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Cyclic photophosphorylation

Photosynthesis can also occur without the involvement of photosystem II. If photosystem I acts independently, neither NADPH nor O2 is formed, and the flow of electrons only generates ATP.

Whether electron transport is cyclic or non-cyclic depends on the cell's need for NADPH, carbohydrates, and extra ATP. If ATP is needed, the chloroplasts close the PS I, and the energy is used in the synthesis of ATP instead of being used to produce NADPH.

Electrons are lifted from chlorophyll P700 to the primary electron acceptor of photosystem I, but they do not follow the series of electronic carriers that lead to NADP+, but instead are diverted into the electron transport chain that connects photosystem I and II and continue down that chain to the chlorophyll molecule P700. ATP is produced on this path.

In photosystem I, a photon is captured that allows an electron to be released from the P 700 reaction center, yielding it to chlorophyll A0, then to phylloquinone (Q), and passes it to ferredoxin (Fd). This gives it to the b6 -f complex , where enough energy is produced to transport the H+ against the electrochemical gradient, and produce ATP as in non-cyclic photophosphorylation. The b6-f complex gives the electrons to plastocyanin (PC), which carries them to the hole left in the P700, so that they can be excited again by a photon and restart cyclic photophosphorylation.

As photosystem II does not intervene, the photolysis of water does not occur and, therefore, NADP+ is not reduced  or oxygen is released, obtaining only ATP.

The main differences between acyclic and cyclic photophosphorylation are:

Acyclic photophosphorylation

Cyclic photophosphorylation

The PS I and PS II intervene.

The PS I spoke.

Electrons follow an open path.

Electrons follow a cyclical path.

Water intervenes as a reducing agent.

Water does not intervene as a reducing agent. There is no photolysis of the water.

NADPH + H + is obtained .

NADPH + H + is not obtained , since the electrons from PS I are transferred to the cit bf complex and not to NADP + .

2 is released .

2 is not released  because the photolysis of the water does not take place, since the PS II does not intervene.

ATP is produced by the gradient formed by the accumulation of H +  in the chloroplast lumen, and by the translocation of H +  by the complete cit bf.

ATP is produced by the translocation of H +  by the complete cit bf.

It is made by plants, algae and cyanobacteria.

It is carried out by sulfobacteria and non-sulfur bacteria.

The earliest photosynthetic mechanisms are thought to function by cyclic photophosphorylation, with cyclic electron flow. Although it is an alternative pathway in eukaryotic cells, it is the way that some bacteria photosynthesize.


Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Extremadura, July 2019, option B, question 2.

Cyclic and non-cyclic photophosphorylation: (1 point each section)

A. Concepts and objective of both.

B. Brief description of the processes.