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Cyclic and Non-Cyclic Photophosphorylation

What is Photophosphorylation

Photophosphorylation is the process by which ADP gets converted into ATP, utilizing light energy from photosynthesis. It synthesizes energy-rich ATP molecules by transferring a phosphate group into an ADP molecule in the presence of light.

The process occurs in the chloroplasts of cells, specifically in the thylakoid membranes.

In photophosphorylation or light-dependent reactions, the light gets absorbed by chlorophyll and other pigment molecules. Then, this light energy is used to create a high-energy electron donor and a lower-energy electron acceptor. As a result, electrons move spontaneously from donor to acceptor through an electron transport chain (ETC).

Based on the path of electrons in the ETC of the primary photochemical reaction, there are two types of photophosphorylation processes. They are cyclic photophosphorylation and non-cyclic photophosphorylation.

1) Cyclic Photophosphorylation

Cyclic photophosphorylation occurs on the stroma lamella or fret channels. By this process, plant cells produce a steady supply of ATP in the presence of sunlight. However, as ATP is a highly reactive molecule, it cannot be readily stored within the cell.

In this process, the high-energy electron released from chlorophyll P700 of Photosystem I flow down in a cyclic pathway.  When the PS I absorbs light energy, the electron gets excited and enters an ETC to produce ATP. The electron starts from a pigment complex called photosystem I, then passes from the primary acceptor to ferredoxin, followed by plastoquinone, then to cytochrome b6f, and then to plastocyanin before returning to photosystem-I.

In this electron transport system, the electron ejected from the P700 molecule is cycled back instead of reducing the NADP. Thus the process is known as cyclic electron transport and phosphorylation as cyclic photophosphorylation. This cyclical movement of electrons from an acceptor to P700 results in the formation of ATP molecules. The process neither produces O2 nor NADPH and occurs in both aerobic and anaerobic conditions.

2) Non-Cyclic Photophosphorylation

Non-cyclic photophosphorylation occurs in the thylakoid membrane. It is a two-stage process involving two different chlorophyll photosystems (PS I and PS II).

When Photosystem II absorbs light, the excited electrons enter into an electron transport chain to produce ATP. Simultaneously, photoactivation of Photosystem I results in the release of electrons which reduce NADP+, forming NADPH.

This process is called non-cyclic photophosphorylation because the lost electrons by P680 of Photosystem II get occupied by P700 of Photosystem I and are not cycled back to P680. Hence, the complete movement of the electrons occurs in a unidirectional or non-cyclic manner.

During non-cyclic photophosphorylation, the electrons freed by P700 get taken away by the primary acceptor and finally get passed on to NADP. Here, the electrons mix with the protons – H+, which is later produced by dividing up the water molecule and reduces NADP to NADPH2.

Thus, the PSI is reduced by electrons coming from PSII, and the PSII gets reduced by electrons coming from water.

Cyclic vs. Noncyclic Photophosphorylation

Both cyclic and non-cyclic phosphorylation contain some differences and share many similarities. Let us explore them below.

Difference Between Cyclic and Non-Cyclic Photophosphorylation

Let us discuss the differences between cyclic and non-cyclic phosphorylation.

BasisCyclic PhotophosphorylationNon-Cyclic Photophosphorylation
OccurrenceOccurs in anaerobic conditionsOccurs mainly in aerobic conditions
ProductsOnly ATPATP, NADPH, and O2
Role of waterWater is not requiredWater is required
Production of ATP And enzymesOnly ATP is producedBoth ATP and reduced coenzymes (2 NADPH2) are produced
No. of ATP produced21
Occurrence conditionWhen the concentration of carbon dioxide is less in the atmosphereWhen the concentration of carbon dioxide is sufficient in the atmosphere
Light intensityLow    High
Oxygen productionNot producedProduced
Photolysis of waterDoes not occurOccur
Electron donorP700P680
Final electron acceptorP700NADP+
Electron returnElectrons return to the P700 after passing through ETSElectrons are not returned to the reaction center of P680 and are accepted by NADP+
Electron expulsionFirst-expelled from the reaction center of  PS I (P700)First-expelled from the reaction center of Ps II (P680)
Photosystem involvedOnly photosystem I is involvedBoth photosystems  I and II are involved
Electron movement patternCyclicLinear
Site of occurrenceIn isolated chloroplasts and photosynthetic bacteriaIn plants, mainly in algae and cyanobacteria
Type of photosynthesisAnoxygenic    Oxygenic
Electron cycleOpenClosed
Effect of DCMUNo effectInhibits the process
SignificanceRequired to produce a steady supply of ATP in the presence of sunlightSupplies assimilatory power in the form of NADPH and ATP for CO2 assimilation and purifies the atmospheric air

Similarities

The similarities between the two types of phosphorylation are as follows:

  • Both occur in light reactions of photosynthesis
  • Both are electron transport systems
  • Both pathways produce assimilatory powers
  • Both phosphorylate ADP, thus producing ATP

FAQs

Q1. What are the products of linear photophosphorylation?

Ans. The products of linear photophosphorylation are ATP and NADPH.

Q2. What is the role of water in non-cyclic photophosphorylation?

Ans. The roles of water in noncyclic photophosphorylation are:
a. Directly generates ATP
b. Harvests light energy
c. Collects the electrons for the Calvin-Benson cycle
d. Provides electrons and protons

Q3. What is the role of water in cyclic photophosphorylation?

Ans.  Cyclic phosphorylation does not require the involvement of water.

Article was last reviewed on Friday, September 3, 2021

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