Carbon and nitrogen cycles – IGCSE Biology exam questions with mark scheme

1Grade 1–34 marks

The diagram shows part of the carbon cycle. The letters A to E show some of the stages.

(a) Name the process shown by arrow A. [1]

(b) Name the process shown by arrow E. [1]

(d) Arrow B shows carbon passing from plants to animals.

State the form in which this carbon is passed on. [1]

(a) photosynthesis; reject respiration(1)
(b) combustion; accept burning (of fossil fuels); ignore 'pollution'(1)
(c) decomposers; accept bacteria / fungi / microorganisms / saprotrophs; ignore 'worms' / detritivores alone(1)
(d) (organic) carbon compounds in food, e.g. carbohydrates / glucose / starch / proteins / fats / lipids; accept 'as food' or 'by eating / feeding' only if a carbon compound is named; reject carbon dioxide(1)
Answer: (a) photosynthesis (b) combustion (c) decomposers, e.g. bacteria or fungi (d) as carbon compounds such as carbohydrates, proteins and fats
Examiner tip: Arrow D runs from dead remains to the air: the process is decomposition, but the question asks for the organisms, so 'decomposition' alone scores nothing in (c).

2Grade 4–55 marks

A leaf falls from a tree onto the soil of a woodland.

(a) Describe how the carbon in the leaf is returned to the air as carbon dioxide. [3]

(b) In a peat bog the soil is waterlogged and acidic. Dead plant material builds up in layers, and over millions of years it can form a fossil fuel.

Explain why the dead plant material builds up in a peat bog. [2]

(a) (leaf is broken down / decomposed) by decomposers / bacteria / fungi; accept saprotrophs / microorganisms(1)
(a) decomposers secrete / release enzymes onto the leaf (extracellular digestion) and absorb the products; accept digest the carbohydrates / cellulose / starch / proteins(1)
(a) decomposers (use the products in) respiration, which releases carbon dioxide; reject 'decomposition releases carbon dioxide' without reference to respiration(1)
(b) little / no oxygen (in waterlogged soil) or the acid conditions, so decomposers cannot survive / grow / respire (aerobically) / their enzymes do not work well; accept enzymes denatured by low pH(1)
(b) so (little) decomposition / decay happens and the carbon (compounds) is not released as carbon dioxide / stays stored in the dead material(1)
Answer: (a) Decomposers (bacteria and fungi) secrete enzymes that digest the leaf, absorb the products and respire, releasing CO₂. (b) Waterlogged/acid conditions give little oxygen or a low pH, so decomposers cannot respire or work; decay is very slow and the carbon stays locked in the material.
Examiner tip: Say that decomposers respire: 'decomposers break it down' is only the first mark — the CO₂ comes from their respiration.

3Grade 4–56 marks

Scientists measure the concentration of carbon dioxide in the atmosphere at a station in the northern hemisphere.

Year	Mean carbon dioxide concentration / parts per million
2000	369.7
2010	389.9
2020	414.2

(a) Calculate the mean increase in carbon dioxide concentration per year between 2010 and 2020. [2]

(b) Within each year the concentration also goes up and down. It is highest in May and lowest in September.

Explain why the carbon dioxide concentration falls between May and September. [2]

Explain why. [2]

(a) 414.2 − 389.9 = 24.3 (ppm)(1)
(a) 24.3 ÷ 10 = 2.43 (parts per million per year); accept 2.4; correct answer with no working scores 2(1)
(b) (in summer) more photosynthesis / plants photosynthesise faster because of more light / longer days / higher temperature / more leaves on trees(1)
(b) carbon dioxide is taken in by photosynthesis faster than it is released by respiration (so concentration falls); accept rate of photosynthesis greater than rate of respiration(1)
(c) combustion / burning of fossil fuels adds (extra) carbon dioxide (from carbon that was stored for millions of years); or deforestation means fewer plants to remove carbon dioxide by photosynthesis; accept named fossil fuel e.g. coal / oil / natural gas; ignore 'pollution' / 'industry' alone(1)
(c) so over a whole year more carbon dioxide is released (by respiration and combustion) than is taken in by photosynthesis; accept the rise in winter is greater than the fall in summer; ORA(1)
Answer: (a) 2.43 ppm per year (b) More photosynthesis in summer removes CO₂ faster than respiration releases it. (c) Burning fossil fuels (and deforestation) add extra CO₂, so over a whole year more CO₂ is released than photosynthesis removes.
Examiner tip: In (b) you must compare the two processes: 'more photosynthesis' alone is one mark; say it removes CO₂ faster than respiration (and decomposition) put it back.

