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Cyclic Photophosphorylation Definition and Meaning


Photophosphorylation is the method of using light energy from photosynthesis to convert ADP to ATP. It is the method of synthesizing energy-rich ATP molecules by carrying the phosphate
group into an ADP molecule. ATP is the essential universal energy “currency” for any known life form. Photosynthesis requires Photolysis and a continuous unidirectional flow of electrons from water to Photosystem II.

Light energy gets used and forms a high-energy electron donor and a lower-energy electron acceptor system. Then the electrons move instantaneously from donor to acceptor through an electron transport chain.

Photophosphorylation exists of two kinds:

  • Cyclic Photophosphorylation
  • Non-cyclic Photophosphorylation

cyclic Photophosphory

Cyclic Photophosphorylation

The photophosphorylation process which results in the movement of the electrons cyclically for synthesizing ATP molecules is called Cyclic Photophosphorylation. It will take place in both aerobic wells as anaerobic conditions.

In this process, plant cells finish the ADP to ATP for instant energy for cells. This process regularly takes place in the thylakoid membrane and uses Photosystem I and the chlorophyll P700.

During Cyclic Photophosphorylation, the electrons are shifted back to P700 rather than moving into the NADP from the electron acceptor. This downward flow of electrons from an acceptor to P700 affects the structure of ATP molecules.

This Photophosphorylation occurs on the stroma lamellae, also called fret channels. When it comes to Cyclic Photophosphorylation, high-energy electrons released from P700 of PS1 flow down on a cyclic path. In cyclic electron flow, an electron starts its journey in a pigment complex called Photosystem-I. It then passes from the first acceptor to ferredoxin and then to plastoquinone. Next it reaches cytochrome b6f and then to plastocyanin before returning Photosystem I.

This chain of transport results in a proton-motive force. It pumps H+ ions across the membrane and produces a concentration gradient used to power ATP synthase during chemiosmosis. This pathway is known as Cyclic Photophosphorylation, and it produces neither O2 nor NADPH. Unlike non-cyclic Photophosphorylation, NADP+ does not accept the electrons; and they are sent back to the cytochrome b6f complex.

In bacterial photosynthesis, a single photosystem is involved in Cyclic Photophosphorylation. It is considered favorable in anaerobic conditions and conditions of CO2 compensation points and of high irradiance.

Features of Cyclic Photophosphorylation

  • Only Photosystem I is needed.
  • P700 is the active reaction center where it takes place.
  • Electrons move cyclically.
  • Electrons can reverse back to Photosystem I.

Cyclic versus Non-Cyclic Photophosphorylation

The moving of electrons in a Non-cyclic manner to produce ATP molecules using the energy from excited electrons provided by Photosystem II is known as Non-cyclic Photophosphorylation.

The difference is:

  • The Cyclic Photophosphorylation can be used to provide a steady supply of ATP in the presence of light
  • Nevertheless, ATP is a highly reactive molecule and hence cannot be store within the cell
  • Non-cyclic Photophosphorylation generates NADPH in addition to ATP (this requires the presence of water)
  • Both NADPH and ATP are required to produce organic molecules via the light-independent reactions
  • Hence, only Non-cyclic Photophosphorylation allows for the synthesis of organic molecules and long term energy storage
  • Only Photosystem I is involved in Cyclic Photophosphorylation
  • Both Photosystem I and II are involved in Non-cyclic Photophosphorylation
  • P700 is the active reaction center in Cyclic Photophosphorylation
  • P680 is the reaction center in Non-cyclic Photophosphorylation
  • Electrons travel cyclically in Cyclic Photophosphorylation
  • Electrons travel in a non – cyclic manner in Non-cyclic Photophosphorylation
  • Electrons revert to Photosystem I in Cyclic Photophosphorylation.
  • Electrons from Photosystem I accepted by NADP in Non-cyclic Photophosphorylation.
  • ATP molecules produced in Cyclic Photophosphorylation.
  • NADPH and ATP molecules created in Non-cyclic Photophosphorylation.
  • No need for Water in Cyclic Photophosphorylation
  • Photolysis of water is present in Non-cyclic Photophosphorylation.
  • No creation of NADPH in Cyclic Photophosphorylation
  • NADPH in Non-cyclic Photophosphorylation.
  • No Oxygen as the by-product in Cyclic Photophosphorylation
  • Oxygen as a by-product in Non-cyclic Photophosphorylation.
  • This process is predominant only in bacteria in Cyclic Photophosphorylation
  • This process is dominant in all green plants in Non-cyclic Photophosphorylation

Advantages of Cyclic Photophosphorylation

  • It allows for different pigments to absorb huge bands of light.
  • Non-cyclic Photophosphorylation is easier for the plant since it uses only one photosystem.
  • Non-cyclic Photophosphorylation produces ATP and NADPH; cyclic produces the only ATP.





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