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Cyclic photophosphorylation refers to the photophosphorylation process which occurs in the cyclic movement of electrons during the synthesis of ATP molecules. In this mechanism, plant cells only convert ADP to ATP to supply instant energy.
- Phosphorylation is considered an essential process during photosynthesis.
- During photosynthesis, the conversion or phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) using photons of sunlight is known as phosphorylation.
- Two types of photophosphorylation occur in cells: Cyclic and Non Cyclic Photophosphorylation.
- In noncyclic photophosphorylation, an electron from chlorophyll travels through the electron transport system before reducing NADP+ to generate a molecule of NADPH.
- Non-cyclic photophosphorylation is the process by which electrons travel non-cyclically in order to synthesize ATP molecules using the energy of excited electrons.
| Table of Content |
Key Terms: Photophosphorylation, Types of photophosphorylation, Cyclic photophosphorylation, Photosynthesis, Adenosine diphosphate, Adenosine triphosphate, Electrons
Process of Photosynthesis
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Photosynthesis is a biological process that converts light energy to chemical energy. This mechanism absorbs light energy and converts carbon dioxide and water into glucose and oxygen gas. Photosynthesis can be carried out by two different processes:
Light Reaction
The light reaction of photosynthesis happens in the presence of light, resulting in the photolysis of water and it occurs in the grana of the chloroplast.
- Here, light energy is transformed into chemical energy in the form of ATP and NADPH.
- Photophosphorylation is the process by which cells synthesize ATP in the presence of light.
Dark Reaction
During the dark reaction, the energy produced by the light reaction is used to convert carbon dioxide into carbohydrates. This occurs in the stroma of chloroplasts.
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What is Photophosphorylation?
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Photophosphorylation is the conversion of ADP to ATP using light energy from photosynthesis. It is the process of producing energy-rich ATP molecules by transferring the phosphate group into an ADP molecule in the presence of light.
There are two types of Photophosphorylation
- Cyclic Photophosphorylation
- Non-cyclic Photophosphorylation
Cyclic Photophosphorylation
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In prokaryotes, photosynthesis is used to produce energy rather than for the production of biological molecules. Cyclic photophosphorylation is the process by which organisms (such as prokaryotes) convert ADP to ATP for immediate energy in their cells.
- This type of photophosphorylation often takes place in the thylakoid membrane.
- In cyclic electron flow, the electron begins in the pigment complex known as photosystem I.
- It then moves from the primary acceptor to ferredoxin, and finally to cytochrome b6f.
- Cytochrome B6f is similar to that found in mitochondria.
- The electron is then transferred to plastocyanin before returning to chlorophyll.
- A proton-motive force is generated throughout this electron transport chain.
- This force pumps H+ ions across the membrane and creates a concentration gradient that can be used to activate ATP synthase during chemiosmosis.
- The complete mechanism is referred to as cyclic photophosphorylation. It generates neither O2 nor NADPH.
Cyclic Photophosphorylation
Difference between Cyclic and Non-Cyclic Photophosphorylation
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The following are the differences between Cyclic and Non-Cyclic Photophosphorylation
| Cyclic Photophosphorylation | Non-cyclic Photophosphorylation |
|---|---|
| Electrons travel in a cyclic manner. | Electrons travel in a non–cyclic manner. |
| The active reaction center is P700. | The active reaction center is P680. |
| Only Photosystem I is involved. | Photosystems I and II both are involved. |
| Electrons return to Photosystem I. | NADP accepts electrons from photosystem I. |
| Water is not required. | Photolysis of water is present. |
| ATP molecules are produced. | Both NADPH and ATP molecules are produced. |
| Oxygen is not produced as a byproduct. | Oxygen is produced as a byproduct. |
| NADPH is not synthesized. | NADPH is synthesized. |
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Things to Remember
- Photophosphorylation is the conversion of ADP to ATP using light energy from photosynthesis.
- Photosynthesis is a biological process that converts light energy to chemical energy.
- Two types of photophosphorylation occur in cells: Noncyclic Photophosphorylation and Cyclic Photophosphorylation.
- Cyclic photophosphorylation is the process by which organisms (such as prokaryotes) convert ADP to ATP for immediate energy in their cells.
- Non-cyclic photophosphorylation is the process by which electrons travel non-cyclically in order to synthesize ATP molecules using the energy of excited electrons.
- During the dark reaction, the energy produced by the light reaction is used to convert carbon dioxide into carbohydrates.
Sample Questions
Ques. What is the process of photophosphorylation? (2 Marks)
Ans. Photophosphorylation occurs in the presence of sunlight when chlorophyll absorbs light energy from the sun. The electrons reach an excited state and leave the chlorophyll center to bind to a protein in the electron transport circuit. This is the mechanism by which the plant cell converts light energy to chemical energy.
Ques. What are the two types of Photophosphorylation? (2 Marks)
Ans. Two types of photophosphorylation occur in cells:
- Noncyclic Photophosphorylation
- Cyclic Photophosphorylation.
Ques. What is the non-cyclic photophosphorylation process? (2 Marks)
Ans. Non-cyclic photophosphorylation occurs when electrons are ejected from the excited photo center but do not rejoin. Only when both photosystems I and II are engaged can this process occur. The photolysis of water releases electrons. As a result, a constant supply of water is required for this activity. This mechanism generates both ATP and NADPH.
Ques. What is the cyclic photophosphorylation process? (2 Marks)
Ans. Cyclic photophosphorylation refers to the photophosphorylation method that results in the cyclic movement of electrons during the creation of ATP molecules. In this mechanism, plant cells merely convert ADP to ATP to supply instant energy.
Ques. In which case Cyclic photophosphorylation is ideal? (1 Mark)
Ans. Cyclic photophosphorylation is ideal only in the case of bacteria.
Ques. How many ATP are formed in cyclic photophosphorylation? (1 Mark)
Ans. There are ATP molecules produced in Cyclic photophosphorylation.
Ques. Where does photophosphorylation occur? (2 Marks)
Ans. Photophosphorylation is a light-dependent process that occurs in plant cells' chloroplasts, especially the thylakoid membranes.
Ques. Why is cyclic photophosphorylation important? (2 Marks)
Ans. Cyclic photophosphorylation is constantly required because it produces ATP at low costs. Only photosystem I is involved in cyclic photophosphorylation.
Ques. What does cyclic photophosphorylation produce? (2 Marks)
Ans. Cyclic photophosphorylation requires the use of photosystem I. When light is absorbed by this photosystem, the excited electron enters the electron transport chain, which produces ATP.
Ques. What is a light reaction? (2 Marks)
Ans. The light reaction occurs in the thylakoid membranes. The light reaction absorbs light and uses energy to drive electrons from water, generating NADPH, and driving protons across the membrane.
Ques. What is a dark reaction? (2 Marks)
Ans. In the dark reaction, plants combine carbon dioxide with ATP and NADPH from the light reactions to make glucose. It takes place in the chloroplast's stroma.
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