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4Grade 6–76 marks

Ecologists estimated the mass of carbon moving in and out of an area of woodland in one year.

Process	Mass of carbon / g per m² per year
taken in by photosynthesis of trees and other plants	1200
released by respiration of plants	550
released by respiration of animals	50
released by respiration of decomposers	450

(a) Calculate the mass of carbon stored in the woodland each year, in g per m². [2]

(b) The woodland has an area of 2.5 hectares. 1 hectare = 10 000 m².

Calculate the total mass of carbon stored by the whole woodland in one year. Give your answer in kg. [2]

Explain how this changes the amount of carbon dioxide in the air. [2]

(a) total released = 550 + 50 + 450 = 1050(1)
(a) 1200 − 1050 = 150 (g per m² per year); correct answer with no working scores 2(1)
(b) 150 × 25 000 = 3 750 000 (g); allow error carried forward from (a)(1)
(b) ÷ 1000 = 3750 (kg); allow ecf; correct answer with no working scores 2(1)
(c) combustion / burning releases carbon dioxide (from the carbon compounds stored in the wood); ignore 'releases carbon' unless carbon dioxide is stated(1)
(c) fewer trees / plants to photosynthesise so less carbon dioxide is taken in / removed from the air; (so carbon dioxide in the air increases)(1)
Answer: (a) 150 g per m² per year (b) 3750 kg (c) Combustion releases the stored carbon as CO₂ and there are fewer trees photosynthesising to remove CO₂, so CO₂ in the air increases.
Examiner tip: Convert both units: hectares to m² first (×10 000), then g to kg (÷1000) — most lost marks come from doing only one of these.

5Grade 1–3Triple only4 marks

Nitrogen gas makes up about 78% of the air, but plants cannot use it directly.

(a) Give the form in which plants absorb nitrogen from the soil. [1]

(b) State why plants need nitrogen. [1]

(d) Name the type of bacteria that convert nitrates in the soil back into nitrogen gas. [1]

(a) nitrate (ions); accept NO₃⁻ / ammonium ions; reject nitrogen / nitrogen gas(1)
(b) to make amino acids / proteins; accept to make DNA / chlorophyll / nucleic acids; ignore 'for growth' alone(1)
(c) nitrogen-fixing (bacteria); ignore named species(1)
(d) denitrifying (bacteria); reject nitrifying(1)
Answer: (a) nitrates (b) to make amino acids and proteins (c) nitrogen-fixing bacteria (d) denitrifying bacteria
Examiner tip: Nitrifying and denitrifying differ by two letters but do opposite jobs: nitrifying bacteria make nitrates, denitrifying bacteria destroy them.

6Grade 8–9Triple only5 marks

The diagram shows part of the nitrogen cycle. The letters P, Q, R and S show some of the stages.

(a) Give the letter of the stage carried out by bacteria living in the root nodules of plants such as peas and beans. [1]

(b) Name the type of bacteria that carry out stage R. [1]

Explain why draining the soil helps the crops to grow. [3]

(a) P(1)
(b) nitrifying (bacteria); reject denitrifying / nitrogen-fixing(1)
(c) draining lets air / oxygen into the soil; accept waterlogged soil has little oxygen / is anaerobic(1)
(c) fewer denitrifying bacteria / less denitrification, so fewer nitrates are converted to nitrogen gas; or nitrifying bacteria need oxygen so more ammonium is converted to nitrates; allow roots get more oxygen for aerobic respiration, so more energy for active transport / uptake of nitrate (mineral) ions(1)
(c) more nitrates are absorbed by the roots to make amino acids / proteins (for growth); ignore 'more nutrients' unless nitrate is named(1)
Answer: (a) P (b) nitrifying bacteria (c) Draining lets oxygen into the soil, so fewer (anaerobic) denitrifying bacteria remove nitrates and aerobic nitrifying bacteria make more nitrates; roots absorb more nitrate to make proteins.
Examiner tip: Link oxygen to a named type of bacteria: denitrifying bacteria thrive without oxygen, nitrifying bacteria need it — 'better for the bacteria' scores nothing.

