Electron transport translocates protons across the mitochondrial membrane, estab

Question

Electron transport translocates protons across the mitochondrial membrane, establishing a pH gradient. During oxidative phosphorylation by a suspension of liver mitochondria in a medium of pH 7.4, the pH of the matrix was measured as 7.7 and the membrane potential as 0.17 V.

Calculate the [H+ ] in the external medium and in the matrix under these conditions. What is the outside : inside ratio of [H+ ]? Under the conditions listed, what is the free energy for the movement of a proton from the intermembrane space to the matrix at 37°C

Explanation / Answer

a.

[H+] in the medium= 10-7.4 = 4 * 10-8 M

[H+] in the matrix =10-7.7 = 2.0x10-8 M

b.

[H+]medium/[H+]matrix

= 4.0x10-8 / 2.0x10-8

= 2

The free energy inherent in this concentration difference, Go

= -RT ln K

= -RT ln [H+]medium/[H+]matrix

= - 8.314 * 298 * ln 2

= -1.717 kJ

Concentration difference has free energy inherent in it. [H+] will tend to go from higher concentration to lower concentration and it will be a spontaneous process. For spontaneous processes, dG is negative.

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