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Inquiry Questions
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Self Test

1). Why is the phospholipid molecule so appropriate as the primary structural component of plasma membranes?
    a). Phospholipids are completely insoluble in water.
    b). Phospholipids form strong chemical bonds between the molecules, forming a stable structure.
    c). Phospholipids form a selectively permeable structure.
    d). Phospholipids form chemical bonds with membrane proteins that keep the proteins within the membrane.
Answer: c

2). Which increases the fluidity of the plasma membrane?
    a). having a large number of membrane proteins
    b). the tight alignment of phospholipids
    c). cholesterol present in the membrane
    d). double bonds between carbon atoms in the fatty acid tails.
Answer: d

3). Which best describes the structure of a plasma membrane?
    a). proteins embedded within two layers of phospholipids
    b). phospholipids sandwiched between two layers of proteins
    c). proteins sandwiched between two layers of phospholipids
    d). a layer of proteins on top of a layer of phospholipids
Answer: a

4). What locks all transmembrane proteins in the bilayer?
    a). chemical bonds that form between the phospholipids and the proteins
    b). hydrophobic interactions between nonpolar amino acids of the proteins and the aqueous environments of the cell
    c). attachment to the cytoskeleton
    d). the addition of sugar molecules to the protein surface facing the external environment
Answer: b

5). The movement of sodium ions from an area of higher concentration to an area of lower concentration is called _____.
    a). active transport
    b). osmosis
    c). diffusion
    d). phagocytosis
Answer: c

6). A cell placed in distilled water will
    a). shrivel up.
    b). swell.
    c). lose water.
    d). result in no net diffusion of water molecules.
Answer: b

7). Sucrose cannot pass through the membrane of a red blood cell (RBC) but water and glucose can. Which solution would cause the RBC to shrink the most?
    a). a hyperosmotic sucrose solution
    b). a hyperosmotic glucose solution
    c). a hypoosmotic sucrose solution
    d). a hypoosmotic glucose solution
Answer: a

8). Which of the following processes requires membrane proteins?
    a). exocytosis
    b). phagocytosis
    c). receptor-mediated endocytosis
    d). pinocytosis
Answer: c

9). Exocytosis involves
    a). the ingestion of large organic molecules or organisms.
    b). the use of ATP.
    c). the uptake of fluids from the environment.
    d). the discharge of materials from cellular vesicles.
Answer: d

10). Molecules that are transported into the cell up their concentration gradients do so by
    a). facilitated diffusion.
    b). osmosis.
    c). coupled transport
    d). none of the above
Answer: c
Test Your Visual Understanding
1). In this figure of a transmembrane protein, what colored area of the protein contains nonpolar amino acids? What colored area contains polar amino acids? What colored area contains amino acids carrying a positive or negative charge?
Answer: The nonpolar amino acids are found within the interior nonpolar region of the lipid bilayer, so the pink colored areas contain nonpolar amino acids. The polar amino acids are found associated with the external environment, so the purple colored areas contain polar amino acids. Charged amino acids are polar and so these amino acids would be found in the purple colored areas of the protein.

2). Match the function of the membrane protein with the appropriate numbered figure:
    a). cell surface receptor
    b). transporter
    c). cell surface identity marker
    d). enzyme

Apply Your Knowledge

1). If during the action of the sodium-potassium pump, 150 molecules of ATP are used, how many sodium ions are transported across the membrane?
Answer: 450 sodium ions.
Solution: Every time the sodium-potassium pump undergoes its conformational change, 3 sodium ions are transported across the membrane and 1 ATP molecule is required. If 150 ATP molecules have been used by the sodium-potassium pump, then the pump has undergone its conformational change 150 times, and has transported 150 x 3 Na+ ions/time, or 450 total sodium ions.

2). If a cell's cytoplasm were hyperosmotic to the extracellular fluid, how would the concentration of solutes in the cytoplasm compare with that in the extracellular fluid? Assuming the membrane was permeably only to water, in which direction would water move?
Answer: The contents of the cell would contain more solutes than the extracellular fluid, which would be hypoosmotic compared to the cell. Assuming that the membrane was permeably only to water, water would move into the cell, toward the area with more solutes.

3). Cholera, a disease caused by a bacterial infection of Vibrio cholerae, results in severe diarrhea leading to dehydration. A toxin released by the bacterium causes the release of chloride ions (Cl-) from cells lining the small intestines and inhibits the uptake of sodium ions (Na+) by these cells. Explain how this disruption of cellular ion concentrations would result in extreme dehydration.
Answer: Under normal conditions, the chloride ions remain in the cells lining the lumen of the small intestine. This creates an overall negative charge within the cells that drives positively charged sodium ions into the cells creating a hyperosmotic condition in the cells. Water moves into the cells by osmosis. The diarrhea that occurs in cholera results from the bacterial infection, which causes the increased concentrations of Cl- and Na+ in the lumen of the small intestine. Elevated concentrations of these two ions results in the lumen of the small intestines becoming hyperosmotic compared to the cells lining the intestines. This in turn causes the diffusion of large amounts of water through osmosis into the lumen of the intestine from the cells and blood stream. The water passes out of the body as diarrhea, leaving the body dehydrated.

4). Cystic fibrosis is a genetic disease that results in thick mucus secretions that clog up air passages in the lungs. Faulty chloride ion channels keep Cl- and Na+ in the cells that line the airways increasing the intracellular ion concentrations. How does this cause the mucus in the airways to become thick?
Answer: This is a similar situation of that seen in cholera but in the opposite direction. Normally, chloride ions (Cl-) pass out of the cells lining the airways through chloride ion channels. Sodium ions (Na+) follow the Cl- out of the cells due to the electrochemical gradient created by the movement of Cl-. The movement of ions out of the cells makes the cells hypoosmotic compared to the lumen of the airways. Water passes out of the cells and blood stream through osmosis and into the airways, thinning the mucus that flushes out impurities. In cystic fibrosis, the chloride channel doesn't function properly so Cl- remains in the cells, likewise Na+ remains in the cells. The cells are hyperosmotic compared to the lumen of the airways so water remains in the cells and bloodstream. Without water passing into the airways, the mucus becomes thick and clogs the airways.

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