7Grade 6–7Triple only5 marks

A farmer grows wheat in two fields. The year before, field 1 was used for wheat and field 2 was used for beans. The remains of the bean plants were ploughed into the soil. No fertiliser was added to either field.

Field	Crop grown the year before	Yield of wheat / tonnes per hectare
1	wheat	6.2
2	beans	7.4

(a) Calculate the percentage increase in the yield of wheat in field 2 compared with field 1. [2]

(b) Bean plants have root nodules.

Explain how growing beans the year before increases the yield of wheat. [3]

(a) 7.4 − 6.2 = 1.2 and 1.2 ÷ 6.2 (× 100)(1)
(a) 19 (%); accept 19.4 / 19.35; correct answer with no working scores 2; reject 16.2 (dividing by 7.4)(1)
(b) root nodules contain nitrogen-fixing bacteria that convert nitrogen gas (from the air) into nitrogen compounds / ammonium; ignore 'take in nitrogen'(1)
(b) decomposers break down the (proteins in) bean remains to ammonium (compounds) and nitrifying bacteria convert this to nitrates(1)
(b) more nitrates (in the soil) are absorbed by the wheat to make amino acids / proteins (for growth); ignore 'more nutrients' unless nitrate is named(1)
Answer: (a) 19% (19.4%) (b) Nitrogen-fixing bacteria in the nodules fix N₂ into nitrogen compounds; decomposers and nitrifying bacteria turn the ploughed-in remains into nitrates; wheat absorbs the nitrates to make proteins.
Examiner tip: Percentage increase divides by the ORIGINAL value (6.2), not the new one.

8Grade 8–9Triple only6 marks

A rabbit dies in a field. Over the following months some of the nitrogen in the proteins of the rabbit's body becomes part of the proteins in the grass growing nearby, and some is returned to the air as nitrogen gas.

Describe how these changes happen.

In your answer refer to the roles of different types of bacteria. [6]

decomposers / bacteria and fungi break down / digest proteins in the dead rabbit (and its urine / faeces)(1)
decomposition (of proteins / amino acids) produces ammonium compounds / ammonia(1)
nitrifying bacteria convert ammonium compounds into nitrites and then nitrates(1)
nitrifying bacteria need oxygen / work in well-aerated soil(1)
nitrates are absorbed by the roots of the grass (by active transport)(1)
grass uses the nitrates (with glucose / sugars from photosynthesis) to make amino acids, which are joined to make proteins(1)
denitrifying bacteria convert nitrates into nitrogen gas, returning it to the air(1)
denitrification happens mostly in waterlogged / anaerobic soil(1)
ignore named species of bacteria; do not credit nitrogen-fixing bacteria as converting nitrates to nitrogen gas(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, correctly ordered description of both routes: decomposition to ammonium compounds, nitrification to nitrates, uptake by roots and use to make amino acids/proteins, and denitrification to nitrogen gas, with each type of bacteria matched to its correct role and some detail on conditions (oxygen / waterlogging).
Level 2	3–4	A description that covers most stages of at least one route with the roles of at least two types of bacteria (or decomposers) correct, but with gaps in the sequence or little detail of conditions.
Level 1	1–2	Isolated correct points, e.g. the rabbit decays, or plants take in nitrates, with little sequence and roles of bacteria missing or confused.
No marks	0	No rewardable content.

Answer: Decomposers break down rabbit proteins to ammonium compounds; nitrifying bacteria (aerobic) convert ammonium to nitrites then nitrates; grass roots absorb nitrates and make amino acids then proteins; in waterlogged soil denitrifying bacteria convert nitrates to nitrogen gas, which returns to the air.
Examiner tip: Keep the order right: proteins → ammonium → nitrates → plant proteins; answers that let decomposers make nitrates directly, or that mix up nitrifying and denitrifying, stay in Level 1–2.

