Plants, algae and certain bacteria use photosynthesis to harness the energy of the Sun and use that to make chemical energy.

Types of photosynthesis

photosynthetic processes are of two types: oxygenic and anoxygenic. The general idea of the two are similar, it’s just that oxygenic photosynthesis is seen more commonly among plants, algae and cyanobacteria. The energy of light transfers electrons from water (H2O) to carbon dioxide (CO2), to
produce carbohydrates during oxygenic photosynthesis. Here, CO2 receives electrons or gets reduced and the water loses electrons or gets oxidized. At last, Oxygen and Carbohydrates are produced.
Oxygenic photosynthesis balances the O2 and CO2 of atmosphere by taking in the carbon dioxide produced by all breathing organisms and reintroducing oxygen to the atmosphere. Electron donors other than water are used in Anoxygenic photosynthesis. The process typically occurs in bacteria such as purple bacteria and green sulphur bacteria, which are primarily found in various aquatic habitats.
“Anoxygenic photosynthesis does not produce oxygen — hence the name,” said David Baum, professor of botany at the University of Wisconsin-Madison. “What is produced depends on the electron donor. For example, many bacteria use the bad-eggs-smelling gas hydrogen sulphide, producing solid sulphur as a by-product.”
Though these steps are a lot complex, but they can be summarized as a relatively smaller equation.

Oxygenic photosynthesis is written as follows:

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Here, six molecules of carbon dioxide (CO2) combine with 12 molecules of water (H2O) using light energy. The end result is the formation of a single carbohydrate molecule (C6H12O6, or glucose) along with six molecules each of breathable oxygen and water.

Similarly, anoxygenic photosynthesis reactions are written as:

CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O

The letter A in the equation is a variable and H2A represents the potential electron donor. For example, A may represent sulfur in the electron donor hydrogen sulfide (H2S), explained Govindjee and John Whitmarsh, plant biologists at the University of Illinois at Urbana-Champaign, in the book “Concepts in Photobiology: Photosynthesis and Photomorphogenesis” (Narosa Publishers and Kluwer Academic, 1999).