# Biophysics - Calculating deltaG

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?

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#### Solution Preview

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 ...

#### Solution Summary

This solution is provided in 250 words. It uses pH, ideal gas constant, and absolute temperature to find deltaG, and calculates moles.