Difference Between Oxygenic and Anoxygenic Photosynthesis

Main Difference – Oxygenic vs Anoxygenic Photosynthesis

The process that converts light energy into chemical energy is known as photosynthesis. This chemical energy is used by organisms in different metabolic processes. The organisms that undergo photosynthesis are called photoautotrophs. Plants, algae, cyanobacteria, and bacteria are photoautotrophs. Oxygen and water are the byproducts of photosynthesis. Oxygenic and anoxygenic photosynthesis are two types of photosynthesis classified based on the ability to produce oxygen. The main difference between oxygenic and anoxygenic photosynthesis is that oxygenic photosynthesis produces oxygen as a byproduct whereas anoxygenic photosynthesis does not produce oxygen as a byproduct.

Key Areas Covered

1. What is Oxygenic Photosynthesis
     – Definition, Process, Significance
2. What is Anoxygenic Photosynthesis
     – Definition, Process, Significance
3. What are the Similarities Between Oxygenic and Anoxygenic Photosynthesis
     – Outline of Common Features
4. What is the Difference Between Oxygenic and Anoxygenic Photosynthesis
     – Comparison of Key Differences

Key Terms: Anoxygenic Photosynthesis, Cyclic Photophosphorylation, Noncyclic Photophosphorylation, Oxygen, Oxygenic Photosynthesis, PS I, PS II

Difference Between Oxygenic and Anoxygenic Photosynthesis - Comparison Summary

What is Oxygenic Photosynthesis

Oxygenic photosynthesis refers to the photosynthesis that occurs in plants, algae, and cyanobacteria in which the final electron acceptor is water. It occurs in two steps: light reaction and dark reaction. The light-trapping pigments used in oxygenic photosynthesis are chlorophyll A and B. The energy trapped by chlorophyll A is passed to the photosystem II (PS II) (P680) and photosystem I (PS I) (P700) in the form of high energy electrons. PS II takes electrons by splitting water molecules into molecular oxygen, generating high energy electrons, which are transferred through a series of electron carriers into PS I. Splitting of water at PS II is called photolysis. PS I also generates high energy electrons by the energy of sunlight. These electrons are used in the formation of NADPH by the enzyme, NADP+ reductase. ATP synthase utilizes H+ ions, which are generated by photolysis in order to produce ATP. The overall reaction of photosynthesis is shown in figure 1.

Main Difference - Oxygenic vs Anoxygenic Photosynthesis

Figure 1: Oxygenic Photosynthesis

During the dark reaction of photosynthesis, glucose is produced from the energy of ATP and NADPH produced in the light reaction.

What is Anoxygenic Photosynthesis

Anoxygenic photosynthesis refers to the photosynthesis in bacteria that occurs under anaerobic conditions, using inorganic molecules as the electron source other than H2O. It occurs in green sulfur and nonsulfur bacteria, purple bacteria, heliobacteria, and acidobacteria. In photosynthetic bacteria, P680 is not present. H2O is too electropositive to be used as an electron source in anoxygenic photosynthesis. Based on the species of the bacteria, the type of pigments present in the PS I may differ. It can be either chlorophyll or bacteriochlorophyll. P870 is the reaction center in purple bacteria. The inorganic electron donor in the PS I may be hydrogen, hydrogen sulfide or ferrous ions. The anoxygenic photosynthesis is shown in figure 2.

Difference Between Oxygenic and Anoxygenic Photosynthesis

Figure 2: Anoxygenic Photosynthesis

In anoxygenic photosynthesis, NADP is not the terminal electron acceptor. The electrons cycle back into the system and ATP is produced by cyclic photophosphorylation.   

Similarities Between Oxygenic and Anoxygenic Photosynthesis

  • Oxygenic and anoxygenic photosynthesis are two types of photosynthesis.
  • Photoautotrophs undergo both oxygenic and anoxygenic photosynthesis.
  • Both oxygenic and anoxygenic photosynthesis occurs in two steps: light dependent reaction and dark reaction.

Difference Between Oxygenic and Anoxygenic Photosynthesis

Definition

Oxygenic Photosynthesis: Oxygenic photosynthesis refers to the photosynthesis that occurs in plants, algae, and cyanobacteria in which the final electron acceptor is water.

Anoxygenic Photosynthesis: Anoxygenic photosynthesis refers to a form of photosynthesis used by certain bacteria, in which oxygen is not produced.

Occurrence

Oxygenic Photosynthesis: Oxygenic photosynthesis occurs in plants, algae, and cyanobacteria.

Anoxygenic Photosynthesis: Anoxygenic photosynthesis occurs in the green sulfur and nonsulfur bacteria, purple bacteria, heliobacteria and acidobacteria.

Photosystems

Oxygenic Photosynthesis: Both photosystem I and II are used in the oxygenic photosynthesis.

Anoxygenic Photosynthesis: Only photosystem I is used in the anoxygenic photosynthesis.

Electron Source

Oxygenic Photosynthesis: H2O is the electron source of the oxygenic photosynthesis.

Anoxygenic Photosynthesis:  Hydrogen, hydrogen sulfide or ferrous ions serves as the electron donor in anoxygenic photosynthesis.

Oxygen

Oxygenic Photosynthesis: Oxygen is produced during the light reaction in oxygenic photosynthesis.

Anoxygenic Photosynthesis: Oxygen is not produced during the light reaction in anoxygenic photosynthesis.

Photosynthetic Pigments

Oxygenic Photosynthesis: Chlorophylls are used in the oxygenic photosynthesis.

Anoxygenic Photosynthesis: Bacteriochlorophylls or chlorophylls are used in anoxygenic photosynthesis.

Mechanism of Generating NADPH

Oxygenic Photosynthesis: NADP serves as the terminal electron acceptor, producing NADPH in oxygenic photosynthesis.

Anoxygenic Photosynthesis: NADPH is not produced in anoxygenic photosynthesis as the electrons are cycled back to the system.

ATP Production

Oxygenic Photosynthesis: ATP is produced by noncyclic photophosphorylation in oxygenic photosynthesis.

Anoxygenic Photosynthesis: ATP is produced by cyclic photophosphorylation in anoxygenic photosynthesis.

Conclusion

Oxygenic and anoxygenic photosynthesis are two types of photosynthesis. Oxygenic photosynthesis occurs in plants, algae, and cyanobacteria. Anoxygenic photosynthesis occurs in cyanobacteria. Oxygen is released as a byproduct of oxygenic photosynthesis. However, oxygen is not produced as a byproduct of anoxygenic photosynthesis. The main difference between oxygenic and anoxygenic photosynthesis is the ability to produce oxygen during each type of photosynthesis.

Reference:

1. “Phototrophy.” Boundless Microbiology, .

Image Courtesy:

1. “Photosynthesis equation” By ZooFari – Own work (Public Domain) via
2. “Anoxygenic Photosynthesis in Green Sulfur Bacteria” By Lithium byproduct – Own work via

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things

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