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Question: A selenium-activated protein **P** catalyses a metabolic reaction that results in release of two H$^...

A selenium-activated protein P catalyses a metabolic reaction that results in release of two H+^+ ions. Expression of P is induced by lactose. In an experiment, cells were grown for 1 hour in selenium and lactose containing media resulting in a net concentration of 10610^6 M H+^+ per cell. Selenium and lactose were removed from the media and cells were grown for another 10 generations. Assuming that all other proteins are expressed at a constant level, the net change in pH inside the cell at the end will be ____.

A

1

B

3

C

11

D

6

Answer

6

Explanation

Solution

The initial statement "net concentration of 10610^6 M H+^+ per cell" is interpreted as the H+^+ concentration increasing by a factor of 10610^6 due to protein P's activity. Assuming a normal intracellular pH of 7 ([H+]=10−7[H^+] = 10^{-7} M), the H+^+ concentration after 1 hour becomes 10−7×106=10−110^{-7} \times 10^6 = 10^{-1} M, resulting in a pH of 1. When selenium and lactose are removed, protein P becomes inactive. The cell's homeostatic mechanisms then restore the pH to its normal physiological level, typically pH 7. The net change in pH is the final pH minus the pH after 1 hour: 7 - 1 = 6.