What is the Difference Between Cyclic and Noncyclic Electron Flow

Cyclic and noncyclic electron flow are two types of electron flows in photosynthesis phosphorylation. Photophosphorylation is the process of using light energy from photosynthesis in order to convert ADP to ATP or for ATP production.  This process synthesizes energy-rich ATP molecules by transferring the phosphate group into the ADP molecule when light is present.

The difference between cyclic and noncyclic electron flow is their result. Cyclic flow produces only ATP, while noncyclic flow produces both ATP and NADPH.

CONTENTS

1. Overview and Key Difference
2. What is Cyclic Electron Flow
3. What is Noncyclic Electron Flow
4. Similarities – Cyclic and Noncyclic Electron Flow
5. Cyclic vs Noncyclic Electron Flow in Tabular Form
6. Summary – Cyclic vs Noncyclic Electron Flow
7. FAQ – Cyclic and Noncyclic Electron Flow

What is Cyclic Electron Flow?

Cyclic electron flow is a type of electron flow that begins with the absorption of a photon by photosystem I (PSI) in photosynthesis. Cyclic electron flow occurs in cyclic phosphorylation. In cyclic phosphorylation, plant cells convert ADP to ATP to provide immediate energy for cells. Cyclic phosphorylation takes place in the thylakoid membrane and uses photosystem I (PSI) and chlorophyll P700.

Figure 01: Cyclic Electron Flow

Moreover, in cyclic phosphorylation, electrons are transferred back to chlorophyll P700 instead of moving into the NADP from the electron acceptor. This downward movement results in the formation of ATP molecules.

What is Noncyclic Electron Flow?

Noncyclic electron flow is a type of electron flow that begins with the absorption of a photon by PSII. Noncyclic electron flow occurs in the noncyclic phosphorylation of photosynthesis. In noncyclic photophosphorylation, the electrons move in a noncyclic manner for synthesizing ATP molecules using the energy from excited electrons provided by photosystem II. Moreover, in noncyclic phosphorylation, the lost electrons by P680 of photosystem II are occupied by P700 of photosystem I and are not reverted to P680. Therefore, the electron flow is unidirectional in the noncyclic phosphorylation.

Figure 02: Noncyclic Electron Flow

Furthermore, in this process, electrons released by P700 are carried by a primary electron acceptor and finally passed on to NADP. Later, these electrons combine with the H+, which is produced by the splitting of water. This reduces NADP to NADPH2.

Similarities Between Cyclic and Noncyclic Electron Flow

1. Cyclic and noncyclic electron flow are two types of electron flow in the phosphorylation.
2. Photosystem I is involved in both electron flow.
3. Both types of electron flow result in the generation of ATP.
4. They are extremely important for the survival of organisms.

Difference Between Cyclic and Noncyclic Electron Flow

Definition

1. Cyclic electron flow is a type of electron flow that occurs in the cyclic phosphorylation of photosynthesis.
2. Noncyclic electron flow is a type of electron flow that occurs in noncyclic phosphorylation of photosynthesis.

Electron Flow

1. Electron flow is cyclic in cyclic electron flow.
2. Electron flow is unidirectional in noncyclic electron flow.

Active Reaction Center

1. P700 is the active reaction center in cyclic electron flow.
2. P680 is the active reaction center in noncyclic electron flow.

Photosystem Involved

1. Only photosystem I is involved in cyclic electron flow.
2. Both photosystem I and II are involved in noncyclic electron flow.

Nature of Electron Flow

1. Electrons revert to photosystem I in cyclic electron flow.
2. Electrons from photosystem I are accepted by NADP in noncyclic electron flow.

Requirement of Water

1. Water is not required in cyclic electron flow.
2. Photolysis of water is present in noncyclic electron flow.

Synthesis of NADPH

1. NADPH is not synthesized after cyclic electron flow.
2. NADPH is synthesized after noncyclic electron flow.

Involvement of Oxygen

1. Oxygen is not evolved as the by-product after cyclic electron flow.
2. Oxygen is evolved as the by-product after noncyclic electron flow.

Examples

1. Cyclic electron flow is predominant only in bacteria.
2. Noncyclic electron flow is predominant in all green plants.

The following infographic summarizes the difference between cyclic and noncyclic electron flow.

Summary – Cyclic vs Noncyclic Electron Flow

Photophosphorylation is the important process of utilizing light energy from photosynthesis in order to convert ADP to ATP. Cyclic and noncyclic electron flow are two types of electron flow in photosynthesis phosphorylation. Cyclic electron flow is present in cyclic phosphorylation, while noncyclic electron flow is present in noncyclic phosphorylation. Moreover, electron flow is cyclic in cyclic electron flow, whereas electron flow is unidirectional in noncyclic electron flow. Further, the cyclic electron flow is only predominant in bacteria. On the other hand, noncyclic electron flow is predominant in all green plants. This is the summary of the difference between cyclic and noncyclic electron flow.

FAQ: Cyclic and Noncyclic Electron Flow

1. What does it mean by phosphorylation in photosynthesis?

• Phosphorylation is a chemical process of adding a phosphate group to an organic compound. In photosynthesis,  phosphorylation occurs by adding a phosphate group of ADP to form ATP using the energy of sunlight.

2. How many ATP are formed in cyclic photophosphorylation?

• Cyclic phosphorylation occurs when electrons move in a cyclic manner to synthesize ATP molecules. Cyclic photophosphorylation generates only two ATP molecules.

3. How many ATPs are produced in noncyclic photophosphorylation?

• Noncyclic phosphorylation occurs when electrons move in a noncyclic manner to synthesize ATP molecules.  Noncyclic photophosphorylation produces 1 ATP and 2 NADPH2 molecules. Overall, 7 ATP molecules are produced during noncyclic photophosphorylation.

4. What happens in cyclic electron flow in photosynthesis?

• A second electron transport pathway called cyclic electron flow produces ATP without the synthesis of NADPH, thereby supplying additional ATP for other metabolic processes. Moreover, in cyclic electron flow, the light energy harvested at photosystem I is used for ATP synthesis rather than NADPH synthesis.

5. What happens in the noncyclic electron transport in photosynthesis?

• Noncyclic electron flow or transport happens in noncyclic photophosphorylation. In this process, electrons that are expelled from the exciting photo center do not return. This process happens when both photosystems I and II are involved. The photolysis of water leads to the release of electrons; hence, a constant water supply is needed for noncyclic photophosphorylation.

Reference:

1. Cooper, Geoffrey M. “Photosynthesis.” The Cell: A Molecular Approach. 2nd Edition., U.S. National Library of Medicine.
2. “Noncyclic Electron Flow.” Encyclopædia Britannica, Encyclopædia Britannica, inc.

Image Courtesy:

1. “Cyclic Photophosphorylation” By David Berard – Own work (CC0) via Commons Wikimedia

2. “Photosynthesis electron transport chain” By Taiz and Zeiger, Plant Physiology, 4th edition – (CC BY 2.0) via Commons Wikimedia

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