Photosynthesis

Photosynthesis ("synthesis with light") is a process in which light energy is captured and stored by an organism, and the stored energy is used to drive cellular processes.
In this process the energy of photons (from sunlight) is absorbed by chlorophyll, and organic compounds are synthesized from carbon dioxide and water (with the release of oxygen). Forms of photosynthesis are found in a large number of organisms (e.g., plants, algae and certain bacteria).

Thus, starting from materials which are abundant in their environment — water (H₂O) and carbon dioxide (CO₂), and certain minerals and other inorganic matter — through a series of physico-chemical reactions — driven by radiation — energy-rich organic molecules are synthesized:
CO₂ provides the carbon source for the reduction mechanisms that generate the organic molecules, and H₂O provides the electrons for the initiating reduction mechanisms, releasing the H₂O´s oxygen into the environment. The photosynthesizing organisms, and the non-photosynthesizing organisms that feed on them, ultimately use those energy-rich organic molecules as cellular building blocks and energy sources, enabling the cellular structures and functions that maintain nearly every living system on Earth in the living state. Living systems on Earth, with notable exceptions, are "solar powered".

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, since without photosynthesis nearly every species on Earth would perish.

Overview

Without photosynthesis the luxuriant, awe-inspiring variety of living systems we see in the terrestrial and marine world about us would not exist. Nearly all living systems on Earth depend directly or indirectly on the energy captured by photosynthesis from light energy radiating to our planet from our sun (see below). For us humans photosynthesis 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 energy-rich carbon-containing molecules we combust as fossil fuels — oil, natural gas, coal and wood — to generate electricity and other forms of energy we use to support human activity.

Nearly every oxygen atom that we inhale from the atmosphere emerged through photosynthetic liberation from a water molecule, among the countless water molecules covering 70% of the Earth´s surface. The chemical energy our bodies generate using that oxygen in biochemically combusting our food represents, almost literally, the energy that photosynthesis secured from the energy in sunlight. Likewise, the energy we generate with that oxygen in combusting fuels — oil, coal, wood, natural gas — owes its origin to photosynthetic capture of the energy of sunlight. The sun offers the energy of life, and the living process of photosynthesis accepts the offer and distributes it.

(CC) Image: Illustrated by Anthony Sebastian
Simpified overview of photosynthesis in plants.

In green plants, the biological process of photosynthesis typically occurs in leaf cells. They capture the energy from photons in sunlight, use it to energize electrons and release a largely 'waste' product, oxygen (O₂) — thus defining 'oxygenic' photosynthesis. The captured and transformed energy is then used to drive a set of biochemical reactions that converts carbon dioxide (CO₂) and water (H₂O) to a carbohydrate compound, a triose, a 3-carbon sugar..

Triose, as triose phosphates, exit the leaf cell's organelles that synthesizes them — viz., a chloroplast, condense themselves into six-carbon hexose phosphates, ultimately forming dimers like sucrose, or polymers like starch or cellulose, the so-called reduced forms of carbon. Thus, the mass of plants, and their predators in the food chain, is from the enrichment of electrons energized by captured photons and the subsequent formation of energy-rich carbon compounds from the carbon dioxide in the air.

Between these two steps, a variety of energy-rich intermediates are formed, some of which can be metabolized by the photosynthetic organism. Light reactions can generate adenosine triphosphate (ATP), a recirculating and recyclable energy currency of cells. 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, to paraphrase the words of Professor Blankenship quoted above.

Organisms that photosynthesize operate 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. As noted above, oxygenic photosynthesis accounts for nearly all of the oxygen in the atmosphere.

Chlorophyll-based photosynthesis

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