La Primera Guerra Anglo-Afgana

12-5 Your friend isolates a new species of yeast that can grow using either ethanol

(CH3CH2OH) or acetate (CH3COO^–) as the yeast’s sole carbon source. He measures the rate of uptake of these carbon sources by the yeast as a function of concentration of carbon source and obtains the following graphs:

Figure Q12-5

A. Is molecule A or B more likely to utilize a carrier protein to mediate its transport?

Why?

B. Given what you know about membrane transport, is molecule A more likely to be ethanol or acetate? Why?

12-6 Name the three main types of active transport.

12-7 Which of the following descriptions are TRUE of the bacterial protein

12-8 The Aeroschmidt weed contains an ATP-driven ion pump in its vacuolar membrane that pumps potentially toxic heavy metal ions such as Zn²⁺ and Pb²⁺ into the vacuole. The pump protein exists in a phosphorylated and an unphosphorylated form and works in a similar way to the Na⁺-K⁺ pump of animal cells. To study its action, you incorporate the unphosphorylated form of the protein into phospholipid vesicles containing K⁺ in their interiors. (You ensure that all of the protein molecules are in the same orientation in the lipid bilayer.) When you add Zn²⁺ and ATP to the solution outside such vesicles, you find that Zn²⁺ is pumped into the vesicle lumen. You then expose vesicles containing the pump protein to the solutes as shown in Table 12-8A.

Table 12-8A

A B C D E F

Outside Zn²⁺ + ATP Zn²⁺ Zn²⁺ + ATP Zn²⁺ ATP ATP

Inside K⁺ K⁺ K⁺

You then determine the amount of phosphorylated and unphosphorylated ATP-driven ion pump protein in each sample. Your results are summarized in Table 12-8B, where “–”

indicates an absence of a type of protein and “+” indicates the presence of a type of protein.

What would you expect to happen if you treat vesicles as in lane F, but before

determining the phosphorylation state of the protein, you wash away the outside buffer and replace it with a buffer containing only Zn²⁺?

(a) Nothing will happen. (No Zn²⁺ will move into the vesicle; no K⁺ will move out of the vesicle; the phosphorylation state of the protein will not change.)

(b) No Zn²⁺ will move into the vesicle; no K⁺ will move out of the vesicle; the protein will become unphosphorylated.

(c) A small amount of Zn²⁺ will move into the vesicle; no K⁺ will move out of the vesicle; the phosphorylation state of the protein will not change.

(d) A small amount of Zn²⁺ will move into the vesicle; no K⁺ will move out of the vesicle; the protein will become unphosphorylated.

(e) A small amount of Zn²⁺ will move into the vesicle; a small amount of K⁺ will move out of the vesicle; the phosphorylation state of the protein will not change.

12-9 Explain why Na⁺ is commonly used to drive the coupled inward transport of nutrients in animal cells.

12-10 Ouabain inhibits the active uptake of glucose into epithelial cells by (a) binding to the glucose-Na⁺ symport.

(b) opening K⁺ channels.

(c) changing the pH of the cell.

(d) increasing the intracellular concentration of Na⁺. (e) depleting the cell of ATP.

12-11 Which of the following statements is TRUE?

(a) Amoebae have carrier proteins that actively pump water molecules from the cytoplasm to the cell exterior.

(b) Bacteria and animal cells rely on the Na⁺–K⁺ pump in the plasma membrane to prevent lysis due to osmotic imbalances.

(c) The Na⁺–K⁺ pump allows animal cells to thrive under conditions of very low ionic strength.

(d) The cell wall surrounding plant cells prevents osmosis.

(e) The Na⁺–K⁺ pump helps to keep both Na⁺ and Cl^– ions out of the cell.

12-12 The Ca²⁺ pumps in the plasma membrane and endoplasmic reticulum are examples of (a) ATP-driven pumps.

(b) coupled transporters.

(c) passive carrier proteins.

(d) ion channels.

(e) symports.

12-13 Ca²⁺ pumps in the plasma membrane and endoplasmic reticulum are important for (a) maintaining osmotic balance.

(b) preventing Ca²⁺ from altering the behavior of molecules in the cytosol.

(c) providing enzymes in the endoplasmic reticulum with Ca²⁺ ions that are necessary for their catalytic activity.

(d) maintaining a negative membrane potential.

(e) helping cells import K⁺.

12-14 Do you agree of disagree with the following statement? Explain your answer.

A symporter would function as an antiporter if its orientation in the membrane were reversed (that is, if the portion of the protein normally exposed to the cytosol faced the outside of the cell instead).

12-15 You have prepared lipid vesicles (spherical lipid bilayers) that contain Na⁺–K⁺ pumps as the sole membrane protein. All of the Na⁺–K⁺ pumps are oriented in such a way that the portion of the molecule that normally faces the cytosol is in the inside of the vesicle and the portion of the molecule that normally faces the extracellular space is on the outside of the vesicle. Assume that each pump transports one Na⁺ ion in one direction and one K⁺ ion in the other direction during each pumping cycle (see Figure Q12-15 for how the Na⁺–K⁺ pump normally functions in the plasma membrane).

Figure 12-15

Predict what would happen in each of the following conditions:

A. The solutions inside and outside the vesicles contain both Na⁺ and K⁺ ions but no ATP.

B. The solution outside the vesicles contains both Na⁺ and K⁺ ions; the solution inside contains both Na⁺ and K⁺ ions and ATP.

C. The solution outside contains Na⁺; the solution inside contains Na⁺ and ATP.

12-16 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once.

For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its __________________ gradient. On the other hand, for a charged molecule, an additional force called the __________________ must also be considered. The net driving force for a charged molecule across a membrane therefore has two components and is referred to as the __________________ gradient. Active transport allows the movement of solutes against this gradient. The carrier proteins called __________________ transporters utilize the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second gradient. When this transporter moves both ions in the same direction across the membrane, it is considered a(n)

__________________; if the ions move in opposite directions, the transporter is considered a(n) __________________.

antiport coupled membrane potential

ATP hydrolysis electrochemical symport

concentration light-driven uniport