4.3 Carbon Cycling Flashcards
What is an autotroph’s role in carbon cycling?
Autotrophs covert carbon dioxide into carbohydrates and other carbon compounds. They reduce the CO2 concentration of the atmosphere.
How do autotrophs in water work?
Autotrophs in aquatic habitats get their carbon dioxide either as a dissolved gas, or sometimes CO2 combines with water to form carbonic acid (H2CO3), which is why carbon dioxide is acidic, and then when it dissociates into H+ and HCO3- the autotrophs can get it from HCO3- which is a hydrogen carbonate ion.
In aquatic habitats carbon dioxide is present as a dissolved gas and hydrogen carbonate ions.
How do autotrophs absorb carbon dioxide?
From the air, they then use this carbon dioxide to make carbon compounds and therefore a concentration gradient is set up and CO2 will keep diffusing in.
In land plants this usually happens through stomata on the underside of leaves. In aquatic plants the entire surface of the leaves and stems is usually permeable to carbon dioxide, so diffusion can be through any part of these plants.
Where does carbon dioxide come from?
Carbon dioxide is a waste product of aerobic cell respiration. It is produced in all cells that carry out aerobic respiration. These can be grouped according to trophic level or the organism.
- Non-photosynthetic cells in producers for example root cells in plants
- Animal cells
- Saprotrophs such as fungi that decompose dead organic matter
Carbon dioxide produced by respiration diffuses out of cells and passes into the atmosphere or water that surrounds these organisms.
What is methanogenesis?
Methanogenesis is the production of methane.
How does methanogenesis occur?
Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans (similar to bacteria) and some diffuses into the atmosphere.
Methane is a waste product of a type of anaerobic respiration by archaeans a type of organism close to bacteria.
Three different types of prokaryotes are involved:
- Bacteria that convert organic matter into a mixture of organic acids, alcohol, hydrogen and carbon dioxide.
- Then a different type of bacteria uses the organic acids and alcohol from the first bacteria to make acetate, carbon dioxide and hydrogen.
- Then archaeans produce methane from carbon dioxide, acetate and hydrogen.
This is the 2 reactions possible:
CO2 + 4H2 = CH4 + 2H2O
CH3COOH = CH4 + CO2
It is therefore this third group, the archaeans that are able to perform methanogenesis and produce methane.
This happens in
- Mud along the shores and in the bed of lakes
- Swamps, mires, mangrove forests and other wetlands where the soil or peat deposits are waterlogged.
- Gut of termites and of ruminant mammals such as cattle and sheep.
- Landfill sites where organic matter is in wastes that have been buried.
Then from these places it diffuses into the environment.
Where does methanogenesis occur?
In archaeans in anaerobic environments, such as:
- Mud along the shores and in the bed of lakes
- Swamps, mires, mangrove forests and other wetlands where the soil or peat deposits are waterlogged.
- Gut of termites and of ruminant mammals such as cattle and sheep.
- Landfill sites where organic matter is in wastes that have been buried.
Then from these places it diffuses into the environment.
What are the 2 reactions to produce methane of archaeans?
CO2 + 4H2 = CH4 + 2H2O
CH3COOH = CH4 + CO2
What happens to methane once it has been produced by archaeans?
It diffuses into the atmosphere. Once in the atmosphere it is oxidised to carbon dioxide and water. Molecules of methane in the air persist there on average for only 12 years because it is naturally oxidised. Monatomic oxygen (O) and highly reactive hydroxyl radicals (OH*) are involved in methane oxidation. This explains why atmospheric concentrations are not high despite large amounts of production of methane by both natural processes and human activities.
Why are atmospheric concentrations of methane low?
Once in the atmosphere it is oxidised to carbon dioxide and water. Molecules of methane in the air persist there on average for only 12 years because it is naturally oxidised. Monatomic oxygen (O) and highly reactive hydroxyl radicals (OH*) are involved in methane oxidation. This explains why atmospheric concentrations are not high despite large amounts of production of methane by both natural processes and human activities.
How does peat form?
Peat forms when organic matter is not fully decomposed because of anaerobic conditions in waterlogged soils.
In many soils all organic matter such as dead leaves from plants is eventually digested by saprotrophic bacteria and fungi. Saprotrophs obtain the oxygen that they need for respiration from air spaces in the soil. In some environments water is unable to drain out of soils so they become water logged and anaerobic. Saprotrophs cannot thrive in these condition so dead organic matter is not fully decomposed. Acidic conditions tend to develop, further inhibiting saprotrophs and also methanogens that might break down organic matter.
Large quantities of partially decomposed organic matter have accumulated in some ecosystems and become compressed to form a dark brown acidic material called peat. About 3% of the Earth’s land surface is covered by peat and as the depth is ten metres or more in some places the total quantities are immense.
What is peat?
Dead organic matter that has not been composed due to lack of oxygen, it is waterlogged and slightly acidic which will stop saprotrophs and methanogens decomposing it further. It slowly becomes accumulated and compressed to form a dark brown acidic material and that is peat.
How is coal formed?
Coal is formed when deposits of peat are buried under other sediments. The peat is compressed and heated, gradually turning into coal. Large deposits were formed during the Pennsylvanian sub period of the Carboniferous. There was a cycle of sea level rises and falls; coastal swamps formed as the level fell and were destroyed and buried when the level rose and the sea spread inland. Each cycle has left a seam of coal.
Basically it is formed when peat is buried, compressed and heated.
How are oil and natural gas formed?
Oil and natural gas are formed in the mud at the bottom of seas and lakes. Conditions are usually anaerobic and so decomposition is often incomplete. As more mud or other sediments are deposited the partially decomposed matter is compressed and heated. Chemical changes occur, which produce complex mixtures of liquid carbon compounds or gases. Methane forms the largest part of natural gas. We call these mixtures crude oil or natural gas. Deposits are found where there are porous rocks that can hold them such as shales and also impervious (don’t allow fluid through) rocks above and below the porous rocks that prevent the deposit’s escape.
What is combustion?
Combustion means burning something in oxygen. If organic compounds are heated in oxygen they will burn, the oxidation reaction that occurs is combustion. The products of complete combustion is carbon dioxide and water.
In some parts of the world where there are forest fires lots of carbon dioxide is released from the combustion of the biomass.
Coal, oil and natural gas are all different forms of fossilised organic matter. They are all burned as fuels. The carbon atoms in carbon dioxide released may have been removed from the atmosphere by photosynthesising plants thousands of millions of years ago.
