2.3. Photosynthesis and Symbiogenesis

Photosynthesis, as its name implies, is a biosynthetic reaction powered by light that can be simplistically summarized as - 

             6CO2 (g) + 6H2O (l)                    C6H12O6 (s)+ 6O2 (g)

where atmospheric carbondioxide diffused in through stomata and water from the roots are combined for cabohydrate synthesis with a concomitant release of gaseous oxygen. As such, photosynthesis is primarily responsible for generating and maintaining oxygen in the atmosphere, and supplies all of the organic compounds and most of the energy requisite for life on Earth. 

In primordial Earth, nearly 3 billion years ago, early photosynthetic organisms called cyanobacteria oxygenated the atmosphere enough so that other organisms, including humans, could respire. In fact, all plant cells incorporate symbiotic cyanobacteria, in the form of chloroplasts, within their cells. Postulated as a theory of Symbiogenesis (from Greek: syn “together”; bios “life”; genesis  “origin”), it holds that chloroplasts and mitochondria evolved in Eukaryotic cells via the engulfment of single-celled Prokaryotes. The fact that both these organelles independently harbor their own genome is cited as the evidence for this endosymbiotic theory.