{Na}^{+}$ than to $\mathrm{K}^{+}$ ? (Note that because few ions need to move across the membrane to change drastically the charge distribution across that membrane, you can safely assume that the ion concentrations on either side of the membrane do not change significantly.) What would you predict would happen next if the $\mathrm{Na}^{+}$ channels closed again?

Sana Riaz

Numerade Educator

Problem 5

Explain as precisely as you can, but in no more than 100 words, the ionic basis of an action potential and how it is passed along an axon.

Bryan Valdivia

Numerade Educator

Problem 6

In the disease myasthenia gravis, the human body makes-by mistakeantibodies to its own acetylcholine receptor molecules. These antibodies bind to and inactivate acetylcholine receptors on the plasma membrane of muscle cells. The disease leads to a devastating progressive weakening of the muscles of people affected. Early on, they may have difficulty opening their eyelids, for example, and, in an animal model of the disease, rabbits have difficulty holding their ears up. As the disease progresses, most muscles weaken, and people with myasthenia gravis have difficulty speaking and swallowing. Eventually, impaired breathing can cause death. Explain which step of muscle function is affected.

Bryan Valdivia

Numerade Educator

Problem 7

When an inhibitory neurotransmitter such as GABA opens Cl' channels in the plasma membrane of a postsynaptic neuron, why does this make it harder for an excitatory neurotransmitter to excite the neuron?

Bryan Valdivia

Numerade Educator

Problem 8

The diagram in Figure $12-9$ shows a transporter that mediates the passive transfer of a solute down its concentration gradient across the membrane. How would you need to change the diagram to convert the transporter into a pump that moves the solute up its concentration gradient by hydrolyzing ATP? Explain the need for each of the steps in your new illustration.

Bryan Valdivia

Numerade Educator

Problem 9

Which of the following statements are correct? Explain your answers.
A. The plasma membrane is highly impermeable to all charged molecules.
B. Channels have specific binding pockets for the solute molecules they allow to pass.
C. Transporters allow solutes to cross a membrane at much faster rates than do channels.
D. Certain $\mathrm{H}^{+}$ pumps are fueled by light energy.
E. The plasma membrane of many animal cells contains open $\mathrm{K}^{+}$ channels, yet the $\mathrm{K}^{+}$ concentration in the cytosol is much higher than outside the cell.
F. A symport would function as an antiport if its orientation in the membrane were reversed (i.e., if the portion of the molecule normally exposed to the cytosol faced the outside of the cell instead).
G. The membrane potential of an axon temporarily becomes more negative when an action potential excites it.

MO

Morgan Ovil

Numerade Educator

Problem 10

List the following compounds in order of decreasing lipidbilayer permeability: RNA, $\mathrm{Ca}^{2+}$, glucose, ethanol, $\mathrm{N}_{2}$, water

Bryan Valdivia

Numerade Educator

Problem 11

Name at least one similarity and at least one difference between the following (it may help to review the definitions of the terms using the Glossary):
A. Symport and antiport
B. Active transport and passive transport
C. Membrane potential and electrochemical gradient
D. Pump and transporter
E. Axon and telephone wire
F. Solute and ion

Bryan Valdivia

Numerade Educator

Problem 12

Discuss the following statement: "The differences between a channel and a transporter are like the differences between a bridge and a ferry."

Aditya Sood

Numerade Educator

Problem 13

The neurotransmitter acetylcholine is made in the cytosol and then transported into synaptic vesicles, where its concentration is more than 100 -fold higher than in the cytosol. When synaptic vesicles are isolated from neurons, they can take up additional acetylcholine added to the solution in which they are suspended, but only when ATP is present. $\mathrm{Na}^{+}$ ions are not required for the uptake, but, curiously, raising the $\mathrm{pH}$ of the solution in which the synaptic vesicles are suspended increases the rate of uptake. Furthermore, transport is inhibited when drugs are added that make the membrane permeable to $\mathrm{H}^{+}$ ions. Suggest a mechanism that is consistent with all of these observations.

Bryan Valdivia

Numerade Educator

Problem 14

The resting membrane potential of a typical animal cell is about $-70 \mathrm{mV}$, and the thickness of a lipid bilayer is about $4.5 \mathrm{nm} .$ What is the strength of the electric field across the membrane in $\mathrm{V} / \mathrm{cm} ?$ What do you suppose would happen if you applied this field strength to two metal electrodes separated by a $1-\mathrm{cm}$ air gap?

