Water Is the Ultimate e Donor for Photosynthetic NADP Reduction

In green plants, water serves as the ultimate electron donor for the photosyn-thetic generation of reducing equivalents. The reaction sequence

describes the process, where nhv symbolizes light energy (n is some number of photons of energy hv, where h is Planck's constant and v is the frequency of

Absence of CO, Os evolved C02 fixation into sugars

Absence of CO, Os evolved C02 fixation into sugars

FIGURE 22.4 • The light-dependent and light-independent reactions of photosynthesis. Light reactions are associated with the thylakoid membranes, and light-independent reactions are associated with the stroma.

the light). Light energy is necessary to make the unfavorable reduction of NADP+ by H2O (A<g°' = -1.136 V; AG°' = +219 kJ/mol NADP+) thermody-namically favorable. Thus, the light energy input, nhv, must exceed 219 kJ/mol NADP+. The stoichiometry of ATP formation depends on the pattern of pho-tophosphorylation operating in the cell at the time and on the ATP yield in terms of the chemiosmotic ratio, ATP/H+, as we will see later. Nevertheless, the stoichiometry of the metabolic pathway of CO2 fixation is certain:

A More Generalized Equation for Photosynthesis

In 1931, comparative study of photosynthesis in bacteria led van Niel to a more general formulation of the overall reaction:

Light

Hydrogen Hydrogen Reduced Oxidized acceptor donor acceptor donor

In photosynthetic bacteria, H2A is variously H2S (photosynthetic green and purple sulfur bacteria), isopropanol, or some similar oxidizable substrate. [(CH2O) symbolizes a carbohydrate unit.]

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