Abstract
The energetics of photosynthesis in plants have been re-analyzed in a framework that represents the relatively high energy of O2 correctly. Starting with the photon energy exciting P680 and “loosening an electron”, the energy transfer and electron transport are represented in a comprehensive, self-explanatory sequence of redox energy transfer and release diagrams. The resulting expanded Z-scheme explicitly shows charge separation as well as important high-energy species such as O2, TyrZ˙, and P680+˙, whose energies are not apparent in the classical Z-scheme of photosynthesis. Crucially, the energetics of the three important forms of P680 and of P700 are clarified. The relative free energies of oxidized and reduced species are shown explicitly in kJ/mol, not encrypted in volts. Of the chemical energy produced in photosynthesis, more is stored in O2 than in glucose. The expanded Z-scheme introduced here provides explanatory power lacking in the classical scheme. It shows that P680* is energetically boosted to P680+˙ by the favorable electron affinity of pheophytin and that Photosystem I (PSI) has insufficient energy to split H2O and produce O2 because P700* is too easily ionized. It also avoids the Z-scheme’s bewildering implication, according to the “electron waterfall” concept, that H2O gives off electrons that spontaneously flow to chlorophyll while releasing energy. The new analysis explains convincingly why plants need two different photosystems in tandem: (i) PSII mostly extracts hydrogen from H2O, producing PQH2 (plastoquinol), and generates the energetically expensive product O2; this step provides little energy directly to the plant; (ii) PSI produces chemical energy for the organism, by pumping protons against a concentration gradient and producing less reluctant hydrogen donors. It also documents that electron transport and energy transfer occur in opposite directions and do not involve redox voltages. The analysis makes it clear that the high-energy species in photosynthesis are unstable, electron-deficient species such as P680+˙ and TyrZ˙, not putative high-energy electrons.
Highlights
Photosynthesis, the light-enabled synthesis of biomolecules from much simpler precursors, is a fascinating and obviously important process
Our analysis reveals that photosynthesis in plants requires two distinct outcomes, each associated with a different photosystem: (i) Photosystem II (PSII) makes hydrogen from H2 O available via plastoquinol for eventual bonding to CO2 and production of carbohydrates, (CH2 O)n, and other biomolecules, while more of the photon energy ends up in O2 than in plastoquinol; and (ii) Photosystem I (PSI) converts solar energy into chemical energy in the organism, for instance stored first in H+ gradients and in ATP; this occurs most prominently in “cyclic electron transport”, whose net effect is summarized by Equation (3) [2]
P680+ ̇ explicitly; it shows O2 correctly at high energy; it explicitly shows both reduced and oxidized species; it demonstrates that PSI has insufficient energy to split water and produce O2 because P700* is easier to ionize than P680* [42], which clarifies why P700 is shifted up relative to P680 in the traditional Z-scheme and why two photosystems are needed; it shows the energy flow from P680+ ̇ to H2 O
Summary
Photosynthesis, the light-enabled synthesis of biomolecules from much simpler precursors, is a fascinating and obviously important process. The overall reaction is usually summarized as the production of glucose and oxygen,. ∆r Go’ = +2870 kJ/mol, ∆r Go = +2875 kJ/mol = ∆r Go”. Where a double prime indicates Alberty’s pH 7 biochemical standard conditions [1], a single prime conventional pH 7 biochemical standard conditions [2], and the absence of a prime chemical standard conditions (e.g., at pH 0); their differences are usually insignificant in this work. The solar energy is captured in a first step, the light reaction, which includes. 12 H2 O + 12 NADP+ → 12 NADPH + 12 H+ + 6 O2
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