In one day the average person secretes about 2.5 litres of gastric juice into the stomach. The pH of the cells in the stomach lining that perform the secretion is about 7 (the same as for all cells) and the pH of the stomach is about 1. Thus, H+ ions must be moved against an H+ gradient. The free energy change for moving molecules against a gradient is:
^G = R T lne(C2/C1) (where ^ = delta)
where C2 is the greater concentration. If the process of pumping H+ ions is 20% efficient, how many moles of ATP are used each day to secrete gastric juice? How many kilograms could be lifted one meter (20% efficiency) wiht this energy?
The H+ concentrations have to be calculated from the pH:
[H+] = 10 ^ -pH (10 to the power of minus pH)
e.g If the pH is 6, [H+] = 10^-6
Now calculating C2/C1 and finding its natural logarithm (ln) should be easy. I get an answer of about 14.
R is the ideal gas constant, which is 8.31 ...
This solution is provided in 250 words. It uses pH, ideal gas constant, and absolute temperature to find deltaG, and calculates moles.