How Do Cyclic and Non-Cyclic Photophosphorylation Differ in Photosynthesis?
Photosynthesis is referred to as the process of converting the light energy of the Sun into chemical energy. During this process, the light energy gets captured and is then used to convert carbon dioxide and water to glucose and oxygen.
Photosynthetic processes can be divided into two categories: oxygenic and anoxygenic. Both work on the same principles, although plants, algae, and cyanobacteria use oxygenic photosynthesis the most.
Light energy transfers electrons from water (H2O) taken up by plant roots to CO2 to make carbohydrates during oxygenic photosynthesis. The CO2 is "reduced," or gains electrons, while the water is "oxidised," or loses electrons, in this process. Along with carbs, oxygen is generated.
6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O
"Anoxygenic Photosynthetic Bacteria," anoxygenic photosynthesis uses electron donors that aren't water and don’t produce oxygen. Green sulfur bacteria and phototrophic purple bacteria are among the microorganisms that undergo this activity.
CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O
However, this entire process of photosynthesis occurs in two different processes:
Light reaction and dark reaction.
Light Reaction
The light reaction of the photosynthesis occurs in the chloroplast inside the grana. In this reaction, the light energy is converted to chemical energy in the form of ATP and NADPH. In this reaction, when phosphate is added in the presence of sunlight or by the process of ATP synthesis by cells, it is referred to as photophosphorylation. Carotenoids make up the accessory pigments. The chlorophyll in the thylakoid membrane of chloroplasts absorbs the energy from the sun. Two-electron transport chains generate ATP and NADPH, which are then transferred to ATP and NADPH. During the process, both water and oxygen are utilised.
Dark Reaction
In the dark reaction of photosynthesis, the energy which is produced in the light reaction is used for converting carbon dioxide into carbohydrates. This reaction happens in the stroma of the chloroplasts. The nighttime reactions of photosynthesis are propelled by the energy provided by ATP (made during the light reactions). The phrase "dark reactions" does not imply that the reactions take place at night or that darkness is required. It means that the reactions can continue regardless of how much light is present. The phrase is solely used to differentiate between dark and light reactions, both of which require light.
Students can refer to the Light Dependent Reactions page for more information.
What is Photophosphorylation?
Otto Kandler published the first experimental evidence for photophosphorylation in vivo in 1950, utilising intact Chlorella cells and interpreting his findings as light-dependent ATP production. With the use of P32, Daniel I. Arnon identified photophosphorylation in isolated chloroplasts in vitro in 1954. In 1956, he released his first review of early photophosphorylation studies.
Photophosphorylation is the process in which light energy is used from photosynthesis to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP). It is the process in which the energy-rich ATP molecules are synthesised by the transfer of the phosphate group to the ADP molecule during the presence of sunlight.
Photophosphorylation is of Two Different Types:
Non-cyclic photophosphorylation
Difference Between Cyclic and Non-Cyclic Photophosphorylation
Cyclic Photophosphorylation
Cyclic photophosphorylation is a process that results in the movement of the electrons in a cyclic way to synthesise the ATP molecules. In this process, the plant cells convert ADP to ATP to gain immediate energy for their cells. The process of cyclic photophosphorylation generally occurs in the thylakoid membrane and makes use of Photosystem I and Chlorophyll P700.
Non-cyclic Photophosphorylation
Non-cyclic photophosphorylation is a process that results in the movement of the electrons in a non-cyclic way to synthesise the ATP molecules by using the energy from the excited electrons that are provided by Photosystem II.
Students Can Check All Other Biology-related Topics for Their Reference-
FAQs on Cyclic vs Non-Cyclic Photophosphorylation: Clear Comparison
1. What is the primary difference between cyclic and non-cyclic photophosphorylation?
The primary difference lies in the electron pathway. In cyclic photophosphorylation, the excited electron that leaves Photosystem I (PS I) eventually returns to it. In contrast, during non-cyclic photophosphorylation, the electron travels in a one-way path from Photosystem II (PS II) to PS I and is finally accepted by NADP+, so it does not return to its starting point.
