13.6.2 Cyclic and Non-cyclic Photo-phosphorylation

Living organisms have the capability of extracting energy from oxidisable substances and store this in the form of bond energy. Special substances like ATP, carry this energy in their chemical bonds. The process of whichATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation.

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. When the two photosystems work in a series, first PS II and then the PS I, a process called non-cyclic photo-phosphorylation occurs.

The two photosystems are connected through an electron transport chain, as seen earlier – in the Z scheme. Both ATP and NADPH + H+ are synthesised by this kind of electron flow. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.

A possible location where this could be happening is in the stroma lamellae. While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme.

The excited electron does not pass on to NADP+ but is cycled back to the PS I complex through the electron transport chain.

The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+ . Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

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