Cyclic And Non Cyclic Photophosphorylation: Difference, Diagram

What Is Photophosphorylation?

Photophosphorylation 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. 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.

There are two major types of photophosphorylation based on the electron flow pathway:

1. Cyclic Photophosphorylation

2. Non-Cyclic Photophosphorylation

Cyclic Photophosphorylation

Cyclic photophosphorylation is the process of ATP synthesis by which electrons cycle around Photosystem I without generating NADPH or oxygen.

Mechanism

• 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 Photons: 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.

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+.

Mechanism

• 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 and NADPH 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

The difference between cyclic and non-cyclic photophosphorylation is:

Role of Photosystem I and II

The PSI and PSII operate in the Z-scheme to help in electron transport and energy conversion.

Importance of Photophosphorylation in Photosynthesis

The importance of photophosphorylation in photosynthesis includes:

• It provides ATP and NADPH for the Calvin cycle or dark reactions.

• It supports the synthesis of glucose and other organic compounds.

• It regulates the energy homeostasis within chloroplasts.

• It balances light harvesting complex and carbon fixation in various environmental conditions.

Cyclic And Non-Cyclic Photophosphorylation NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Mechanism of Cyclic and Non-cyclic Photophosphorylation

• Cyclic vs Non-cyclic Photophosphorylation

Practice Questions for NEET

Q1. Cyclic photophosphorylation results in the formation of

1. NADPH + H+

2. ATP AND NADPH + H+

3. ADP and NADP

4. ATP

Correct answer: 4) ATP

Explanation:

Cyclic Photophosphorylation -

Operates at low light intensity, low CO2 availability and anaerobic condition, and only ATP is produced.

- wherein

It requires no external electron donor, and no photolysis of H2O

Hence, the correct answer is option 4) ATP.

Q2. Where does Cyclic Photophosphorylation occur in chloroplasts?

1. Intermembrane space

2. Stroma lamellae

3. Grana lamellae

4. Thylakoid lumen

Correct answer: 2) Stroma lamellae

Explanation:

The chloroplasts' Stroma lamellae are where cyclic photophosphorylation takes place.

The grana, or stacked thylakoid discs, are connected via the stroma lamellae, portions of the thylakoid membranes. Photosystem I (PSI), which is found in the stroma lamellae, is the primary site of cyclic photophosphorylation.
Through the cyclic passage of electrons through PSI, ATP is produced in this process without the formation of oxygen or NADPH.
Both ATP and NADPH are produced by non-cyclic photophosphorylation, which is carried out by grana lamellae, or the stacked thylakoid membranes.

Hence, the correct answer is option 2)Stroma lamellae

Q3. Cyclic photophosphorylation is functional under

1. Low light intensity

2. High CO2 availability

3. Aerobic conditions

4. Anaerobic conditions

5. Low CO2 availability

6. High light intensity

Choose the correct option

1. I, IV and V

2. I. III and IV

3. II, III and V

4. I and VI

Correct answer: 1) I, IV and V

Explanation:

It operates under aerobic conditions coupled with optimum light intensity in the functioning of cyclic photophosphorylation. The process mainly takes place in the thylakoid membranes of chloroplasts and involves only photosystem I, where re-excited electrons are recycled back to the photosystem, resulting in the production of ATP without oxygen or NADPH release. Multiple sources indicate that it does not operate effectively under low light intensity or anaerobic conditions.

Hence, the correct answer is option 1) I, IV, and V

Also Read:

• MCQs on Cyclic Photophosphorylation

• Carbon Fixation

• C4 and CAM Pathway