Ch. 3 - Plasma Membrane and membrane potential

Question 1

1. Draw how phospholipid molecules align themselves into a lipid bilayer when in water.

Answer 1

1. See Figure 3-2, p. 57.

Question 2

2. Explain what accounts for the appearance of the plasma membrane under an electron microscope.

Answer 2

2. The two dark layers are the hydrophilic polar regions of the lipid and protein molecules that take up a stain, whereas the light middle layer is the poorly stained hydrophobic core made up of the nonpolar regions of these molecules.

Question 3

3. List the specialized functions of the different types of membrane proteins.

Answer 3

3. (1) form channels, (2) serve as carriers, (3) serve as docking-marker acceptors, (4) function as membrane-bound enzymes, (5) serve as receptors, (6) serve as cell adhesion molecules (CAMs), and (7) are important in “self ” recognition

Question 4

FIGURE FOCUS: Different types of specialized membrane proteins such as channels, carriers, or enzymes are localized at the luminal membrane or at the basolateral membrane. What keeps these proteins from migrating to the wrong part of the membrane?

Answer 4

Figure 3-5 (p. 62): Tight junctions prevent specialized membrane proteins from migrating between the luminal and basolateral parts of the plasma membrane of epithelial cells.

Question 5

1. Describe the extracellular matrix.

Answer 5

1. the biological “glue” that holds neighboring cells together; consists of an intricate meshwork of proteins in a watery, gel-like substance (interstitial fluid)

Question 6

2. List the three types of specialized cell junctions and indicate their primary role.

Answer 6

(1) desmosome (adhering junction that spot-rivets two adjacent but nontouching cells, anchoring them together in tissues subject to considerable stretching);
(2) tight junction (impermeable junction that joins the lateral edges of epithelial cells near their luminal borders, thus preventing movement of materials between the cells); and
(3) gap junction (communicating junction made up of small connecting tunnels that permit movement of charge-carrying ions and small molecules between two adjacent cells)

Question 7

3. Draw a desmosome.

Answer 7

3. See Figure 3-4, p. 61. 3.3 (Questions on p. 63.)

Question 8

1. Explain how both highly lipid-soluble substances of any size and small water-soluble substances are able to permeate the plasma membrane without assistance.

Answer 8

1. Lipid-soluble substances of any size can permeate the plasma membrane without assistance by dissolving in the lipid bilayer. Small water-soluble substances (ions) can pass through the membrane without assistance through open channels specific for them.

Question 9

2. Distinguish between passive and active forces that produce movement of substances across the plasma membrane.

Answer 9

2. Passive forces do not require energy and active forces require energy to produce movement across the plasma membrane.

Question 10

1. List the means of unassisted membrane transport.

Answer 10

1. movement down a concentration gradient (including osmosis) and movement along an electrical gradient

Question 11

2. Compare osmotic pressure and hydrostatic pressure.

Answer 11

2. Osmotic pressure is a “pulling” pressure; it is a measure of the tendency for osmotic flow of water into a solution resulting from its relative concentration of nonpenetrating solutes and water. Hydrostatic (fluid) pressure is a “pushing” pressure; it is the pressure exerted by a stationary fluid on an object.

Question 12

3. Draw the relative volume of a cell surrounded by (a) an isotonic, (b) a hypotonic, and (c) a hypertonic solution.

Answer 12

3. See Figure 3-13, p. 69.

Question 13

1. Draw a graph comparing simple diffusion down a concentration gradient and carrier-mediated transport.

Answer 13

1. See Figure 3-15, p. 71.

Question 14

FIGURE FOCUS: If ATP production was to sharply fall, what would happen to the Na+ and K+ concentrations in the ECF and the ICF?

Answer 14

Figure 3-16 (p. 74): If insufficient ATP were available to run the Na+–K+ pump, the Na+ concentration would fall in the ECF and rise in the ICF, whereas the K+ concentration would rise in the ECF and fall in the ICF. Passive movement of these ions down their concentration gradients across the plasma membrane through leak channels would not be adequately counterbalanced by active pumping of these ions across the membrane against their concentration gradients.

Question 15

FIGURE FOCUS: By what chain of events does SGLT lead to water absorption from the digestive tract lumen into the blood to promote rehydration when a child with dehydrating diarrhea sips a salt and glucose solution such as Pedialyte or a homemade version?

Answer 15

Figure 3-18 (p. 76): When both Na+ and glucose are present in the digestive tract lumen, they are cotransported via SGLT across the digestive tract wall into the blood. Water follows osmotically, helping rehydrate a child who has dehydrating diarrhea.

Question 16

1. Define membrane potential.

Answer 16

1. a separation of opposite charges across the membrane, or a difference in the relative number of cations and anions in the ECF and ICF

Question 17

2. Describe what causes the carrier to change shape to expose binding sites for passengers to opposite sides of the membrane in facilitated diffusion, primary active transport, and secondary active transport.

Answer 17

2. In facilitated diffusion, the carrier undergoes spontaneous changes in shape as a result of thermal energy. In primary active transport, phosphorylation (binding of the phosphate group derived from the carrier splitting ATP) increases affinity of the carrier for its passenger ion; this binding causes the carrier to change its shape. In secondary active transport, the change in shape of a cotransport carrier that binds both Na+ and the transported solute is driven by a Na+ concentration gradient established by a primary active transport mechanism.

Question 18

3. Distinguish between steady state and dynamic equilibrium.

Answer 18

3. In a steady state, opposing passive and active forces exactly counterbalance each other. In a dynamic equilibrium, opposing passive forces exactly counterbalance each other. In both cases, no net change takes place, but energy is used to maintain this constancy in a steady state, but no energy is needed in a dynamic equilibrium.

Question 19

FIGURE FOCUS: If the ECF concentration of K+ decreases, does E K+ become more negative, less negative, or stay the same?

Answer 19

Figure 3-20 (p. 81): more negative. Because the ECF K+ concentration is lower but its ICF concentration is the same, the concentration gradient for K+ to exit the cell is greater than normal. Therefore the opposing electrical gradient at E K+ must be greater than normal to exactly counterbalance the larger concentration gradient. That is, E K+ must be more negative. The same conclusion can be reached by using the Nernst equation. Plugging a value less than the normal 5mM extracellular concentration of K+ into the equation yields a value more negative than 290mV.

Question 20

2. Describe the relative contributions of K+ and Na+ to the resting membrane potential.

Answer 20

2. Because the resting membrane is 25 to 30 times more permeable to K+ than to Na+, K+ passes through more readily than Na+. The substantially larger movement of K+ out of the cell influences the resting membrane potential to a much greater extent than the smaller movement of Na+ into the cell does. As a result, the resting potential (270 mV) is closer to the equilibrium potential for K+ (290 mV) than to the equilibrium potential for Na+ (160 mV). The resting potential is less than the K+ equilibrium potential because the limited entry of Na+ neutralizes some of the potential that would be created by K+ alone.