Photosynthesis is a chemical reaction (or more accurately, a chain of chemical reactions) which takes place in green plants, some protists (e.g. algae) and some prokaryotes (e.g. cyanobacteria). Photosynthesis produces glucose, a carbohydrate which can then be broken down to release energy (respiration), or used to synthesize other molecules, including lipids (fats and oils) and proteins. Photosynthesis is therefore crucial to all living things (except a few prokaryotes which can synthesize molecules using energy from things like underwater volcanic vents), either directly (plants) or indirectly (animals, fungi).
Oxygen gas is given off as a waste product of this reaction. This reaction is the exact reverse of the simplified respiration reaction.
 The extended version
Photosynthesis, however, is not quite this simple and instead occurs in a multiple step chain reaction. Each step must be catalyzed by different enzymes. In plants photosynthesis occurs in organelles called chloroplasts, and the steps of the reactions are classified into "light dependent reactions" and "light independent reactions" (or "carbon reactions").
 The light dependent reactions
These organelles contain pigments including chlorophyll (which makes plants green) and carotenids. These pigments are capable of absorbing sunlight, and the energy absorbed causes the pigment to lose an electron (energy carrying sub-atomic particle).
 The evolution of photosynthesis
A substantial literature on origin and evolution of photosynthesis has recently developed. The conclusion of experts is that photosynthesis similar to that used by modern cyanobacteria developed at least 3.5 billion years ago from an earlier non-oxygen evolving phase. Its evolution likely involved both lateral gene transfer between organisms and a duplication event resulting in two linked photosystems.
 Creationists' take
Creationists claim that the process of photosynthesis could not have evolved since it is irreducibly complex. Answers in Genesis poses the question, "Why would evolution produce a series of enzymes that only generate useless intermediates until all of the enzymes needed for the end product have evolved?" They conclude, "It defies common sense to imagine that the irreducible complexity of photosynthetic systems would have arisen according to evolutionary theory. Rather, the incredible organization and intricacy evident in photosynthesis—a process man has yet to fully understand, let alone copy—shrieks of having been designed."
Answers in Genesis also challenges the gene duplication event in the evolution of photosynthesis on the grounds that it does not add "information." They note, "It is then proposed that a 'simple duplication' of the genes encoding one photosystem occurred, followed by mutation leading to the formation of another photosystem. What is needed is new information to form another photosystem, not duplication of already present genes followed by mutation."
Adherents of Breatharianism believe they can get all the nourishment they need from air and sunlight.
If they were plants, that might be true. But humans, sadly, are not photosynthetic organisms.
- ↑ See WikiPedia:Electron
- ↑ Swindell, Rick (Dec. 2003). "Shining light on the evolution of photosynthesis." Answers in Genesis.
- ↑ Purdom, Georgia (May 16, 2007). "Origin of photosynthesis: A theory out of thin air." Answers in Genesis.
- Taiz, L. & Zeiger, E. 2002. Plant Physiology 3rd ed.. Mass: Sinauer.
- "Abstract: Origin and early evolution of photosynthesis." PubMed.
 Further Reading (popular science)
- King, J. How Plants Work. Cambridge University Press.