The Z-scheme of oxygenic photosynthesis is usually drawn as a ladder of redox…

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Molecular machines, transport and bioenergetics · Molecular machines, transport and bioenergetics

Forces, Machines and Signals, Ch Proton gradients, ATP synthesis and photosynthesis

Problem

The Z-scheme of oxygenic photosynthesis is usually drawn as a ladder of redox potentials: water is oxidised at

E^{\circ\prime}(\mathrm{O_2/H_2O}) = +0.82\,\mathrm{V}

; absorption of a photon at photosystem II (

680\,\mathrm{nm}

) lifts an electron high enough to pass down an electron-transport chain to photosystem I; absorption of a second photon there, at

700\,\mathrm{nm}

, lifts the electron again and delivers it finally to

\mathrm{NADP^+/NADPH}

E^{\circ\prime} = -0.32\,\mathrm{V}

. Measured quantum requirements show that, in practice, about

10

photons in total (roughly

5

absorbed at each photosystem) are used for every

\mathrm{O_2}

released (four electrons transferred), rather than the theoretical minimum of

8

. ((a)) Translate the description above into numbers: state, in

\mathrm{eV}

, the energy of one

680\,\mathrm{nm}

photon, one

700\,\mathrm{nm}

photon, and the free-energy span of the water-to-

\mathrm{NADP^+}

step that the two photons together must drive. ((b)) Using the measured quantum requirement of

10

photons per

\mathrm{O_2}

(not the theoretical minimum), calculate the percentage of the total absorbed photon energy that ends up stored across that span, per

\mathrm{O_2}

released.

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