The reaction centre in PS II absorbs 680 nm wavelength of red light. This causes electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor and sent to an electrons transport system consisting of cytochromes.
In terms of an oxidation-reduction potential scale, this movement of electrons is downhill. The electrons are not used up as they pass through the electron transport chain. They are passed on to the pigments of PS I.
At the same time, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm. They are then transferred to another acceptor molecule which has a greater redox potential.
Z Scheme: These electrons are then moved downhill again, to a molecule of energy-rich NADP+. The addition of these electrons reduces NADP+ to NADPH+H+. This whole transfer of electrons from PS II to the acceptor, to PS I, to another acceptor and finally to NADP+ is called the Z scheme, because of its characteristic shape.
Water is split into H+, [O] and electrons. The splitting of water is associated with PS II. This creates oxygen. Photosystem II provides replacement for electrons removed from PS I.
2H2O → 4H+ + O2 + 4e-
Synthesis of ATP in chloroplast can be explained by chemiosmotic hypothesis. The way it happens in respiration, ATP synthesis during photosynthesis happens because of development of a proton gradient across a membrane, i.e. membrane of the thylakoid. The following steps are involved in development of proton gradient across the thylakoid membrane.
When the electrons move through the photosystems, protons are transported across the membrane. The primary acceptor of electron is located towards the outer side of the membrane. It transfers its electrons not to an electron carrier but to an H carrier. Due to this, it removes a proton from the stroma while transporting an electron. When electron is passed to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.
The NADP reductase enzyme is located on the stroma side of the membrane. Protons are also necessary for the reduction of NADP+ to NADPH+H+. These protons are also removed from the stroma.
Thus, protons in the stroma decrease in number and accumulate in the lumen. This results in development of a proton gradient across the thylakoid membrane. Additionally, there is a measurable decrease in pH in the lumen.
The breakdown of this gradient leads to release of energy. The movement of protons across the membrane to the stroma results in breakdown of this gradient. The movement of protons takes place through the transmembrane channel of the F0 of the ATPase.
The ATPase enzyme consists of two parts. One part is called the F0 and is embedded in the membrane. This forms a transmembrane channel which carries out facilitated diffusion of protons across the membrane. The other portion is called F1. It protrudes on the outer surface of thylakoid membrane on the lumen side.
The breakdown of the gradient provides enough energy to cause a change in the F1 particle of the ATPase which results in synthesis of several molecules of energy-packed ATP.
To summarise, it can be said that chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is utilised to pump protons across a membrane, to create a gradient of protons within the thylakoid membrane. ATPase has a channel. This channel allows diffusion of protons back across the membrane. The diffusion of protons releases enough energy to activate ATPase enzyme. The ATPase enzyme catalyses the formation of ATP. NADPH and the ATP are used in the biosynthetic reaction which takes place in the stroma. This reaction is responsible for fixing CO2 and for synthesis of sugars.
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