4.3- carbon cycling

Question 1

carbon fixation

Answer 1

• autotrophs convert carbon dioxide into carbohydrates and other carbon compounds
• this reduces CO2 concentration in the atmosphere

Question 2

carbon dioxide in solution

Answer 2

• CO2 is soluble, so is present as a dissolved gas and hydrogen carbonate ions in aquatic habitats
• can also combine with water to form carbonic acid (H2CO3) and then dissociate to form H+ and HCO3-
• both dissolved CO2 and hydrogen carbonate ions are absorbed by aquatic plants/other autotrophs and used to make carbs and other C compounds

Question 3

absorption of carbon dioxide

Answer 3

autotrophs use CO2 in photosynthesis
- this reduces the concentration of CO2 inside autotrophs and sets up a concentration gradient between cells in autotrophs and the surrounding air/water, causing it to diffuse

Question 4

release of carbon dioxide from cell respiration

Answer 4

by animal cells, saprotrophs that decompose dead organic matter, and non-photosynthetic cells in producers (eg plant root cells)

Question 5

Anaerobic conditions where methanogens may be found include:

Answer 5

• Wetlands (e.g. swamps and marshes)
• Marine sediments (e.g. in the mud of lake beds)
• Digestive tract of ruminant animals (e.g. cows, sheep, goats)

Question 6

3 steps of methanogenesis

Answer 6

1. Bacteria convert organic matter into a mixture of organic acids, alcohol, hydrogen, CO2
2. Bacteria use organic acids and alcohols to produce acetate, CO2, hydrogen
3. Archaeans produce methane from CO2, hydrogen and acetate:

CO2 + 4H2 -> CH4 + 2H2O
CH3COOH -> CH4 + CO2

Question 7

Methanogenesis

Answer 7

Methanogens (archaean microorganisms) produce methane (CH4) as a metabolic by-product of anaerobic digestion, principally acetic acid and carbon dioxide - this then diffuses into the atmosphere

Question 8

oxidation of methane

Answer 8

• methane is oxidised to carbon dioxide and water in the atmosphere
• monatomic oxygen and highly reactive hydroxyl radicals are involved in this oxidation

Question 9

peat formation

Answer 9

peat forms the organic matter is not fully decomposed because of anaerobic conditions in waterlogged soils, which inhibit saprotroph activity

Question 10

partially decomposed organic matter from past geological eras was converted into

Answer 10

oil and gas in porous rocks or coal

Question 11

coal formation

Answer 11

• deposits of peat are buried under other sediments
• peat is compressed and heated, gradually turning into coal

Question 12

oil and natural gas formation

Answer 12

• formed in the mud at the bottom of seas and lakes as a result of the decay of marine organisms
• sediments (e.g. clay and mud) are deposited on top of the organic matter, creating anoxic conditions that prevent decomposition
• partially decomposed matter is compressed and heated, causing chemical changes into crude oil and natural gas

Question 13

combustion

Answer 13

carbon dioxide is produced by the combustion of biomass and fossilised organic matter

Question 14

limestone

Answer 14

• animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate
• when the animals die:
  ACID CONDITIONS: calcium carbonate dissolves away
  NEUTRAL/ALKALINE CONDITIONS: it is stable and deposits of it form on the sea bed. the result is these hard parts becoming fossilised in limestone rock

Question 15

draw a carbon cycle diagram

Question 16

describe the trends in atmospheric carbon dioxide

Answer 16

• CO2 levels fluctuate annually (lower in the summer months when long days and more light increase photosynthetic rates)
• Global CO2 trends will conform to northern hemisphere patterns as it contains more of the planet’s land mass (i.e. more trees)
• CO2 levels are steadily increasing year on year since the industrial revolution (due to increased burning of fossil fuels)

Question 17

estimation of carbon fluxes due to processes in the carbon cycle

Answer 17

Carbon fluxes describe the rate of exchange of carbon between the various carbon sinks / reservoirs

It is not possible to directly measure the size of the carbon sinks or the fluxes between them – instead estimates are made