Photosynthesis is the process by which autotrophs—plants, algae, and some bacteria—use the energy of sunlight to convert carbon dioxide (CO2) into the organic matter that makes up their cells, releasing oxygen (O2) in the process. The production of organic matter through photosynthesis—known as primary production—makes up the base of most food webs and is the ultimate source of energy for most life on Earth. (Some autotrophs, referred to as chemoautotrophs, get the energy for sugar synthesis not from sunlight but from chemical energy, such as oxidation of the hydrogen sulfide that emerges from deep-sea vents; this is still primary production but it is not photosynthesis.) Photosynthesis is often represented chemically as the production of a simple sugar (C6H12O6): 6CO2 + 6H2O –> C6H12O6 + 6O2.
Note that terrestrial plants take up CO2 (a gas) directly from the air, while aquatic plants use CO2 dissolved in water. Dissolved CO2 can take various forms, including H2CO3, HCO3-, and CO32-, which are collectively referred to as dissolved inorganic carbon (DIC).
Photosynthesis is a redox (reduction-oxidation) reaction, in which carbon is being reduced (gaining electrons) and oxygen is being oxidized (losing electrons). Don’t fret if that language doesn’t mean too much to you; it’s just trying to convey that, in photosynthesis, oxygen is giving electrons to carbon. Oxygen, being an electronegative atom, is “an electron hog” and doesn’t part with its electrons readily, which is why photosynthesis requires external energy input from sunlight (i.e., it is endothermic).
The sugars produced by photosynthesis are converted into the complex array of organic molecules that cells require: carbohydrates, fats, nucleic acids, proteins, and others. Photosynthesis thus requires the availability of other chemical elements that are important constituents of these molecules. For example, proteins are high in nitrogen, and nucleic acids are high in phosphorus, so both N and P are essential nutrients for primary production.
Respiration is the process by which the organic matter produced by photosynthesis is broken back down into CO2 (and inorganic forms of other elements such as N and P), releasing energy to drive cellular processes. Even autotrophs must carry out respiration to power their cells, but most respiration is carried out by heterotrophs: animals, bacteria, and fungi who consume organic matter of various kinds. The dominant form of respiration is aerobic respiration, which is simply the reverse of the photosynthesis reaction above: C6H12O6 + 6O2 –> 6CO2 + 6H2O.
In aerobic respiration, carbon is being oxidized (losing electrons) and oxygen is being reduced (gaining electrons). This type of reaction produces a lot of energy (i.e., it is very exothermic), because O2 really “wants” to grab electrons by oxidizing carbon. Oxygen’s tendency to gain electrons is also reflected in other oxidations that it carries out, such as the formation of rust (oxidation of iron to iron oxides).
Settings that lack oxygen (such as many wetland soils) are referred to as reducing environments, because the absence of the strong oxidant O2 allows many chemicals to be present in their more chemically-reduced (electron-rich) forms. In these settings, aerobic respiration can’t happen and organisms that depend on it can’t survive. However, certain microbes can break down organic matter using various forms of anaerobic respiration, in which chemicals other than O2 (e.g., NO3-) serve as electron acceptors to allow the oxidation of organic matter.