Living entities necessitate energy to perform different activities on a daily basis. This energy is derived from the oxidation of proteins, carbohydrates, and fat, as seen in green plants that possess some chlorophyll-containing compounds used in the synthesis of food from water and carbon dioxide.
The phenomenon by which entities trap photons and store them as ATP (chemical energy) or/and NADPH is phototrophy. Phototrophy broadly could be of two kinds – rhodopsin-based retinal photography and chlorophyll-based chloro phototrophy.
Further, Chloro Phototrophy can be of the following 2 types –
- Oxygenic photosynthesis
- Anoxygenic photosynthesis
Oxygenic Photosynthesis
- The oxidation of H2O releases molecular oxygen as a by-product. Since oxygen is released, the photosynthesis in such entities is referred to as oxygenic photosynthesis
- Cyanobacteria and algae – examples
- Here, in the reaction carbon dioxide gets reduced or receives water and electrons and becomes oxidized, losing electrons. Finally, oxygen is released with carbon dioxide
- It occurs in two stages – light dependant reaction and light-independent reaction
- Light-dependent reaction – sequence of reactions occurring in the chloroplast (grana) necessitating solar energy. Light is used for the synthesis of ATP, water is photolysed into oxygen and hydrogen ion and free-electron carrier
- Light independent reaction – sequence of reactions occurring in the chloroplast (stroma). It aids in producing carbohydrates from carbon dioxide molecules by the process of reduction
The equation for oxygenic photosynthesis is as follows –
6C2O + 12 H2O + Light ——-> C6H12O6 + 6O2 + 6 H2O
Anoxygenic Photosynthesis
- In this type of photosynthesis, H2O is not oxidized, hence O2 is not released. As a result the process is called anoxygenic photosynthesis
- Green sulphur bacteria, green and red filamentous anoxygenic phototrophs (FAPs) – examples
- This form of photosynthesis is seen occurring frequently in microbes that are seen in the aquatic habitat
- In purple bacteria and green sulphur bacteria, the reducing agent used is sulphur
- The anoxygenic photosynthetic entities seem to have occurred in the bacteria domain
- This type of photosynthesis is based on electron donors, such as molecular hydrogen, sulphur compounds or organic compounds
- They can be seen in brackish water, freshwater, hypersaline and marine water bodies
Key Difference between Oxygenic and Anoxygenic Photosynthesis
The table below depicts the difference between Oxygenic and Anoxygenic Photosynthesis.
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Phenomena converting light energy into chemical energy by some autotrophs involving the liberation of molecular oxygen |
Phenomena converting light energy into chemical energy by some bacteria, involving no release of molecular oxygen |
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Yes, as a by-product |
No |
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As an initial electron donor |
Water is not used as an electron donor. Hydrogen, ferrous ions or hydrogen sulphide are used as electron donors |
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Both, Photosystem I and II are involved |
Photosystem I is present, photosystem II is absent |
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Through electron carriers |
Through cyclic photosynthetic electron chain |
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Yes |
Chlorophyll or bacteriochlorophyll is used |
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Non-cyclic photophosphorylation |
Cyclic photophosphorylation |
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Yes |
No |
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Cyanobacteria, algae |
Green sulphur bacteria, green and red filamentous anoxygenic phototrophs (FAPs) |
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