4.3 Photosynthesis, respiration and decay

Green plants absorb solar radiation and use its energy to fuel photosynthesis — a chemical reaction in which carbon dioxide (CO₂) from the atmosphere is combined with water (H₂ O) to form carbohydrates with the general formula C_nH_2nO_n. One of the simplest carbohydrates, glucose, has the chemical formula C₆H₁₂O₆, so in its simplest form photosynthesis can be represented by the balanced chemical equation:

Equation 1.1 Balanced chemical equation representing photosynthesis in its simplest form

The oxygen produced by this reaction is released by plants into the atmosphere. Carbohydrates act as a store of energy for plants and also for other organisms that eat them. Such organisms use oxygen from the air to react with the carbohydrates (and other substances) to liberate energy by a process called respiration. During respiration, carbon dioxide and water are returned to the atmosphere. Expressed in the simplest chemical terms, the balanced reaction is:

Equation 1.2 is the exact reverse of the photosynthesis reaction in Equation 1.1

Carbon exchanges or fluxes link the chemistry of the atmosphere with plant and animal chemistry. The carbon taken from the atmosphere (fixed) by plants enables them to grow, but in addition much of it enters the food chain as either living or dead material. Living plants are eaten by herbivores which themselves may become food for carnivores. The dead material provides food for the decomposers (bacteria and fungi) that live in plant detritus, in the soil, and on the rotting remains of dead animals. Almost all organisms return some carbon to the atmosphere through respiration, but by far the greatest contribution comes from the activities of the decomposers. The timescale by which this takes place is measured in months and years, so plant and animal material is not normally available to be preserved as fossil fuels.

However, if organic matter decays in an environment where the oxygen supply is limited, carbohydrates cannot be broken down completely to form water and carbon dioxide. In this special oxygen-poor (anoxic) environment, a carbohydrate comparatively enriched in carbon may be produced. For example, within the waterlogged environment of a swamp (mire), cellulose (a common constituent of plants) can be broken down according to the following reaction:

Equation 1.3 Within the waterlogged environment of a swamp (mire), cellulose (a common constituent of plants) can be broken down according to the reaction shown in this equation.

The residue produced, C₈H₁₀O₅ is relatively enriched in carbon compared with the original cellulose (C₆H₁₀O₅). This breakdown reaction releases methane (CH₄), as well as carbon dioxide and water. Methane is an organic compound containing carbon and hydrogen but no oxygen; one of a family of organic compounds known as hydrocarbons. So anoxic environments prevent some fixed carbon returning to the atmosphere as CO₂, and these hydrocarbons together with carbon-rich residues represent a chemical half-way-house within the carbon cycle, which make carbon available to form the basis for fossil fuels.

Although significant layers of decaying plant debris are found on the floor of modern forests, these are often oxygen-rich environments thanks to the constant reworking of decaying material by plants and animals, fungi and bacteria. However, one modern environment with which you are probably familiar does contain plant material decaying in anoxic conditions — the peat bog.This is where useful preservation of terrestrial carbon occurs.