Cyclic And Non Cyclic Photophosphorylation: Difference, Diagram

Light Reactions

Light reactions take place in the thylakoid membranes of the chloroplast. These processes capture light energy via chlorophyll and other pigments and then store it chemically in the form of ATP and NADPH. Water molecules are split in this process, which results in oxygen as a byproduct. Hence, the light reactions result in products, ATP and NADPH that are both utilised by the following dark reactions in the stroma of the chloroplast.

Dark Reactions

The dark reactions are also known as the Calvin cycle or the light-independent reactions and take place in the stroma of chloroplasts. The dark reactions themselves do not need direct light. Instead, they drive the carbon dioxide from the atmosphere into glucose with the ATP and NADPH obtained in the light reactions. The Calvin cycle is very essential for the photosynthetic production of compounds necessary for providing energy and acting as basic blocks for growth and development in plants.

Definition Of Photophosphorylation

It is an important biochemical process, whereby light energy is converted into a form of chemical energy, specifically ATP, in the chloroplasts of plant cells. During photophosphorylation, electrons flow down a gradient of several proteins in the thylakoid membrane to eventually result in the production of ATP during the light-dependent reactions of photosynthesis. Photophosphorylation is the step providing energy for the subsequent stages of photosynthesis, by which carbon dioxide is fixed into glucose.

Apart from synthesising ATP, photophosphorylation plays a significant role in the homeostasis of energy and reducing powers required during sugar synthesis. It does this through two major types of photophosphorylation processes: cyclic and non-cyclic. This would account for the way plants harness light energy to be stored in the form of chemical energy while maintaining energy homeostasis for their growth and development.

Cyclic Photophosphorylation

Light-dependent reaction in photosynthesis in which electrons are recycled in the photosystem (PSI), leading to the formation of ATP but neither NADPH nor oxygen is formed.

• Photon Absorption: Absorption of light photons by Photosystem I, exciting an electron.

• Transport of Electron: Flow of excited electrons in the electron transport chain.

• Energy in Electron Flow: This energy from the electron flow is used in pumping protons across the thylakoid membrane, generating a proton gradient that drives ATP synthesis.

• Return of Electron: Electrons ultimately return to PSI to complete the cycle.

• Absorption of Photon: In PSI, the photons get absorbed, exciting electrons into higher energy states.

• Electron Transfer: Electron transport entails a series of proteins in the thylakoid membrane, including plastoquinone, the cytochrome b6f complex, and plastocyanin.

• ATP Formation: Electron flow gives the energy to pump protons into the thylakoid lumen, thus developing a proton gradient. It is this gradient that is harnessed by the enzyme ATP synthase to produce ATP from ADP and inorganic phosphate.

• Recycling: Electrons get passed back to PSI, so the cycle starts all over again.

Role Of PSI

• PSI is central to cyclic photophosphorylation; under this pathway, it absorbs light and facilitates the recycling of electrons to generate ATP.

• ATP Production: One of the primary products is ATP, which acts as the energy source for cellular processes.

• No NADPH Production or Oxygen Evolution: Cyclic photophosphorylation does not produce NADPH or oxygen.

Noncyclic Photophosphorylation

Noncyclic photophosphorylation refers to the process in which light energy is used to generate both ATP and NADPH as well as the formation of oxygen due to the linear electron flow from water to NADP+.

• Photon Absorption: Light absorbance by Photosystem II (PSII) excites electrons.

• Water Splitting: Water molecules are split to provide electrons and to release oxygen.

• Electron Transport: The electron transport chain is passed through from PSII to PSI.

• ATP and NADPH Formation: Use of the energy of electron flow in generating ATP and NADPH.

• Photon Absorption, PSII: Photons are absorbed by PSII, which excites an electron and starts its transport.

• Splitting of Water: In the oxygen-evolving complex of PSII, water is split into oxygen, protons, and electrons.

• Electron transport chain: Electron transport, including plastoquinone, the b6f complex of cytochrome, and plastocyanin, passes electrons over to PSI.

• Photon Absorption (PSI): More photons absorb in PSI, further exciting electrons. These electrons then reduce NADP+ to NADPH.

• ATP Formation: A proton gradient created by electron transport drives ATP synthesis with the help of an enzyme, namely ATP synthase.

Differences Between Cyclic And Non-Cyclic Photophosphorylation

Roles Of Photosystem II (PSII) And Photosystem I (PSI)

• PSII: The process starts with the absorption of light and photolytic cleavage of water, releasing electrons, protons, and oxygen.

• PSI: The remaining light is absorbed and results in the reduction of NADP+ into NADPH, closing the electron flow.

• Results: ATP and NADPH production. Both ATP and NADPH are products of this process; these will be crucial in the Calvin cycle.

• Oxygen evolution: It is the process whereby oxygen is evolved as a by-product of water-splitting. Importance:

• Significance of the Calvin Cycle: The products NADPH and ATP from the light-dependent reaction are consumed during the Calvin cycle to fix atmospheric carbon dioxide into glucose.

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