Question

The standard free energy that is required for the sodium-potassium ATPase to pump two K+ ions into the cell and three Na+ ions out is +43.8 kJ/mol but the standard free energy change of hydrolysis of ATP is only -32 kJ/mol. This apparent imbalance of free energy can be accounted for because:

A) the movement of Na+ ions is down the concentration gradient

. B) the movement of K+ ions is down the concentration gradient.

C) the free energy provided from the hydrolysis of one ATP is sufficient under physiological conditions.

D) more than one ATP is hydrolyzed.

E) the membrane potential favours the flow of ions.

Answer 1

Na-K ATPase pump works by transporting K+ ions and Na+ ions against their concentration gradient. To provide energy for this transport, a molecule of ATP is hydrolyzed for the movement of 2 K+ ions out of the cell and 3 Na+ ions into the cell.

This function of the pump is essential to restore the Na+ and K+ concentrations outside and inside the cell to allow the action potential and hence the neuronal function to occur smoothly.

The energy required for this process is +43.8kJ/mol. Hence 43.8kJ/mol are expended in this transport. However, the standard free energy change for ATP hydrolysis is only -32kJ/mol.

This discrepancy is accounted for by the physiological conditions present inside the cell. Under physiological conditions, the ATP hydrolysis provides energy in the range of 50-70kJ/mol. This solves the problem.

Hence, the correct answer is C.