Question 1

Which of the following best describes secondary active transport?

Accepted Answer

It couples the movement of a substrate against its electrochemical gradient to the movement of another ion down its gradient.

Question 2

A cell is exporting Na$$^+$$ ions against both a concentration gradient and a transmembrane electrical potential of $$+65 mV ($$inside negative). If the free energy change for ATP hydrolysis is $$-30 kJ/$$mol, what is the maximum concentration gradient (Na$$^+$$ outside/inside) that can be established at $$37^	ext{o}C$$? Use $$R = 8.314 J/($$mol·K), $$T = 310 K$$, $$F = 96.5 kJ/($$mol·V), and $$z = +1$$ for Na$$^+.$$

Accepted Answer

The maximum gradient is achieved when $$\Delta G_t + \Delta G_{ATP} = 0$$, so $$\frac{[Na^+]_{out}}{[Na^+]_{in}} = \exp\left(\frac{-\Delta G_{ATP} - zF\Delta \psi}{RT}ight). $$Plugging in the values: $$\exp\left(\frac{30,000 - (1)(96,500)(0.065)}{8.314 	imes 310}ight) \approx 1.6$$

Question 3

Which of the following is NOT a characteristic of P-type ATPases?

Accepted Answer

They are always antiporters.

Question 4

The free energy change for transporting an ion across a membrane is given by $$\Delta G_t = RT \ln \left( \frac{C_2$}{C_1$} \right) + zF \Delta \psi. $$Which term accounts for the electrical gradient?

Accepted Answer

$$zF \Delta \psi$$

Question 5

A patient is given digitalis, a cardiotonic steroid. What is the direct effect of this drug on the Na$$^+/K$^+$$ ATPase?

Accepted Answer

It inhibits the dephosphorylation step, preventing the release of K$$^+$$ into the cell.

Question 6

Which of the following transporters is a symporter involved in secondary active transport in the intestine?

Accepted Answer

Na$$^+-$$glucose symporter

Question 7

In the lactose permease system of E. coli, which process provides the energy for lactose import against its concentration gradient?

Accepted Answer

Movement of H$$^+$$ down its electrochemical gradient

Question 8

Which of the following statements about the Na$$^+/K$^+$$ ATPase is TRUE?

Accepted Answer

It creates a net negative membrane potential by exporting more positive charge than it imports.

Question 9

A molecule is transported from a region of lower concentration ($$C_1) to $$higher concentration ($$C_2) $$with $$z=0$$ and $$\Delta \psi = 0. $$What is the sign of $$\Delta G_t$$ for this process?

Accepted Answer

Positive, because the process is endergonic.

Question 10

Which of the following best explains why transporters (active or passive) are generally slower than ion channels?

Accepted Answer

Transporters require conformational changes for each transport cycle.

Question 11

In the equation $$\Delta G_t = RT \ln \left( \frac{C_2$}{C_1$} \right) + zF \Delta \psi$$, what does $$\Delta \psi$$ represent?

Accepted Answer

The electrical potential difference across the membrane.

Question 12

Which of the following is an example of primary active transport?

Accepted Answer

Na$$^+/K$^+$$ ATPase

Question 13

A cell uses a secondary active transporter to import glucose. What is the immediate energy source for this process?

Accepted Answer

The Na$$^+$$ gradient established by the Na$$^+/K$^+$$ ATPase

Question 14

Which of the following would decrease the energy cost of transporting an ion against its concentration gradient?

Accepted Answer

Increasing the transmembrane electrical potential in the direction of transport for a positively charged ion.

Question 15

Which of the following is TRUE about the SERCA Ca$$^{2+}$$ pump?

Accepted Answer

It pumps Ca$$^{2+}$$ into the sarcoplasmic reticulum using ATP hydrolysis.

Question 16

In the context of membrane transport, what is the main difference between a symporter and an antiporter?

Accepted Answer

A symporter moves two substrates in the same direction, while an antiporter moves them in opposite directions.

Question 17

Which of the following scenarios would make the transport of a positively charged ion ($$z=+1) $$from inside ($$V_1=0$ mV) to outside (V_2$=-75 mV) $$more favorable?

Accepted Answer

Making the outside potential more negative.

Question 18

Which of the following is a direct consequence of inhibiting the dephosphorylation step in the Na$$^+/K$^+$$ ATPase?

Accepted Answer

The transporter cannot release K$$^+$$ into the cell.

Question 19

Which of the following is NOT a requirement for active transport?

Accepted Answer

Transport is always mediated by a symporter.

Membrane Transport II: Active Transport and Transport Energetics

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