Gastric juice is made up of substances secreted from parietal cells, chief cells

Question

Gastric juice is made up of substances secreted from parietal cells, chief cells, and mucous-secreting cells. The cells secrete HCl, proteolytic enzyme zymogens, mucin, and intrinsic factor. The pH of gastric juice is acidic, between 1-3.

If the pH of gastric juice is 2.2, what is the amount of energy (delta G) required for the transport of hydrogen ions from a cell (internal pH of 7.4) into the stomach lumen? Assume that the potential difference across the membrane separating the cell and the interior of the stomach is -70.0 mV (inside of cells negative relative to the lumen of the stomach). Assume thatthe temperature is 37.0 degrees C.

The Faraday constant is 96.5 kJV^-1 mol^-1 and the gas constant is 8.314x10^-3 kJmol^-1 K^-1

Express your answer in KJ/mol.

Explanation / Answer

Given E0 = - 70.0 mV

[H+]cell = 10-7.4

[H+]stomach = 10-2.2

Applying Nernst equation

E = E0 - (RT/nF) x log[H+]stomach / [H+]cell

=> E = - 70.0 mV - (8.314 Jmol-1K-1 x 310K / 1x96500 JV-1) x log(10-2.2 / 10-7.4)

=> E = - 0.2089 V

Now Gibbs free energy can be calculated from the following formulae

G = - nFE

=> G = - 1 x (96.5 kJ/V) x ( - 0.2089 V ) = +20.16 kJ/mol (answer)

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