Possibly the most important process in all ecosystems. Plants, algae and other organisms use photosynthesis to convert light energy into chemical energy. The chemical energy is stored as carbohydrate molecules and can be released later to fuel organism activities. Carbohydrates are biomolecules consisting of Carbon(C), Hydrogen(H), and Oxygen(O); such as sugar (C6H12O6). 


Photosynthesis has another important role, for producing and maintaining the oxygen content of the Earth’s atmosphere. Although photosynthesis can be performed differently by different species, the fundamentals remain the same where the process always begins with light energy being absorbed by proteins called reaction centres that contain pigments. Pigments are molecules that bestow colour on plants, algae and bacteria. Different colour pigments absorb different wavelengths of light. The three main groups of pigments are as below: 


Chlorophylls: Are green pigments that are capable of trapping red and blue lights. There are three subtypes of chlorophylls including chlorophyll a, chlorophyll b and chlorophyll c. Chlorophyll a are found in all plants, algae and other organisms that perform photosynthesis.


Phycobilins: Are red and blue pigments; they absorb wavelengths of light that are not as well absorbed by chlorophylls and carotenoids. They can be seen in red algae and cyanobacteria. 


Carotenoids: Are red, orange and yellow-coloured pigments that absorb bluish-green light. Carotenoids give the characteristic colour to many everyday vegetables and fruits including pumpkins, carrots, oranges, corn, lemon, and tomatoes. 


There are two types of photosynthetic processes; oxygen photosynthesis and anoxygenic photosynthesis. Although the general principle of oxygen photosynthesis and anoxygenic photosynthesis are pretty much the same, they do have their differences. Most plants, algae and cyanobacteria use oxygenic photosynthesis. 


Oxygen Photosynthesis: The light energy absorbed transfer electrons from water (H2O) to carbon dioxide (CO2) to produce carbohydrates that can be stored as chemical energy. During this shift, CO2 is eliminated by the chemical equations below:


CO2 + 2(H2O) + Photons = CH2O + O2 + H2O


Anoxygenic photosynthesis: Unlike oxygen photosynthesis, anoxygenic photosynthesis does not produce any oxygen, hence the name. This is because anoxygenic photosynthesis does not use water as it’s electron donor. Instead, it uses other molecules such as the bad egg smelling gas, hydrogen sulphide. This is shown in the chemical equation below:


CO2 + 2H2A + Photons = CH2O + 2A +H2O


Where H2A could be H2S, H2 or another electron donor.



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