Oxygenic Photosynthesis

The light reaction of oxygenic photosynthesis in plants occurs in chloroplasts, which has the following general structure and distribution of components:

The classical explanation involves deterministic electron transfer concepts (quite similar to the classical mechanistic perspectives in xenobiotic metabolism and aerobic respiration) with the Z-scheme Electron Transport Chain, Kok-Joliot oxygenesis cycle, Q-cycle, and Chemiosmotic Rotary ATP Synthesis (CRAS). The salient aspects of these explanations are shown in the following figures.

Quite like the classical explanation for cellular respiration, the classical paradigm also does not accommodate the reactivity and mobility of oxygen or diffusible reactive species; seeking a deterministic/affinity driven relay of electrons and pumping of protons to synthesize ATP harnessing a trans-membrane potential. There are several fundamental flaws and impossible facets within the classical proposal. Kok-Joliot cycle does not agree with structural aspects known of PS II and also, cannot explain oxygen evolution in the 2nd light pulse (post dark acclimatization). Q-cycle is an improbable multimolecular scheme that is thermodynamically and kinetically disadvantaged. Z-scheme violates basic notions of electrochemistry in steady-state as electrons seem to take criss-cross route in various phases, with scant regard to polarities (as a comparison with a simple electrochemical setup shows above). Further, proton is also supposed to move criss-cross at the same membrane. The serial circuitry cannot work because the purported ETC can get shunted or discontinued owing to multiple reasons, as shown in figure below. In a functional ETC, the pertinent redox centers would be placed at suitable distances, and would have the correct redox states and potentials. These requirements are unmet in reality (see figure below), as we know with the current awareness of components' structures.

Further, there are several mechanistic similarities/commonalities (like CRAS) with the disclaimed ideas seen in aerobic respiration.

Interestingly, the uncoupling phenomenon is common to mXM, OxPhos and Pl-Pp systems (of which two have already been shown to occur via murburn mechanism).

Therefore, the underpinning murburn model of Pl-Pp was floated, as shown below in three different perspectives.

The murburn model efficiently explains known observations and fundamental theoretical aspects of photochemistry and structure-function correlations and distributions of components in chloroplasts. For example: the large bulbous extension on the lumen side of PS II is a source of DRS and has several ADP binding sites. Further, the Emerson enhancement effect (synergtic functioning of PS I and II can be explained with the parallel functioning of components in the murburn model.