4.3 Carbon cycling

Question 1

carbon fixation

Answer 1

autotrophs convert carbon dioxide into carbohydrates and other carbon compounds.

Question 2

carbon dioxide in solution

Answer 2

In aquatic habitats carbon dioxide is present as a
dissolved gas and hydrogen carbonate ions.
Carbon dioxide is soluble in water. It can either remain in water as a dissolved gas or it can combine with water to form carbonic acid (H2CO3). Carbonic acid can dissociate to form hydrogen and hydrogen carbonate ions (H+ and HCO3 ). This explains how carbon dioxide can reduce the pH of water.

Question 3

how is carbon dioxide absorbed into autotrophs?

Answer 3

Autotrophs use carbon dioxide in the production of carbon compounds by photosynthesis or other processes. This reduces the concentration of carbon dioxide inside autotrophs and sets up a concentration
gradient between cells in autotrophs and the air or water around.
Carbon dioxide therefore diffuses from the atmosphere or water into
autotrophs.

Question 4

how is carbon dioxide released from cell respiration?

Answer 4

Carbon dioxide is a waste product of aerobic cell respiration. It is produced in all cells that carry out aerobic cell respiration.

Question 5

methanogenesis

Answer 5

Methane is produced from organic matter in anaerobic

conditions by methanogenic archaeans and some diffuses into the atmosphere.

Question 6

what and how are the three different groups of anaerobic prokaryotes involved in methanogenesis?

Answer 6

Bacteria that convert organic matter into a mixture of organic acids,
alcohol, hydrogen and carbon dioxide.
Bacteria that use the organic acids and alcohol to produce acetate,
carbon dioxide and hydrogen.
Archaeans that produce methane from carbon dioxide, hydrogen and
acetate. They do this by two chemical reactions:
CO2 + 4H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2

Question 7

oxidation of methane

Answer 7

Methane is oxidized to carbon dioxide and water

in the atmosphere.

Question 8

peat formation

Answer 8

peat forms when organic matter is not fully decomposed because of anaerobic conditions in waterlogged soils.

Question 9

combustion

Answer 9

If organic matter is heated to its ignition temperature in the presence of oxygen it will set light and burn, creating oxidation reactions

Question 10

limestone

Answer 10

Animals such as reef-building corals and molluscs have
hard parts that are composed of calcium carbonate and
can become fossilised, such as:
- mollusc shells contain calcium carbonate;
- hard corals that build reefs produce their exoskeletons by secreting calcium carbonate.

Question 11

Environmental monitoring

Answer 11

Carbon dioxide and methane concentrations in the atmosphere have very important effects. Carbon dioxide concentrations affect photosynthesis rates and the pH of seawater. Both gases influence global temperatures, heat energy in the ocean currents, the distribution of rainfall and frequency and severity of extreme weather events.
–> Reliable data and realistic measurements are thus essential to evaluate hypotheses and predictions about carbon dioxide and methane concentrations in the atmosphere.

Question 12

Trends in atmospheric carbon dioxide

Answer 12

fluctuating