1.  Shared traits. Name some of the features common to all membranes. ✓ 1

2.  Simple diffusion. What conditions are required for a small molecule to spontaneously pass through a membrane? ✓ 2

3.  Bread and jam. Match each term with its description.

4.  Solubility matters. Arrange the following substances in order of increasing permeability through a lipid bilayer: (a) glucose; (b) glycerol; (c) Cl⁻; (d) indole; (e) tryptophan. ✓ 2

5.  A helping hand. Differentiate between simple diffusion and facilitated diffusion. ✓ 2

6.  Gradients. Differentiate between passive transport and active transport. ✓ 3

7.  The golden mean. Proper membrane fluidity is vital to membrane-protein function. Suggest how a loss of fluidity and how too much fluidity might affect membrane-protein function. ✓ 3

8.  Heart beats. Outline the relation between the Na⁺–K⁺ ATPase and the strength of a heart contraction. Identify the relevant primary and secondary active-transport components. How do cardiotonic steroids affect the strength of a heartbeat? ✓ 3

9.  Hunting hippos. Somali hunters use arrows that have been dipped in high concentrations of the cardiac glycoside ouabain to kill game. Indeed, there are reports that animals the size of a hippopotamus can be killed by ouabain-treated arrows. Suggest a biochemical basis for the lethal action of ouabain. ✓ 3

10.  Only so much energy. Consider Figure 12.20, which illustrates the relation between the sodium–glucose symporter and the Na⁺–K⁺ ATPase. If the symporter were inhibited, what effect, if any, would such inhibition have on the ATPase? ✓ 3

11.  Commonalities. What are two fundamental properties of all ion channels? ✓ 3

12.  Opening channels. Differentiate between ligand-gated and voltage-gated channels. ✓ 3

13.  Behind the scenes. Is the following statement true or false? Explain. ✓ 3

The sodium–glucose linked transporter does not depend on the hydrolysis of ATP.

14.  Powering movement. List two forms of energy that can power active transport. ✓ 3

15.  Greasy patch. A stretch of 20 amino acids is sufficient to form an α helix long enough to span the lipid bilayer of a membrane. How could this piece of information be used to search for membrane proteins in a data bank of primary sequences of proteins?

16.  Water-fearing. Lipid bilayers are self-sealing. If a hole is introduced, the hole is filled in immediately. What is the energetic basis of this self-sealing? ✓ 1

17.  Embedded or not. Differentiate between peripheral proteins and integral proteins.

18.  Water-loving. All biological membranes are asymmetric. What is the energetic basis of this asymmetry? ✓ 1

19.  Pumping sodium. Design an experiment to show that the action of the sodium-glucose linked transporter can be reversed in vitro to pump sodium ions across a membrane. ✓ 3

20.  Different directions. The K⁺ channel and the Na⁺ channel have similar structures and are arranged in the same orientation in the cell membrane. Yet the Na⁺ channel allows sodium ions to flow into the cell and the K⁺ channel allows potassium ions to flow out of the cell. Explain. ✓ 3

21.  Energy considerations. Explain why an a helix is especially suitable for a transmembrane-protein segment.

22.  Speed and efficiency matter. The neurotransmitter acetylcholine, which activates a ligand-gated ion channel, is rapidly destroyed by the enzyme acetylcholinesterase. This enzyme, which has a turnover number of 25,000 per second, has attained catalytic perfection with a k_cat/K_M of 2 × 10⁸ M⁻¹ s⁻¹. Why is the efficiency of this enzyme physiologically crucial?

23.  Relief for sore joints. Both aspirin and ibuprofen inhibit prostaglandin H₂ synthase-1 and relieve inflammation. Aspirin functions by blocking a channel in the enzyme, thereby preventing access to the substrate. Ibuprofen does not block this channel but still inhibits the synthase. How might ibuprofen function?

24.  Cholesterol effects. The red curve on the following graph shows the fluidity of the fatty acids of a phospholipid bilayer as a function of temperature. The blue curve shows the fluidity in the presence of cholesterol. ✓ 2



(a) What is the effect of cholesterol?

(b) Why might this effect be biologically important?

25.  Transport differences. The rate of transport of two molecules, indole and glucose, across a cell membrane is shown in the following illustration. What are the differences between the transport mechanisms of the two m olecules? Suppose that ouabain, a specific inhibitor of the Na⁺–K⁺ ATPase, inhibited the transport of glucose. What would this inhibition suggest about the mechanism of transport? ✓ 3

26.  Desert fish. Certain fish living in desert streams alter their membrane-lipid composition in the transition from the heat of the day to the cool of the night. Predict the nature of the changes. ✓ 2

27.  A handy protective device. The multidrug-resistance protein, which can lead to drug resistance in cancer and other pathological situations, is present in most normal cells. What purpose might this protein serve in normal cells? ✓ 3

28.  Looking-glass structures. Phospholipids form lipid bilayers in water. What structure might form if phospholipids were placed in an organic solvent? ✓ 1

30.  Channel problems 1. A number of pathological conditions result from mutations in the acetylcholine receptor channel, an ion channel that is activated by the binding of acetylcholine. One such mutation causes muscle weakness and rapid fatigue. An investigation of the acetylcholine-generated currents through the acetylcholine receptor channel for both a control and a patient yielded the following results. What is the effect of the mutation on channel function? Suggest some possible biochemical explanations for the effect. ✓ 3

31.  Channel problems 2. The acetylcholine receptor channel can undergo mutation leading to fast-channel syndrome (FCS), with clinical manifestations similar to those of slow-channel syndrome (SCS). What would the recordings of ion movement look like in this syndrome? Suggest a biochemical explanation. ✓ 3

32.  A dangerous snail. Cone snails are carnivores that inject a powerful set of toxins into their prey, leading to rapid paralysis. Many of these toxins are found to bind to specific ion-channel proteins. Why are such molecules so toxic? How might such toxins be useful for biochemical studies? ✓ 3

33.  A perfect fit. Provide an energetic explanation for how the potassium channel allows passage of the potassium ion but not the smaller sodium ion. ✓ 3