Difference Between Cyclic and Noncyclic Photophosphorylation

Main Difference – Cyclic vs Noncyclic Photophosphorylation

During the light reaction of photosynthesis, the high energy electrons are produced by the capturing of light energy by the photosystems. These high energy electrons are expelled from the photosystems and are passed through a series of molecular complexes known as electron transport system (ETS), synthesizing ATP. This process is referred as the photophosphorylation. The two types of photophosphorylation are cyclic and noncyclic phosphorylation. Cyclic photophosphorylation occurs during anoxygenic photosynthesis while noncyclic photophosphorylation occurs in oxygenic photosynthesis. The main difference between cyclic and noncyclic photophosphorylation is that in cyclic photophosphorylation, the electrons move in a circular pattern whereas, in noncyclic photophosphorylation, the electrons move in a linear pattern. 

Key Areas Covered

1. What is Cyclic Photophosphorylation
     – Definition, Mechanism, Significance
2. What is Noncyclic Photophosphorylation
     – Definition, Mechanism, Significance
3. What are the Similarities Between Cyclic and Noncyclic Photophosphorylation
     – Outline of Common Features
4. What is the Difference Between Cyclic and Noncyclic Photophosphorylation
     – Comparison of Key Differences

Key Terms: Cyclic  Photophosphorylation, Electron Transport System (ETS), NADP, Noncyclic  Photophosphorylation, Oxygen, PS I, PS II

What is Cyclic  Photophosphorylation

Cyclic photophosphorylation refers to the synthesis of ATP during the light reaction of photosynthesis, coupling to a cyclic passage of electrons to and from photosystem I (P700). Thus, only a single type of photosystem is involved in cyclic photophosphorylation. The expelled high energy electrons pass through the ETS and return back to the P700. Hence, NADP+ is not used as the final electron acceptor. As photosystem II is not used during cyclic photophosphorylation, no oxygen is produced in the cyclic photophosphorylation. Cyclic photophosphorylation is shown in figure 1.

Figure 1: Cyclic Photophosphorylation

Generally, cyclic photophosphorylation occurs in photosynthetic bacteria such as green sulfur and nonsulfur bacteria, purple bacteria, heliobacteria, and acidobacteria. When the ATP supply drops and under high NADPH concentrations, chloroplasts also shift into cyclic photophosphorylation.

What is Noncyclic  Photophosphorylation

Noncyclic photophosphorylation refers to the synthesis of ATP during the light reaction of photosynthesis where an electron donor is required and oxygen is produced as a byproduct. Both photosystem I (P700) and photosystem II (P680) are used in noncyclic photophosphorylation. The high energy electrons expelled from P680 pass through the ETS and return to P700. At P700, these electrons are taken up by NADP+, producing NADPH. At P680, photolysis occurs, splitting water to replace the released electrons of P680. During this process, oxygen is produced as a byproduct. The noncyclic photophosphorylation is shown in figure 2.

Figure 2: Noncyclic  Photophosphorylation

Generally, noncyclic photophosphorylation occurs in plants, algae, and cyanobacteria. During noncyclic photophosphorylation, both ATP and NADPH are produced.

Similarities Between Cyclic and Noncyclic Photophosphorylation

• Both cyclic and noncyclic photophosphorylation occur during the light reaction of photosynthesis.
• Cyclic and noncyclic photophosphorylation are two types of ETS.
• Both cyclic and noncyclic photophosphorylation are light-dependent.
• Both cyclic and noncyclic photophosphorylation generate ATP.

Difference Between Cyclic and Noncyclic Photophosphorylation

Definition

Cyclic Photophosphorylation: Cyclic photophosphorylation refers to the synthesis of ATP during the light reaction of photosynthesis, coupling to a cyclic passage of electrons to and from P700.

Noncyclic Photophosphorylation: Noncyclic photophosphorylation refers to the synthesis of ATP during the light reaction of photosynthesis in which an electron donor is required and oxygen is produced as a byproduct.