9Grade 4–56 marks

A student investigated the decomposition of oak leaves on a woodland floor.

She dried some leaves and put 5.00 g into each of two nylon bags.
Bag X had large holes (5 mm), so earthworms and other small animals could get in.
Bag Y had tiny holes (0.1 mm), so only microorganisms could get in.
She left both bags on the woodland floor for 12 weeks, then dried and reweighed the leaves.

Bag	Dry mass at start / g	Dry mass after 12 weeks / g
X (large holes)	5.00	1.85
Y (tiny holes)	5.00	3.40

(a) Calculate the percentage decrease in dry mass of the leaves in bag X. [2]

(b) Suggest why the leaves in bag X decreased in mass more than the leaves in bag Y. [2]

(d) State how the loss of mass from the leaves adds carbon dioxide to the air. [1]

(a) 5.00 − 1.85 = 3.15 and 3.15 ÷ 5.00 (× 100)(1)
(a) 63 (%); accept 63.0; correct answer with no working scores 2(1)
(b) earthworms / small animals / detritivores (could get in and) eat / break up the leaves; ignore 'more organisms' alone(1)
(b) (break leaves into smaller pieces giving) a larger surface area for decomposers / microorganisms / enzymes to act on; accept more decomposers / bacteria / fungi could enter with larger holes(1)
(c) water content of leaves varies / changes (with weather), so only the dry mass shows the mass of material decomposed; accept so mass change is not due to water gained / lost(1)
(d) decomposers / microorganisms respire (the digested leaf material), releasing carbon dioxide; reject 'the leaves respire'(1)
Answer: (a) 63% (b) Earthworms and other animals could enter bag X and eat/break up the leaves, giving a larger surface area for microorganisms. (c) Water content varies, so dry mass gives a fair measure of material broken down. (d) Decomposers respire the leaf material, releasing CO₂.
Examiner tip: Percentage decrease uses the change (3.15 g), not the final mass: 1.85 ÷ 5.00 gives 37%, the percentage remaining.

10Grade 6–77 marks

Hydrogencarbonate indicator changes colour with the concentration of carbon dioxide in the water.

Carbon dioxide concentration	Colour of indicator
higher than in normal air	yellow
same as normal air	orange-red
lower than in normal air	purple

A student set up four sealed tubes, each containing the same volume of orange-red indicator. She left them for 4 hours.

Tube	Contents	Conditions	Colour after 4 hours
1	indicator only	light	orange-red
2	water snail	light	yellow
3	water snail and pondweed	light	orange-red
4	water snail and pondweed	dark	?

(a) Explain the colour in tube 2. [2]

(b) Explain why the indicator in tube 3 stayed orange-red. [2]

(d) Give the purpose of tube 1. [1]

(a) the snail respires, releasing / producing carbon dioxide(1)
(a) so the carbon dioxide concentration rises above that in normal air (turning the indicator yellow); ignore 'no plant to photosynthesise' alone(1)
(b) the pondweed photosynthesises, taking in / using carbon dioxide(1)
(b) at the same rate as carbon dioxide is released by respiration of the snail and pondweed; accept CO₂ used = CO₂ produced / rates balanced(1)
(c) yellow(1)
(c) no light so no photosynthesis (by the pondweed), but both the snail and pondweed still respire, releasing carbon dioxide; the explanation mark depends on yellow or 'more yellow than tube 2'(1)
(d) a control: to show that the colour change is caused by the organisms / living things; accept to show the indicator does not change on its own / light alone does not change it(1)
Answer: (a) Snail respires, releasing CO₂, so CO₂ rises (yellow). (b) Pondweed photosynthesis removes CO₂ at the same rate as respiration releases it. (c) Yellow: in the dark there is no photosynthesis but both organisms respire. (d) Control — shows the change is caused by the organisms.
Examiner tip: Plants respire all the time, light or dark: in (c) mention respiration by the pondweed as well as the snail.

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