Mechanisms of Photophosphorylation in Chloroplasts

Noncyclic Phosphorylation

The arrangement of the pigment-protein complexes, Photosystems I and II, both electron carriers in the electronic transport chains and ATP synthase enzymes, is necessary to maintain a specific spatial arrangement in the thylakoid membrane, which enables ATP synthesis. The mechanism of ATP synthesis is nearly identical to that of the mitochondria; it depends on the existence of the ATP synthase enzyme complex on the stromal face of the thylakoid membrane. Proton pumping into the thylakoid, performed by several protein complexes of the light phase, generates a proton-motive force gradient necessary for synthesis. In the chloroplast, it is known that each ATP synthesis catalyzed by the stromal portion F1 is associated with the flow through four protons of the particle. As the energy comes ultimately from light, it is named photophosphorylation.

Cyclic Phosphorylation

There is a possibility of a cyclic electron transport regardless of PSII. The process starts with the absorption of energy by PSI and electron transfer to ferredoxin, which transfers them to cytochrome b6-f so the process becomes a cycle, where only PSI acts. Continued proton pumping will continue to accumulate energy that will make the synthesis possible.

Cellular Energy Requirements

These characteristics depend on the need for NADPH, carbohydrates, and extra ATP in the cell. When the chloroplast needs ATP, it closes on PSI energy so that it is used for the synthesis of ATP instead of the production of NADPH.

Light-Dependent Phase Scheme

Absorbed water by the roots reaches the chloroplasts. In an enzymatic process there, the molecule breaks; O₂ yields electrons and protons that make the existence of aerobic life possible. Electrons are collected by PSII to be excited by light. Pheophytin A yields these to several plastoquinone molecules induced in the thylakoid membrane. From there, they are transferred to cytochrome b6-f. The protons pumped into the luminal space, added to the photolysis of water, increase the acidity of the lumen. Via this process, the electrochemical gradient is generated, which will have important consequences for photophosphorylation. It is possible for ferredoxin not to transfer the electrons to cytochrome b6-f, thus the process becomes a cycle.