4.3 Carbon cycling

Question 1

Autotrophs absorb CO2 from the atmosphere and convert it into

Answer 1

organic compounds.

Reduces the concentration of CO2 in the atmosphere (approximately 0.039% or 390 micromoles/mole)

Question 2

Some CO2 will directly dissolve

Answer 2

in water (soluble), but most will combine with water to become carbonic acid.

Question 3

Carbonic acid can dissociate to

Answer 3

form H+ and HCO3- (bicarbonate/ hydrogen carbonate)

Question 4

H+ ions explains how carbon dioxide reduces

Answer 4

pH of water.

Question 5

Both dissolved carbon dioxide and hydrogen carbonate ions are absorbed by

Answer 5

aquatic plants and other autotrophs that live in water.

Question 6

Photosynthesis uses CO2 keeping the

Answer 6

concentration of CO2 inside the leaf low,
maintains concentration gradient of CO2

Question 7

CO2 from outside the leaf diffuses down the concentration gradient into

Answer 7

the leaf

Question 8

CO2 moves through the

Answer 8

stomatal pores in the leaves of land plants

In many aquatic plants diffusion happens directly through the surface tissues.

Question 9

CO2 is a

Answer 9

waste product of aerobic cell respiration

Question 10

Methanogens

Answer 10

are archaean microorganisms that produce methane as a metabolic byproduct in anaerobic respiration (methane is produced from carbon dioxide)

Question 11

Methanogens are found in a variety of anaerobic environments:

Answer 11

Wetlands (paddies, swamps and mangroves)
Digestive tracts of animals (cows, humans, termites)
Marine & freshwater sediments (mud in lakebeds)
Landfill sites (in which organic matter has been buried)

Question 12

Measurements indicate that the levels of atmospheric methane are

Answer 12

increasing

Question 13

It is estimated that, on average, methane persists in the atmosphere for

Answer 13

12 years (naturally oxidized in the stratosphere)

Question 14

The most important process of methane removal is

Answer 14

oxidation by hydroxyl radicals.
methane + hydroxyl radical → carbon dioxide + water

Question 15

Partially decomposed organic matter can be compressed to form

Answer 15

brown soil-like peat

Question 16

Peat is a highly effective carbon sink,

Answer 16

it is estimated that the world’s peat contains 550 Gt of carbon

it is estimated that the world’s peat contains 550 Gt of carbon

Question 17

Peat is a highly effective carbon sink,

Answer 17

it is estimated that the world’s peat contains 550 Gt of carbon

Once dried peat burns easily and can be used as a fuel.

Question 18

How does peat form?

Answer 18

Organic matter, e.g. dead leaves

Saprotrophs assimilate some carbon for growth and release as carbon dioxide during aerobic respiration.

Waterlogged soils are an anaerobic environment

saprotrophs and methanogens are inhibited

Organic matter is only partially decomposed

Partial decomposition causes acidic conditions

Large quantities of (partially decomposed) organic matter build up.

the organic matter is compressed to form peat

Question 19

There are _____________– of carbon from past geological eras

Answer 19

large deposits

Question 20

These deposits of carbon are the result of

Answer 20

incomplete decomposition of organic matter and its burial in sediment that become rock

Question 21

Coal is formed when

Answer 21

deposits of peat are buried under other sediments.
The peat is compressed and heated over millions of years eventually becoming coal.

Question 22

Oil and natural gas are formed in the

Answer 22

mud at the bottom of seas and lakes

Anaerobic conditions mean decomposition is incomplete

As mud and sediment are deposited, the partially decomposed organic matter is compressed and heated

Chemical changes produce mixtures of liquid carbon compounds or gases

Methane forms the largest part of natural gas

Deposits are mostly found in porous rocks

Question 23

Combustion

Answer 23

heated dried biomass or fossilized fuels will burn in the presence of oxygen
Fossil/Biomass fuel + O2 → CO2 + H2O

Natural forest/grass fires occur in some regions (organisms adapted to periodic fires)

Human-caused fires also occur to clear areas for agriculture or as a method of harvesting

Question 24

Some animals are composed of

Answer 24

calcium carbonate (CaCO3) structures made by secretion:
Mollusc shells
Hard corals exoskeletons

Question 25

When the animals die the soft body parts decompose, but the calcium carbonate remains to ____________ on the ocean floor (in acidic conditions CaCO3 dissolves)

Answer 25

form deposits

Question 26

The deposits are buried and eventually

Answer 26

form limestone rock.

Limestone rock is a huge carbon sink

Question 27

Imprints of the hard body parts remain in rock as

Answer 27

fossils.

Question 28

The carbon cycle explains the

Answer 28

exchange of carbon between the biosphere, lithosphere, hydrosphere, and atmosphere.

Question 29

Pool

Answer 29

reserve of C (can be organic or inorganic)

Question 30

Flux

Answer 30

transfer of C from one pool to another