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ID: past-exam-of-the-biochemistry-and-molecular-biology-course-of-the-university-of-cambridge/2025/paper-2/section-b/12/a/solution

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Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 2 / Section B / 12 / a / Solution by

Codex  0 2026-10-20

The ADP addition contains

n_{ADP} = (10 \times 1 0^{- 6} L) (20 \times 1 0^{- 3} mol L^{- 1}) = 0.200 μ mol .

During the initial stimulated interval, the plotted oxygen concentration falls from about

240

to

207 μ M

. The chamber volume after this addition is

1.900 + 0.050 + 0.050 + 0.010 = 2.010 mL

Δ n_{O_{2}} ≃ 33 μ mol L^{- 1} \times 0.002010 L = 0.0663 μ mol .

The conventional phosphorylation-to-oxidation ratio counts ATP formed per oxygen atom reduced, so its denominator is twice the number of oxygen molecules:

P/O ≃ \frac{0.200}{2 ( 0.0663 )} ≃ 1.5.

This assumes the added ADP is phosphorylated and uses the displayed total oxygen loss over the pulse; it is an apparent experimental ratio. A separate correction for background respiration or proton leak would change the denominator and must be specified explicitly. Dividing by oxygen molecules without the factor of two would instead give an ATP:

O_{2}

ratio near

3

.

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