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Cyclic and Non-Cyclic Photophosphorylation are the two types of Photophosphorylation. Photophosphorylation is the addition of phosphate in the presence of light or the production of ATP by cells in this relatively simple mechanism. The light reaction takes place in the chloroplast's grana where light energy is converted to chemical energy in the form of ATP and NADPH. The energy created earlier in the light reaction is used to repair carbon dioxide into carbohydrates in the dark reaction.
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What is Photophosphorylation?
Photophosphorylation is a process for phosphorylating ADP to create ATP using sunlight as a source of energy. Living organisms have access to only two sources of energy: sunlight and reduction-oxidation (redox) reactions. The common energy currency of life is ATP, which is produced by all species. Water photolysis, or photodissociation, occurs during photosynthesis, as does a constant unidirectional flow of electrons from water to photosystem II.
Light energy is employed in the photophosphorylation process to create a high-energy electron donor and a lower-energy electron acceptor. Electrons are then abruptly transferred from source to acceptor via an electron transport chain.
Photophosphorylation
Cyclic Photophosphorylation
- The process of cyclic photophosphorylation involves the recycling of electrons. A photosystem, which is filled with chlorophyll, is one of the elements of the thylakoid membrane. The light energy is absorbed by the chlorophyll and used to activate the electron. After then, the electron is transmitted along with an electron transport channel to an electron acceptor protein.
- The electron loses energy as it travels along the transport channel, which is subsequently used to generate ATP from ATP and Pi. After that, the electron is recycled and reintroduced into the photosystem. The stroma lamellae or frets are where photophosphorylation takes place. The high-energy electron is liberated from P700 to ps1 and flows via a cyclic channel in cyclic photophosphorylation.
- The electron in cyclic electron flow begins in a pigment complex termed photosystem I, then travels from the primary acceptor to ferredoxin, plastoquinone, cytochrome b6f (a homologous complex also found in mitochondria), and finally plastocyanin before returning to chlorophyll.
- This transfer or shifting channel generates a proton-motive force (PMF) by pumping H+ ions across the membrane, resulting in a concentration gradient that can be used to activate ATP synthase during the chemiosmosis process. Cyclic photophosphorylation is the name given to this pathway, which produces neither oxygen (O2) nor NADPH. Non-cyclic photophosphorylation, on the other hand, does not remove the electrons; instead, they are returned to the cytochrome b6f complex.
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Cyclic Photophosphorylation
Non-Cyclic Photophosphorylation
- Non-cyclic photophosphorylation, the other light reaction pathway, is a two-stage process involving two separate chlorophyll photosystems. Non-cyclic photophosphorylation occurs in the thylakoid membrane because it is a light response. A water molecule is initially broken down into 2H+ + 1/2 O2 + 2e through a process known as photolysis (light-splitting).
- The two water molecule electrons are then maintained in photosystem II, while the 2H+ and 1/2 O2 are released for other purposes. The photon is then absorbed by chlorophyll pigments that surround the photosystem's reaction core center. The light excites the electrons in each pigment, resulting in a chain reaction that transmits energy to the photosystem II core, stimulating the two electrons that are transmitted to the primary electron acceptor, pheophytin.
- The electron shortfall is filled by stealing electrons from another water molecule. The electrons transfer from pheophytin to plastoquinone, which takes two electrons from pheophytin and two hydrogen ions from the stroma to form PQH2, which is then broken down into PQ, the two electrons are released to the Cytochrome b6f complex, and the two hydrogen ions are released into the thylakoid lumen.
- After that, the electrons pass through the Cyt b6 and Cyt f channels. They are then transmitted along plastocyanin, which provides the energy needed to drive hydrogen ions (H+) into the thylakoid region. By providing the energy for ATP regeneration, a gradient is created, causing hydrogen ions to flow back into the stroma of the chloroplast.
Non-Cyclic Photophosphorylation
Difference Between Cyclic and Non-Cyclic Photophosphorylation
- In the presence of sunlight, cyclic photophosphorylation can be exploited to generate a consistent supply of ATP.
- However, because ATP is a highly reactive chemical, it cannot be stored easily within the cell.
- In addition to ATP, non-cyclic photophosphorylation produces NADPH (this requires the presence of water)
- The light-independent processes require both NADPH and ATP to generate organic compounds.
