Difference Between Cyclic And Noncyclic Photophosphorylation: Noncyclic Photophosphorylation

What Is Photophosphorylation?

Photophosphorylation is a process in which light energy is used in the light-dependent reactions to synthesise ATP from ADP and inorganic phosphate. This activity is an essential part of the light energy conversion into chemical energy stored in the form of ATP and occurs within the membranes of thylakoids inside chloroplasts.

Photophosphorylation is a part of the photosynthesis by which ATP and NADPH are produced, the essential energy carriers for the subsequent cycle of Calvin. These molecules carry energy and reduce power to drive carbon dioxide into glucose and other organic compounds supporting plants' energy needs for survival and growth.

There are two different pathways of photophosphorylation producing ATP in the light reactions of photosynthesis: cyclic and noncyclic photophosphorylation. Both are essential steps of light-dependent reaction, although the two processes play different roles in supporting the overall photosynthetic process.

Definition Of Cyclic Photophosphorylation

Cyclic photophosphorylation is light-dependent and the process by which ATP is produced using only Photosystem I and involves the cyclic flow of electrons. This does not yield NADPH or oxygen and seems to be mainly implicated in replenishing ATP levels when their levels are needed more than those of NADPH.

Definition Of Noncyclic Photophosphorylation

Noncyclic photophosphorylation involves both photosystems, PS II and PS I, and the result is the formation of ATP and NADPH with the release of oxygen gas. It forms a part of the light-dependent reaction of photosynthesis and provides energy and reducing power to the Calvin cycle.

Steps In Cyclic Photophosphorylation

The steps of cyclic photophosphorylation includes:

• Light Absorption: Light absorption by photosystem I excites electrons.

• Electron Transport: The electrons transfer through a series of transport proteins in the thylakoid membrane.

• ATP Synthesis: The energy from electron transport is used by ATP synthase to drive the synthesis of ATP.

• Return of Electron: This step is completed by cycling back the electrons to Photosystem I.

Steps In Noncyclic Photophosphorylation

The steps of non-cyclic photophosphorylation includes:

• Light Absorption: Light is absorbed by photosystem II and results in the splitting of the water molecule.

• Electron Transport: The electrons are passed through the electron transport chain, thus creating a proton gradient.

• ATP and NADPH Synthesis: The energy from this gradient of protons drives the process of synthesising ATP. These electrons reduce NADP+ into NADPH.

• Oxygen Release: Water is split to release oxygen as a byproduct.

Difference Between Cyclic And Noncyclic Photophosphorylation

The difference between cyclic and noncyclic photophosphorylation in given in the table below:

Role And Significance

Cyclic and non-cyclic photophosphorylation play different roles.

Cyclic Photophosphorylation

• ATP Supply: Provides ATP for other cellular processes.

• Balancing ATP and NADPH: Maintains the level of ATP when there is excess NADPH.

• Acts in Stress: Acts as a protective mechanism under high light intensity.

Noncyclic Photophosphorylation

• ATP and NADPH: The Calvin cycle is supplied with energy and reducing power.

• Oxygen Production: Releases oxygen which is necessary for respiration in aerobic organisms.

• Main Pathway: Acts as a major pathway during the normal photosynthesis.

Interrelation Between Both Pathways

Although both pathways are different, they are interrelated in following ways:

• Both pathways operate simultaneously in chloroplasts.

• When the demand of ATP is more than NADPH, cyclic flow is increased to fulfill the demand.

• Together they maintain the optimal ratio of ATP and NADPH i.e., 3:2 for carbon fixation.

Cyclic vs 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 occurs only when cells are in a need of

1. NADPH

2. Oxygen

3. ATP

4. Both ATP and NADPH

Correct answer: 3) ATP

Explanation:

Cyclic photophosphorylation takes place only if the plant cells need ATP.
Key Points Regarding Cyclic Photophosphorylation:
Photosystem Involvement: This process is only involved by Photosystem I.
Products: The prime product is ATP and doesn't produce NADPH or O₂ as a byproduct.
Function: It is used when the cell needs to produce more ATP for such processes as the Calvin cycle especially when NADPH is high or in low light intensity.
Cyclic photophosphorylation is important for the efficient manner in which plants can fulfil their energy needs under given circumstances.

Hence, the correct answer is option 3. 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

• C4 and CAM Pathway

• Photorespiration