Photosynthesis

Generically, in the biological process of photosynthesis, certain organisms (e.g., certain plants, algae and bacteria) capture the energy of photons in sunlight, and through a series of physico-chemical reactions, transform that radiant energy into the biochemical energy-rich molecules that the photosynthesizing organisms, and the organisms that feed on them, need to perform the cellular work that maintains them in the living state.

Inasmuch as the etymological definition of photosynthesis, "synthesis with light", can apply to other light generating phenomena outside biology, a leading researcher in photosynthesis, Robert E. Blankenship, in the Department of Chemistry and Biochemistry at Arizona State University, suggests this succinct definition of photosynthesis:

Photosynthesis is a process in which light energy is captured and stored by an organism, and the stored energy is used to drive cellular processes.

Nearly all living systems on Earth depend directly or indirectly on photosynthesis (see below), and for us humans it indirectly provides essentially all of our food-energy, as well the bulk of our non-food energy resources, inasmuch as ancient photosynthesizing organisms produced the fossil fuels we burn to generate electricity and other forms of energy we use to support human activity.

In the biological process of photosynthesis in green plants, the leaves capture energy from photons in sunlight and use that captured energy as the essential primary energy source to drive a set of biochemical reactions that converts carbon dioxide (CO₂) and water (H₂O) to oxygen (O₂) and a carbohydrate compound, a triose, a 3-carbon sugar — defining 'oxygenic' photosynthesis.. Triose, as triose phosphates, exit the leaf cell's organelles that synthesizes them — viz., the chloroplast. They then condense into six-carbon hexose phosphates and ultimately form [Dimer|dimers]] like sucrose, and polymers like starch or cellulose — so-called [[Redox|reduced] forms of carbon enriched in electrons energized by light energy — viz., the formation of energy-rich carbon compounds that, as mentioned, the photosynthesizing organisms and their predators need to perform the cellular work that maintains them in the living state. The process effectively stores energy in such sugar molecules, which the photosynthetic organism can metabolize to generate adenosine triphosphate (ATP), the universal circulating and recyclable energy currency of cells, so-called because it can provide the energy needed to drive many of the biochemical reactions necessary to synthesize the macromolecules and molecular intermediates required to maintain the cell in a living state. The process also produces other recyclable forms of circulating energy currency (e.g., NADPH). Photosynthesizing cells thus convert light energy to the life-sustaining chemical energy that drives life-sustaining cellular processes.

Organisms that photosynthesize function as autotrophs — viz., organisms that generate their own source of food-energy — specifically referred to as photoautotrophs. They draw on minerals and other inorganic compounds from the environment and produce an ultimately photon-energy-derived complement of carbohydrates, proteins and lipids that self-organize the photoautotophic organism. In doing so they directly, though blindly, offer themselves as a source of food-energy (e.g., as vegetables, fruits) for consumption by us humans and other organisms, so-called heterotrophs — viz., organisms that feed on other organisms or on their energy-rich structural components — and indirectly provide a source of food-energy in the form of the non-human heterotrophs that we humans consume (e.g., chicken, fish and other animals). Photosynthesizing cells also supply the sufficient amounts of oxygen they and we need to generate ATP and NADPH, and they consume the 'waste' CO₂ produced in the process of generating ATP.

Not all photosynthesizing organisms produce oxygen. The specific physico-chemical reactions of those that do biologists refer to as oxygenic photosynthesis, and those that do not as 'anoxygenic' photosynthesis. Oxygenic photosynthesis accounts for most of the oxygen in the atmosphere.

This article will classify the differing types of photosynthesizing organisms, and describe the details of the differing photosynthetic mechanisms employed by them. It will also discuss the implications of photosynthesis in the sciences of biology, geology, oceanography, climatology, and other areas of importance to the life of planet Earth.

Preliminarily, the reader might refer to the following: