Question

The number of ATP molecules formed from 8 molecules of H20 due to noncyclic electron transport and subsequent photophosphorylation is ?

Assume that quinone cycle facilitates the transfer 4H+ to the lumen of thylakoid membrane for every two electrons passing through electron transport system and one ATP is formed for every 3H+ moving down the proton gradient by the mediation of Fo - F, ATPase.

Answer 1

Answer on Question #57384 - Biology - Biochemistry

Question:

The number of ATP molecules formed from 8 molecules of $\mathsf{H}_2\mathsf{O}$ due to noncyclic electron transport and subsequent photophosphorylation is?

Assume that quinone cycle facilitates the transfer $4\mathrm{H}+$ to the lumen of thylakoid membrane for every two electrons passing through electron transport system and one ATP is formed for every $3\mathrm{H}+$ moving down the proton gradient by the mediation of Fo - F, ATPase.

Solution:

To calculate the ATP watch a detailed diagram of the Z-scheme of photosynthesis:

FIGURE 22-16. A detailed diagram of the Z-scheme of photosynthesis. Electrons ejected from P680 by the absorption of photons are replaced with electrons abstracted from $\mathrm{H}_2\mathrm{O}$ by an Mn complex (OEC), thereby forming $\mathrm{O}_2$ and four $\mathrm{H}^+$ . Each ejected electron is passed through a chain of electron carriers to a pool of plastoquinone molecules (Q). The resulting plastoquinol, in turn, reduces the cytochrome $b_{\mathrm{a}} /$ particle (yellow box) that transfers electrons with the concomitant

translocation of protons into the thylakoid space. Cytochrome $b_{\mathrm{a}}f$ then transfers the electrons, via a poorly characterized pathway, to plastocyanin (PC). The plastocyanin regenerates photooxidized P700. The electron ejected from P700, through the intermediacy of a chain of electron carriers, reduces $\mathrm{NADP^{+}}$ to NADPH in noncyclic electron transport. Alternatively, the electron may be returned to the cytochrome $b_{\mathrm{a}}f$ complex in a cyclic process that translocates protons into the thylakoid space.

ATP productions in noncyclic pathways:

- $12\mathrm{H}+$ (4 from $2\mathrm{H}_{2}\mathrm{O}$ and 8 from $4\mathrm{PQH}_{2}$ in Cyt b6-f) are transported by one $\mathrm{O}_{2}$ production, $12\mathrm{H}+$ /O2.

- $3\mathrm{H}+$ produce one ATP ( $\sim$ 1 ATP/ $3\mathrm{H}+$ ), thus, 4ATP/O2.

- Each NADPH produced has the free energy to produce 3ATP (we have 2NADPH).

- Thus if the NADPH oxidation is taken into account, 10ATP can be produced (4ATP by proton gradient and 6ATP by 2NADPH oxidation).

So this is calculation for 2H₂O: 4ATP (or 10 ATP if the NADPH oxidation is taken into account)

Now we can calculate ATP productions for 8H₂O: 16ATP (or 40 ATP if the NADPH oxidation is taken into account)

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