Question

Assume a cell with the following conditions.

K⁺ Equilibrium Potential = -81 mV

Na⁺ Equilibrium Potential = +58 mV

The plasma membrane of the cell is permeable only to K⁺
The membrane potential is constant over time

Which of the following statements is/are correct?

	a.	The cell must expend energy to maintain a K+ concentration gradient.
	b.	The membrane potential is -70mV.
	c.	The rate of K+ exiting the cell is equal to the rate of K+ entering the cell.
	d.	both a) and b) are correct, and c) is incorrect
	e.	a), b) and c) are all correct

Which of the following statements is/are correct for an endotherm when ambient temperature decreases below the thermoneutral zone?

	a.	The driving force for heat loss to the environment increases
	b.	Unit metabolic rate increases
	c.	Body temperature decreases in proportional with ambient temperature
	d.	both a) and b) are correct, and c) is incorrect
	e.	a), b) and c) are all correct

Answer 1

e. a), b) and c) are all correct. At the steady state membrane potential, the total flux of charge across the membrane taking into account all the potential ions, must be zero. If the total charge flux were not zero, the membrane potential would be changing with time, not steady. If only Na+  and k+ contribute, there will be a struggle between Na+ equilibrium potential (+58mV) and k+ on the othar hand, pushing the membrane potential toward the k+ equilibrium potential (-80mV). The balance will be struck when Na+ influx exactly balances k+ efflux. K+ permeability of the membrane is usually much larger than Na+ permeable, so that a relatively large flux of k+ results from a relatively small disparity between the actual value of the membrane potential and the k+ equilibrium potential.  

a. The driving force for heat loss to the environment increase.

At most ambient temperatures, the driving force for heat exchange between an organism and the environment is greater for endotherms than for ectotherms. For ambient temperature within the thermoneutral zone of an endotherm, a decrease in environment temperature leads to increased vasoconstriction without a change in metabolic rate.