2. What happens during cyclic photophosphorylation?
During cyclic photophosphorylation, only Photosystem I (PS I) is active. Light excites an electron from the PS I reaction centre, which then passes through an electron transport chain and eventually cycles back to the same PS I. This electron flow generates a proton gradient that drives the synthesis of ATP, but it does not produce NADPH or release oxygen.
3. What are the key steps in non-cyclic photophosphorylation?
Non-cyclic photophosphorylation, also known as the Z-scheme, involves several key steps:
- Light energy excites an electron from Photosystem II (PS II).
- The photolysis of water occurs, splitting water to release electrons, protons (H+), and oxygen. The electrons replace those lost from PS II.
- The excited electron from PS II moves down an electron transport chain to Photosystem I (PS I), generating ATP in the process.
- PS I absorbs more light, re-energising the electron.
- The final electron acceptor, NADP+, is reduced to form NADPH.
4. Why are the products of cyclic and non-cyclic photophosphorylation different?
The products differ because of their distinct electron pathways and components. Cyclic photophosphorylation only involves PS I in a short electron circuit, so its sole purpose is to produce additional ATP when the cell's energy demand is high. Non-cyclic photophosphorylation involves both PS II and PS I, the photolysis of water, and the reduction of NADP+. This complete, linear pathway results in the production of ATP, NADPH, and oxygen, which are all essential for the subsequent Calvin cycle.
5. How is the Z-scheme related to non-cyclic photophosphorylation?
The Z-scheme is the descriptive name given to the graphical representation of the electron flow in non-cyclic photophosphorylation. When the redox potential or energy level of the electrons is plotted as they move from PS II, down the electron transport chain, up to PS I, and finally to NADP+, the resulting shape resembles the letter 'Z'. It visually illustrates the entire energy journey of the electron in this process.
6. Which type of photophosphorylation produces oxygen and why?
Non-cyclic photophosphorylation is the process that produces oxygen. This happens because it involves the photolysis (splitting) of water molecules at Photosystem II to replace the electrons lost from its reaction centre. Oxygen is released as a crucial byproduct of this water-splitting reaction. Cyclic photophosphorylation does not involve PS II or water splitting, so no oxygen is generated.
7. What are the main similarities between cyclic and non-cyclic photophosphorylation?
Both processes share several fundamental characteristics:
- Both occur in the thylakoid membranes of chloroplasts.
- Both are part of the light-dependent reactions of photosynthesis.
- Both are initiated by the absorption of light energy by photosystems.
- Both utilise an electron transport chain to facilitate chemiosmosis.
- Both processes ultimately result in the synthesis of ATP (phosphorylation).
8. Under what conditions does a plant favour cyclic over non-cyclic photophosphorylation?
A plant performs non-cyclic photophosphorylation under normal conditions. However, it switches to or supplements with cyclic photophosphorylation when the Calvin cycle consumes more ATP than NADPH, creating an imbalance. This can happen under conditions of high light intensity or when low CO₂ levels limit the Calvin cycle's consumption of NADPH, leading to a build-up of reducing power. The cyclic pathway helps balance the ATP/NADPH ratio by producing only ATP.
9. Why is the production of both ATP and NADPH in the non-cyclic pathway so important?
The production of both ATP and NADPH is crucial because they are the two essential products required for the next stage of photosynthesis, the Calvin cycle. In the Calvin cycle, ATP provides the necessary energy, and NADPH provides the reducing power to convert atmospheric carbon dioxide into glucose and other organic molecules. Without both, the synthesis of carbohydrates would not be possible.
10. Which photosystems (PS I and PS II) are involved in each type of photophosphorylation?
The involvement of photosystems is a key distinguishing factor. Cyclic photophosphorylation exclusively uses Photosystem I (PS I). In contrast, non-cyclic photophosphorylation requires the coordinated action of both Photosystem II (PS II) and Photosystem I (PS I) to move electrons from water to NADP+.
