Cyclic Photophosphorylation: Types And Process

What Is Photophosphorylation?

Photophosphorylation is the process by which light energy is converted into chemical energy in the form of ATP. This process takes place during photosynthesis in the chloroplasts of plant cells and the thylakoid membranes of cyanobacteria. In the presence of light, it further drives the synthesis of ATP from ADP and inorganic phosphate Pi.

There are two major types of photophosphorylation: cyclic and noncyclic photophosphorylation. Both include the transfer of electrons through a series of proteins in the thylakoid membrane. Non-cyclic photophosphorylation uses both photosystems I and II of the process and yields ATP and NADPH, whereas cyclic photophosphorylation involves only photosystem I, yielding only ATP.

Significance In Photosynthesis

• Photophosphorylation is the process by which ATP and NADPH are created for the Calvin cycle.

• In this cycle, incoming carbon dioxide is converted and fixed into organic molecules, fracturing glucose.

• Through photosynthesis, this energy-conversion process supports the growth and development of plants and their ability to bear fruit through reproduction.

• In addition to the production of vital products, photosynthetic activity helps maintain the balance of carbon dioxide and oxygen in the atmosphere.

What Is Cyclic Photophosphorylation?

Cyclic photophosphorylation is the process of ATP synthesis by which electrons cycle around Photosystem I without generating NADPH or oxygen. This mainly occurs when the cell has a higher demand for ATP over NADPH. It is a photodependent process that occurs in chloroplasts' thylakoid membranes especially in stromal lamellae and involves only Photosystem I.

This process is highly important in maintaining the proper ATP to NADPH ratio in the chloroplast during photosynthesis, particularly at times when the Calvin cycle requires more ATP than NADPH. It is active in higher plants, algae and cyanobacteria under low light or limited NADPH availability.

Mechanism Of Cyclic Photophosphorylation

This mechanism includes light absorption, excitation of electrons, transfer of electrons to the primary electron acceptor and finally synthesis of ATP.

Step 1 – Excitation Of Photosystem I (P700)

• Photosystem I functions after PSII in the electron transport chain.

• Energy from light is absorbed by PSI to transfer the electrons of the P700 chlorophyll to a higher state.

• Electrons are passed to the primary electron acceptor, ferredoxin.

Step 2 – Electron Transport Chain (ETC)

• Electrons then travel from ferredoxin to cytochrome b6f complex and finally to plastocyanin (Pc).

• As they move through the chain, protons are pumped into the thylakoid lumen, generating a proton gradient.

Step 3 – ATP Formation (Photophosphorylation)

• The ATP synthesis is driven through an enzyme called ATP synthase by the ETC proton gradient produced.

• The electrons return to P700, completing the cyclic flow.

Difference Between Cyclic And Non-Cyclic Photophosphorylation

The difference between cyclic and non-cyclic photophosphorylation is:

Significance Of Cyclic Photophosphorylation

The significance of the cyclic photophosphorylation is:

• It plays an important role in the energy balance of the light reactions taking place in the chloroplasts, mainly under light conditions where there is an ATP demand greater than that of NADPH.

• It provides additional ATP for the Calvin cycle since its metabolic demands go beyond the available light-energised reactions.

• It controls the ATP/NADPH ratio being produced in the chloroplast.

• It ensure a continuous supply of energy by the light variation.

Factors Affecting Cyclic Photophosphorylation

The rate and efficiency of cyclic photophosphorylation mainly depend on the light intensity, availability of NADP+, and a few other environmental characteristics that may affect its functioning.

Light Intensity

• If the intensity of light is high, then electrons in.

• Low light provides an opportunity to slow the rate of the process while still resulting in ATP.

• Optimum light helps increase the overall effectiveness of the process.

NADP+ Availability

• If there is a lack of NADP+, cyclic photophosphorylation is preferred to produce extra ATP.

• It maintains a ready supply of energy thereby allowing utilisation for the Calvin cycle.

Temperature & Water Stress

• Cyclic photophosphorylation is affected by temperature, water, and other environmental parameters.

• Plants can switch over a photophosphorylation pathway, and the stress conditions be adopted.

Cyclic Photophosphorylation In Different Organisms

Cyclic photophosphorylation is carried out in different photosynthetic organisms: cyanobacteria, algae, and higher plants. Each of the above groups has modified the process for their specific energy needs and their peculiar environmental situations.

Cyanobacteria

• Carry out cyclic photophosphorylation with the help of specialised thylakoid membranes.

• They occupy a vast range of varied and, in places, adverse habitats.

• They work in nitrogen fixation and primary production.

Algae

• Use cyclic photophosphorylation to balance the amounts of ATP and NADPH produced.

• Respond to changing light conditions in aquatic systems.

• Make significant contributions to global photosynthesis and carbon cycling.

Higher Plants

• Use cyclic photophosphorylation to provide extra ATP supply when metabolic need is high.

• Respond to changes in the intensity of light and other environmental changes

• Contract and role to perform an important function in the maintenance of efficient energy metabolism inside chloroplasts.

Cyclic Photophosphorylation NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Mechanism of Cyclic Photophosphorylation

• Factors affecting 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