Understanding the Two Types of Photophosphorylation in Photosynthesis
Photophosphorylation is a process that occurs in the thylakoid membranes of the chloroplasts during photosynthesis. It involves the absorption of light energy by photosystems to produce ATP, which is used to power the light-independent reactions of photosynthesis. There are two types of photophosphorylation, cyclic and noncyclic, which differ in the way they produce ATP.
Last updated date: 27th Sep 2023
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What is Cyclic and Noncyclic Photophosphorylation?
The process of cyclic photophosphorylation involves the transfer of electrons in a cyclic manner to produce ATP molecules. It occurs in the thylakoid membrane of chloroplasts during photosynthesis. The process starts when light energy is absorbed by photosystem I, which contains chlorophyll P700. The light energy excites electrons in the chlorophyll P700, which are then transferred to an electron acceptor molecule. In cyclic photophosphorylation, the electron acceptor molecule is the cytochrome b6f complex.
The electrons are then transferred to the electron transport chain, which passes them back to photosystem I, where they are used to produce ATP molecules. The movement of electrons during cyclic photophosphorylation is cyclic, meaning that the electrons are transferred in a loop rather than a linear pathway. This process produces ATP molecules that are used for energy, and it is also important for balancing the ratio of ATP to NADPH in the cell especially under conditions of low light intensity. Cyclic photophosphorylation is important for the survival of plants and other photosynthetic organisms, as it allows them to produce ATP even when there is not enough light to drive the noncyclic photophosphorylation process.
Noncyclic photophosphorylation is a process that occurs in the thylakoid membrane of chloroplasts during photosynthesis. It involves the transfer of electrons from water molecules to NADP+ to produce NADPH, while also generating ATP. The process begins with the absorption of light by photosystem II, which contains chlorophyll P680. The light energy excites electrons in the chlorophyll, which are then transferred to an electron acceptor molecule. This process generates a proton gradient that is used to produce ATP through the process of chemiosmosis. The electrons from the electron acceptor molecule are then passed through a series of electron carriers, including the cytochrome b6f complex, which also generates a proton gradient used to produce ATP molecules. Finally, the electrons are transferred to photosystem I, which contains chlorophyll P700. The electrons are re-energised by light and used to produce NADPH molecules from NADP+. This process also produces oxygen as a byproduct. Now, lets see cyclic and noncyclic photophosphorylation examples.
Cyclic and Noncyclic Photophosphorylation Examples
Examples of Cyclic Photophosphorylation
Examples of organisms that use cyclic photophosphorylation include purple bacteria, green sulfur bacteria, and heliobacteria. Within these organisms, the process occurs in specialized structures called chromatophores, which contain pigments such as bacteriochlorophyll that absorb light energy and transfer it to the photosystems involved in the process. Unlike noncyclic photophosphorylation, cyclic photophosphorylation does not produce NADPH or oxygen as byproducts. Instead, it recycles electrons through the electron transport chain to generate a proton gradient and ATP.
Examples of Noncyclic Photophosphorylation
Examples of organisms that use noncyclic photophosphorylation include green plants, algae, and cyanobacteria. Within these organisms, the process occurs in specialized organelles called chloroplasts, which contain pigments such as chlorophyll and carotenoids that absorb light energy and transfer it to the photosystems involved in the process. Now, lets differentiate between cyclic and noncyclic photophosphorylation.
Cyclic and Noncyclic Photophosphorylation Difference
The table presented below highlights the main differences between cyclic and non cyclic photophosphorylation.
ATP and NADPH
Electrons are recycled through the ETC
Electrons are transferred from water to Photosystem II and then passed through the ETC to Photosystem I
Movement of electrons
Photosystem I and II
Photosystem reaction centers
Photosystem I (P700|)
Photosystem II (P680) and Photosystem I (P700)
Role of water
Water is not split
Water is split to replace the electrons lost by Photosystem II
Occurs in some photosynthetic bacteria and in some eukaryotic cells
Occurs in all photosynthetic organisms
The main difference between cyclic and noncyclic photophosphorylation is the source of electrons that drives the electron transport chain. In noncyclic photophosphorylation, electrons are transferred from water to NADP+ to generate NADPH, while ATP is generated by the proton gradient across the thylakoid membrane. In cyclic photophosphorylation, electrons are recycled through the electron transport chain to generate a proton gradient and ATP, but no NADPH or oxygen is produced. Another difference is the type of organisms that use each process. Noncyclic photophosphorylation is used by green plants, algae, and cyanobacteria, while cyclic photophosphorylation is used by certain types of bacteria such as purple bacteria, green sulfur bacteria, and heliobacteria.
FAQs on Difference Between Cyclic and Noncyclic Photophosphorylation
1. What is the difference between cyclic and noncyclic photophosphorylation?
The main difference between cyclic and non-cyclic photophosphorylation is the pathway of electron flow in the light-dependent reactions of photosynthesis. Non-cyclic photophosphorylation involves the transfer of electrons from water to Photosystem II (PSII) to Photosystem I (PSI) to NADP+ to form NADPH, while also generating ATP via ATP synthase. This process is the primary pathway for ATP and NADPH production during photosynthesis, and it also produces oxygen as a byproduct of water oxidation. Cyclic photophosphorylation, on the other hand, involves a circular pathway of electrons from PSI back to the cytochrome b6f complex, which generates a proton gradient that drives ATP synthesis via ATP synthase. This process generates only ATP and does not produce oxygen.
2. Is non-cyclic photophosphorylation light dependent?
Yes, non-cyclic photophosphorylation is light-dependent. During non-cyclic photophosphorylation, light energy is absorbed by pigments such as chlorophyll in Photosystem II, which excites electrons that are transferred to Photosystem I via an electron transport chain. In Photosystem I, the electrons are re-energized by light energy and are used to reduce NADP+ to NADPH. The electron transport chain generates a proton gradient that drives ATP synthesis via ATP synthase. Therefore, non-cyclic photophosphorylation requires light energy to excite electrons and generate a proton gradient for ATP synthesis.
3. How many ATP are produced in cyclic photophosphorylation?
Cyclic photophosphorylation generates only ATP, but not NADPH or oxygen. The exact number of ATP molecules produced in cyclic photophosphorylation is not fixed and can vary depending on the conditions. In general, a single cycle of cyclic photophosphorylation can produce 1-2 molecules of ATP, which is much less than the yield of ATP from non-cyclic photophosphorylation. However, cyclic photophosphorylation can be important for balancing the ATP/NADPH ratio in the chloroplast and can help regulate the electron flow in the thylakoid membrane.