Sana Riaz

Numerade Educator

Problem 15

Phospholipid bilayers form sealed, spherical vesicles in water (discussed in Chapter 11 ). Assume you have constructed lipid vesicles that contain $\mathrm{Na}^{+}$ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one $\mathrm{Na}^{+}$ one way and one $\mathrm{K}^{+}$ the other way in each pumping cycle. All the $\mathrm{Na}^{+}$ pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of Figures $12-11$ and $12-12$, determine what would happen in each of the following cases.
A. Your vesicles were suspended in a solution containing both $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ ions and had a solution with the same ionic composition inside them.
B. You add ATP to the suspension described in (A).
C. You add ATP, but the solution-outside as well as inside the vesicles-contains only $\mathrm{Na}^{+}$ ions and no $\mathrm{K}^{+}$ ions.
D. The concentrations of $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ were as in $(\mathrm{A}),$ but half of the pump molecules embedded in the membrane of each vesicle were oriented the other way around, so that the normally cytosolic portions of these molecules faced the inside of the vesicles. You then add ATP to the suspension.
E. You add ATP to the suspension described in (A), but in addition to $\mathrm{Na}^{+}$ pumps, the membrane of your vesicles also contains $K^{+}$ leak channels.

Patina Herring

Numerade Educator

Problem 16

Name the three ways in which an ion channel can be gated.

Aditya Sood

Numerade Educator

Problem 17

One thousand $\mathrm{Ca}^{2+}$ channels open in the plasma membrane of a cell that is $1000 \mu \mathrm{m}^{3}$ in size and has a cytosolic $\mathrm{Ca}^{2+}$ concentration of $100 \mathrm{nM}$. For how long would the channels need to stay open in order for the cytosolic $\mathrm{Ca}^{2+}$ concentration to rise to $5 \mu \mathrm{M} ?$ There is virtually unlimited $\mathrm{Ca}^{2+}$ available in the outside medium (the extracellular $\mathrm{Ca}^{2+}$ concentration in which most animal cells live is a few millimolar), and each channel passes $10^{6} \mathrm{Ca}^{2+}$ ions per second.

Sana Riaz

Numerade Educator

Problem 18

Amino acids are taken up by animal cells using a symport in the plasma membrane. What is the most likely ion whose electrochemical gradient drives the import? Is ATP consumed in the process? If $s o,$ how?

Bryan Valdivia

Numerade Educator

Problem 19

We will see in Chapter 15 that endosomes, which are membrane-enclosed intracellular organelles, need an acidic lumen in order to function. Acidification is achieved by an $\mathrm{H}^{+}$ pump in the endosomal membrane, which also contains $\mathrm{Cl}^{-}$ channels. If the channels do not function properly (e.g., because of a mutation in the genes encoding the channel proteins), acidification is also impaired.
A. Can you explain how Cl' channels might help acidification?
B. According to your explanation, would the Cl- channels be absolutely required to lower the pH inside the endosome?

Bryan Valdivia

Numerade Educator

Problem 20

Some bacterial cells can grow on either ethanol $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right)$ or acetate $\left(\mathrm{CH}_{3} \mathrm{COO}^{-}\right)$ as their only carbon
source. Dr. Schwips measured the rate at which the two compounds traverse the bacterial plasma membrane but, due to excessive inhalation of one of the compounds (which one?), failed to label his data accurately.
A. Plot the data from the table below.
B. Determine from your graph whether the data describing compound A correspond to the uptake of ethanol or acetate.
Explain your answers

Sana Riaz

Numerade Educator

Problem 21

Acetylcholine-gated cation channels do not discriminate between $\mathrm{Na}^{+}, \mathrm{K}^{+},$ and $\mathrm{Ca}^{2+}$ ions, allowing all to pass through them freely. So why is it that when acetylcholine binds to this protein in the plasma membrane of muscle cells, the channel opens and there is a large net influx of primarily $\mathrm{Na}^{+}$ ions?

Bryan Valdivia

Numerade Educator

Problem 22

The ion channels that are regulated by binding of neurotransmitters, such as acetylcholine, glutamate, GABA, or glycine, have a similar overall structure. Yet each class of these channels consists of a very diverse set of subtypes with different transmitter affinities, different channel conductances, and different rates of opening and closing. Do you suppose that such extreme diversity is a good or a bad thing from the standpoint of the pharmaceutical industry?

Sana Riaz

Numerade Educator