What is the role of cyclic photophosphorylation and under what conditions does it occur?

Answer 1

The first stage of the photosynthetic system is the light-dependent reaction, which converts solar energy into chemical energy.

Light absorbed by chlorophyll or other photosynthetic pigments such as carotene is used to drive a transfer of electrons and hydrogen from water (or some other donor molecule) to an acceptor called NADP+, reducing it to the form of NADPH by adding a pair of electrons and a single proton (hydrogen nucleus). The water or some other donor molecule is split in the process; it is the light reaction which produces waste oxygen.

The light reaction also generates ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation. ATP is a versatile source of chemical energy used in most biological processes. The ATP produced in this part of photosynthesis is used to build up carbohydrates in the next part of photosythesis, known as the light independent stage or the Calvin Cycle. Note, however, that the light reaction produces no carbohydrates such as sugars. The light reaction occurs in the stacked membranes of the grana in the thylakoid membrane. Oxygen is a byproduct.

The process of synthesizing ATP and NADPH are accomplished via the mechanism of an electron transport chain. This is a series of proteins embedded in a biological membrane that transfers high-energy electrons from one to another, accomplishing various activities along the way as the electron drops in energy level. When sunlight strikes a chlorophyll complex (cluster of chlorophyll), the molecule is excited, and an electron is transferred to a higher energy level of the molecule.

The chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.

Cyclic photophosphorylation
In cyclic electron flow, the electron begins in a pigment complex called photosystem I, passes from the primary acceptor to ferredoxin, then to a complex of two cytochromes (similar to those found in mitochondria), and then to plastocyanin before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H+ ions across the membrane; this produces a concentration gradient which can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosporylation.

Answer 2

Plant chloroplasts (the site of photosynthesis) contain two different reaction centers that respond to light at slightly different wavelengths. These are known as photosystems I and II, or PSI and PSII. There are also two different mechanisms of photosynthesis, both of which were discovered in Arnon's laboratory more than 25 years ago. In the first, called cyclic photophosphorylation, light energy interacts with PSI to form adenosine triphosphate (ATP), the cell's primary energy carrier. In the second, called noncyclic or linear photophosphorylation, ATP is formed, free oxygen is liberated from water, and ferredoxin (an iron-containing protein that carries electrons) is reduced. The reduced ferredoxin is used in the conversion of carbon dioxide to sugars, in a reaction that takes place in the dark.