9.12 - 9.13 - Material cycling and carbon

9.12 — Reading material cycles

An ecosystem contains a community of organisms and the non-living environment with which they interact. Its biotic components are living organisms, such as plants, animals and microorganisms. Its abiotic components include the atmosphere, water and soil, together with the non-living substances stored in them.

A material cycle is a set of transfers between stores. A store is a place or form in which a material remains for a time; a process moves it from a source store to a sink store. The same store can be a source in one transfer and a sink in another. For example:

soil water -> root uptake -> water inside a plant

Here, soil water is the source, root uptake is the transfer process, and the plant is the sink. Reversing the arrow would make a different claim, so every arrow in a cycle must be read from its tail to its head.

Atoms and molecules are transferred and reused, so matter cycles. Energy is different: light energy enters ecosystems, is transferred, and is eventually dissipated to the surroundings. Energy therefore does not cycle in the same way as carbon, water and nitrogen.

9.13 — Carbon through an ecosystem

Carbon is part of carbohydrates, lipids, proteins and DNA. The carbon cycle is important because it keeps carbon moving into forms that organisms can use instead of leaving all of it locked in one store.

One clear route begins with carbon dioxide in the atmosphere:

1. Photosynthesis: green plants and algae take in carbon dioxide. Using light energy, they make glucose, so carbon moves from an abiotic gas into carbon compounds in producer biomass.
2. Feeding: an animal eats a plant, or another animal, and digests and absorbs carbon compounds. Carbon moves between biotic stores along a food chain.
3. Respiration: plants, animals and microorganisms respire carbon compounds. Respiration releases energy for metabolic processes and produces carbon dioxide, which returns carbon to the surroundings.
4. Death, waste and decomposition: dead organisms and waste contain carbon compounds. Decomposer microorganisms, mainly bacteria and fungi, digest this material and absorb soluble products. Carbon enters the microorganisms; when they respire, carbon dioxide is released.
5. Long-term storage and combustion: if some dead material is buried where decomposition is incomplete, its carbon can eventually become part of fossil fuels over millions of years. Combustion of fossil fuels or biomass transfers this stored carbon back to atmospheric carbon dioxide.