chapter 6

26) What is the most important factor in explaining why osmosis occurs spontaneously?

A) It leads to an increase in entropy.

20) You have a planar bilayer with equal amounts of saturated and unsaturated phospholipids. After testing the permeability of this membrane to glucose, you increase the proportion of unsaturated phospholipids in the bilayer. What will happen to the membrane's permeability to glucose?

A) Permeability to glucose will increase.

15) Which aspect of phospholipids is most important to the formation of bilayers?

A) They are amphipathic.

23) Which of the following factors would tend to increase membrane fluidity?

A) a greater proportion of unsaturated phospholipids

18) Which of the following crosses lipid bilayers the slowest?

A) a sodium ion

46) A membrane protein that spans the phospholipid bilayer one or more times is _____.

A) a transmembrane protein

5) Lipids _____.

A) are insoluble in water

56) In some cells, there are many ion electrochemical gradients across the plasma membrane even though there are usually only one or two proton pumps present in the membrane. The gradients of the other ions are most likely accounted for by _____.

A) cotransport proteins

3) Cooking oil and gasoline (a hydrocarbon) are not amphipathic molecules because they _____.

A) do not have a polar or charged region

8) The presence of cholesterol in the plasma membranes of some animal cells _____.

A) enables the membranes to stay fluid when cell temperature drops

21) The membranes of winter wheat are able to remain fluid when it is extremely cold by _____.

A) increasing the percentage of unsaturated phospholipids in the membrane

25) The text states that ribonucleotides can diffuse through some types of liposomes. It is likely that the lipids present early in chemical evolution had short chains. Would liposomes formed from these types of lipids be more or less permeable to ribonucleotides than if early cells formed from long-chained lipids?

A) more permeable

53) In what way do the membranes of a eukaryotic cell vary?

B) Certain proteins are unique to each membrane.

48) Which of the following is a characteristic feature of a carrier protein in a plasma membrane?

B) It exhibits a specificity for a particular type of molecule.

12) If you mechanically shook a mixture of phospholipids and water, what would you expect to see when you observe the solution using an electron microscope?

B) Some lipids will have formed tiny vesicles filled with water.

58) Steroid hormones are large communication molecules that are modified cholesterol molecules. How do you think they enter a cell?

B) Their lipid nature probably allows them to diffuse through the plasma membrane.

7) Steroids are considered to be lipids because they _____.

B) are not soluble in water

1) A phospholipid is a _____.

B) nonpolar lipid molecule that is made amphipathic by the addition of a phosphate

2) What region of a steroid is hydrophilic?

B) the terminal hydroxyl group

37) Under what circumstances does membrane transport require energy?

B) whenever a solute is moved against its electrochemical gradient

14) Which of the following is the best explanation for why cholesterol decreases the permeability of biological membranes?

C) Because cholesterol is amphipathic, it fits in between the phospholipids and blocks diffusion through the membrane.

27) Which of the following is true of osmosis?

C) In osmosis, water moves across a membrane from areas of lower solute concentration to areas of higher solute concentration.

39) Gramicidin is an antibiotic that increases the permeability of bacterial cell walls to inorganic ions. What is the most likely mode of action of gramicidin?

C) It forms a channel in the membrane.

19) You have just discovered an organism that lives in extremely cold environments. Which of the following would you predict to be true about the phospholipids in its membranes, compared to phospholipids in the membranes of organisms that live in warmer environments?

C) The membrane phospholipids of cold-adapted organisms will have more unsaturated hydrocarbon tails.

35) A patient was involved a serious accident and lost a large quantity of blood. In an attempt to replenish body fluids, distilled water—equal to the volume of blood lost—is added to the blood directly via one of his veins. What will be the most probable result of this transfusion?

C) The patient's red blood cells will swell and possibly burst because the blood has become hypotonic compared to the cells.

44) For a protein to be an integral membrane protein, it would have to be _____.

C) amphipathic, with at least one hydrophobic region

4) Phospholipids and triglycerides both _____.

C) have a glycerol backbone

28) Celery stalks that are immersed in fresh water for several hours become stiff. Similar stalks left in a 0.15 M salt solution become limp. From this we can deduce that the fresh water_____.

C) is hypotonic and the salt solution is hypertonic to the cells of the celery stalks

55) Which of the following membrane activities requires energy from ATP?

C) movement of Na+ ions from a lower concentration in a mammalian cell to a higher concentration in the extracellular fluid

42) Which of the following types of molecules are the major structural components of the cell membrane?

C) phospholipids and proteins

22) Which of the following would likely move through the lipid bilayer of a plasma membrane most rapidly?

CO2

13) In an experiment involving planar bilayers, a solution of table salt (sodium and chloride ions in water) is added on the left side of the membrane while pure water is added on the right side. After 30 minutes the researchers test for the presence of ions on each side of the membrane. The right side tests negative for ions. What can you conclude?

D) Ions cannot cross planar bilayers.

47) Why are lipids and proteins free to move laterally in membranes?

D) There are only weak hydrophobic interactions in the interior of the membrane.

17) Which of the following crosses lipid bilayers the fastest?

D) a small, nonpolar molecule like oxygen (O2)

16) Which of the following increases the strength of the hydrophobic interactions in lipid bilayers and thus makes them less permeable to polar molecules?

D) increasing length of the hydrocarbon chains

45) When a membrane is freeze-fractured, the bilayer splits down the middle between the two layers of phospholipids. In an electron micrograph of a freeze-fractured membrane, the bumps seen on the fractured surface of the membrane are _____.

D) integral proteins

59) Which of the following processes includes all others?

D) passive transport

11) Lipids that form membranes have what kind of structure?

D) polar heads and nonpolar tails; the polar heads interact with water

38) Where would you most likely find an integral membrane protein?

D) spanning the cell membrane, with parts of the protein visible from both the inside and the outside of the cell

29) Ions diffuse across membranes through specific ion channels down _____.

D) their electrochemical gradients

57) Which of the following is most likely true of a protein that cotransports glucose and sodium ions into the intestinal cells of an animal?

E) A substance that blocks sodium ions from binding to the cotransport protein will also block the transport of glucose.

49) Which of the following allows water to move much faster across cell membranes?

E) aquaporins

51) Glucose diffuses slowly through artificial phospholipid bilayers. The cells lining the small intestine, however, rapidly move large quantities of glucose from the glucose-rich food into their glucose-poor cytoplasm. Using this information, which transport mechanism is most probably functioning in the intestinal cells?

E) facilitated diffusion

43) When biological membranes are frozen and then fractured, they tend to break along the middle of the bilayer. The best explanation for this is that _____.

E) the hydrophobic interactions that hold the membrane together are weakest at this point

24) Which of the following affects the osmolarity of a solution?

II) concentration of anions III) concentration of water

50) The phosphate transport system in bacteria imports phosphate into the cell even when the concentration of phosphate outside the cell is much lower than the cytoplasmic phosphate concentration. Phosphate import depends on a pH gradient across the membrane—more acidic outside the cell than inside the cell. Phosphate transport is an example of _____.

cotransport

6) Which of the following is a large organic molecule that is NOT assembled by polymerization of a few kinds of simple subunits?

steroid