- As a result, only non-cyclic photophosphorylation allows for organic molecule synthesis and long-term energy storage.
- The following table summarises the key differences between cyclic and non-cyclic photophosphorylation:
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| Cyclic Phosphorylation | Non-Cyclic Photophosphorylation |
|---|---|
| Photosystem I is involved in the cyclic photophosphorylation process. | Both Photosystem I and II are involved in the non-cyclic photophosphorylation process |
| In cyclic photophosphorylation, P700 is known to be the active reaction center. | In the non-cyclic photophosphorylation, P680 is known to be the active reaction centre. |
| Electrons tend to pass in a cyclic manner. | Electrons tend to pass in a non–cyclic manner. |
| Electrons return back to Photosystem I. | Electrons from Photosystem I am accepted by NADP and it does not return back. |
| ATP molecules get generated in this process. | Both ATP and NADPH molecules get formed. |
| Water is not needed in the cyclic photophosphorylation process. | Water is needed in the process and the process of photolysis takes place as well. |
| Oxygen does not get produced as a by-product | Oxygen gets produced as a by-product. |
| This process is ideal only in the case of bacteria. | This process is ideal amongst all the green plants. |
Things to Remember
- The light reaction takes place in the chloroplast's grana where light energy is converted to chemical energy in the form of ATP and NADPH.
- The energy created earlier in the light reaction is used to repair carbon dioxide into carbohydrates in the dark reaction.
- Photophosphorylation is a process for phosphorylating ADP to create ATP using sunlight as a source of energy.
- Only non-cyclic photophosphorylation allows for organic molecule synthesis and long-term energy storage.
- The light-independent processes require both NADPH and ATP to generate organic compounds.
Important Questions
Ques. How does non-cyclic photophosphorylation work? (3 marks)
Ans. Non-cyclic photophosphorylation (the "normal" form of the light-dependent processes) involves removing electrons from water and passing them via PSII and PSI before ending at NADPH.
- Light absorption in PSII
- ATP synthesis
- Light absorption in PSI
- NADPH formation
Ques. In non-cyclic photophosphorylation, what is the initial step? (2 marks)
Ans. Non-cyclic photophosphorylation happens in the thylakoid membrane because it is a light response. A process termed photolysis (or light-splitting) first breaks down a water molecule into 2H+ + 1/2 O2 + 2e. The photon is then absorbed by the chlorophyll pigments that surround the photosystem's reaction core centre.
Ques. What is cyclic phosphorylation? (2 marks)
Ans. Cyclic photophosphorylation is defined as the production of ATP that is related to electron transport that is only activated by Photosystem I and may thus be carried out in long-wavelength light (03BB 2265 700 nm).
Ques. What is the significance of cyclic photophosphorylation? (2 marks)
Ans. It is concluded that cyclic photophosphorylation is essential to replenish the Calvin cycle's pools of phosphorylated intermediates at a time when noncyclic photophosphorylation is inefficient. There is yet no proof that cyclic photophosphorylation plays a stoichiometric role in photosynthesis.
Ques. Is oxygen produced by cyclic photophosphorylation? (2 marks)
Ans. The electrons are instead transported back to P700. The production of ATP arises from the downward passage of electrons from an electron acceptor to P700, which is known as cyclic photophosphorylation. It's vital to note that during cyclic photophosphorylation, oxygen and NADPH2 aren't produced.
Ques. What are the conditions in which cyclic photophosphorylation occurs? (2 marks)
Ans. Both aerobic and anaerobic circumstances result in cyclic photophosphorylation. The electron that was ejected from the P700 molecule is cycled again in this electron transport system, giving rise to the terms cyclic electron transport and cyclic photophosphorylation.
Ques. What is the inactivation of photosynthesis known as? ( 2 marks)
Ans. When there is a high intensity of light, photosynthesis inhibition happens due to the block in the electron transport chain. This is known as photoinhibition. This photosynthetic capacity is reduced, which in turn reduces the normal metabolism of all plants.
Ques. Which photosystem does Cyclic photophosphorylation have? (1 mark)
Ans. Photophosphorylation contains only photosystem I. Because there is only a transfer of electrons, but there’s no reduction of NADP+.
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