Occurrence

Cyclic Photophosphorylation: Cyclic photophosphorylation occurs in isolated chloroplasts and photosynthetic bacteria.

Noncyclic Photophosphorylation: Noncyclic photophosphorylation occurs in plants, algae, and cyanobacteria.

Type of  Photosynthesis

Cyclic Photophosphorylation: Cyclic photophosphorylation occurs in anoxygenic photosynthesis.

Noncyclic Photophosphorylation: Noncyclic photophosphorylation occurs in oxygenic photosynthesis.

Electron Movement

Cyclic Photophosphorylation: Electrons move in a cyclic pattern in cyclic photophosphorylation.

Noncyclic Photophosphorylation: Electrons move in a linear pattern in noncyclic photophosphorylation.

Photosystems

Cyclic Photophosphorylation: Only photosystem I is involved in the cyclic photophosphorylation.

Noncyclic Photophosphorylation: Both photosystems I and II are involved in the noncyclic photophosphorylation.

Electrons First Expelled from

Cyclic Photophosphorylation: Electrons are first-expelled from the reaction center of PS I in cyclic photophosphorylation.

Noncyclic Photophosphorylation: Electrons are first-expelled from the reaction center of PS II in noncyclic photophosphorylation.

Fate of the Electrons

Cyclic Photophosphorylation: Electrons return to the P700 after passing through ETS in cyclic photophosphorylation.

Noncyclic Photophosphorylation: Electrons return to the reaction center of P680 and they are accepted by NADP+ in noncyclic photophosphorylation.

Final Electron Acceptor

Cyclic Photophosphorylation: The final electron acceptor of the cyclic Photophosphorylation is P700.

Noncyclic Photophosphorylation: The final electron acceptor of the noncyclic photophosphorylation is NADP+.

Photolysis

Cyclic Photophosphorylation: Photolysis does not occur in cyclic photophosphorylation.

Noncyclic Photophosphorylation: Photolysis occurs in noncyclic photophosphorylation.

Oxygen

Cyclic Photophosphorylation: Oxygen is not produced in cyclic photophosphorylation.

Noncyclic Photophosphorylation: Oxygen is produced in noncyclic photophosphorylation.

Result

Cyclic Photophosphorylation: Only ATP is produced in cyclic photophosphorylation.

Noncyclic Photophosphorylation: Both ATP and reduced coenzymes are produced in noncyclic photophosphorylation.

Effect of Light

Cyclic Photophosphorylation: Cyclic photophosphorylation occurs under low light intensity.

Noncyclic Photophosphorylation: Noncyclic photophosphorylation occurs under higher light intensity.

Anaerobic/Aerobic

Cyclic Photophosphorylation: Cyclic photophosphorylation mainly occurs in anaerobic conditions.

Noncyclic Photophosphorylation: Noncyclic photophosphorylation mainly occurs in aerobic conditions.

Inhibition

Cyclic Photophosphorylation: Cyclic photophosphorylation cannot be inhibited by Diuron.

Noncyclic Photophosphorylation: Noncyclic photophosphorylation is inhibited by Diuron.

Conclusion

Cyclic and noncyclic photophosphorylation are the two mechanisms of photophosphorylation that occur during the light reaction of photosynthesis. Cyclic photophosphorylation occurs in photosynthetic bacteria during anoxygenic photosynthesis.  Noncyclic photophosphorylation occurs in plants, algae, and cyanobacteria during oxygenic photosynthesis. Electrons move in a cycle during cyclic photophosphorylation while they are not recycled in noncyclic photophosphorylation. The main difference between cyclic and noncyclic photophosphorylation is their movement of electrons.

Reference:

1. “Cyclic vs. Non-Cyclic Electron Flow.” Mandeville High School, Available here.

Image Courtesy:

1. “Thylakoid membrane 3” By Somepics – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Cyclic Photophosphorylation” By David Berard – Own work (CC0) via Commons Wikimedia

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