Carbon cycle: four processes move CO₂
Carbon exists as carbon dioxide (CO₂). Four processes cycle it: photosynthesis REMOVES it (chlorophyll builds glucose in light); respiration RETURNS it (all organisms break glucose down); decomposition RETURNS it (decomposers respire on dead matter); combustion RETURNS it (burning wood and fossil fuels). Equations — photosynthesis: carbon dioxide + water → glucose + oxygen; respiration is the reverse.
Nitrogen cycle: four bacterial roles
Most organisms cannot use nitrogen gas () directly; four bacterial groups transform it. Nitrogen-fixing bacteria convert → ammonium (), free in soil or in legume root nodules. Decomposers break dead matter and waste into ammonium. Nitrifying bacteria convert ammonium → nitrate (), which roots absorb (aerobic). Denitrifying bacteria convert nitrate → in waterlogged (anaerobic) soils.
Decomposition links the two cycles
The carbon and nitrogen cycles meet at decomposition: decomposers break down dead matter, releasing (carbon cycle) and ammonium (nitrogen cycle). Plants take in and nitrate to build organic molecules; animals gain carbon and nitrogen by eating; all respire and eventually die and are decomposed. Exam cue: the process that REMOVES is photosynthesis; respiration, combustion and decomposition release it.
Drawn from real examiner reports.
Carbon-cycle arrows: which process?
On a carbon-cycle diagram, arrows FROM the atmosphere () TO organisms are photosynthesis; arrows FROM organisms, dead matter or fossil fuels TO the atmosphere are respiration, decomposition or combustion. A classic slip is naming an atmosphere-to-plant arrow 'respiration' — it is photosynthesis.
June 2023 Paper 1BR Q8(a)(ii): asked to name process W (atmosphere → plants), many wrote respiration or combustion; the answer was photosynthesis.
Photosynthesis equation errors
Balanced equation: (light, chlorophyll). Errors: glucose as or (missing subscript 6); not balancing the 6s; saying light is absorbed by 'the leaf' when the scheme needs chlorophyll; omitting oxygen.
June 2023 Paper 1BR Q6(a): asked for the balanced photosynthesis equation; errors included the wrong glucose formula and an unbalanced equation.
Nitrogen-fixing vs nitrifying bacteria
These two are constantly swapped. Nitrogen-fixing bacteria fix atmospheric into ammonium — the entry point, in legume root nodules or free in soil. Nitrifying bacteria do NOT touch air nitrogen; they convert ammonium → nitrate. Wrong: nitrifying bacteria 'add nitrogen from the air' (that is fixing); fixing bacteria 'make nitrate' (that is nitrifying).
Decomposers vs nitrifying bacteria
Both act on soil nitrogen at different stages. Decomposers (bacteria and fungi) break down proteins and urea in dead matter, releasing ammonium. Nitrifying bacteria then convert ammonium → nitrate, the form roots absorb. Confusing them — decomposers 'make nitrate', or nitrifying bacteria 'break down dead organisms' — loses the mark.
Plants respire too, not just photosynthesise
A common carbon-cycle error is 'only animals respire — plants just photosynthesise'. All living organisms respire — plants, fungi and bacteria — all the time. Plants photosynthesise only in light but respire continuously, releasing . Do not treat plants as absorbers only.
Denitrifying bacteria reduce soil nitrogen
Denitrifying bacteria REMOVE usable nitrogen: they convert nitrate () back to nitrogen gas (), which escapes to air. They thrive in waterlogged, anaerobic soils, so poor drainage lowers fertility. Do not confuse them with nitrifying bacteria (which make nitrate) — denitrifying bacteria run the cycle in reverse.
Naming a cycle-diagram arrow
For 'name the process at arrow X', use the direction: atmosphere → plant = photosynthesis; organism/dead matter → atmosphere = respiration, decomposition or combustion; → soil = fixation; ammonium → nitrate = nitrification; nitrate → = denitrification.
Explain a nitrogen-cycle sequence
For "explain how nitrogen from a dead organism returns to the air", give the full chain: decomposers break proteins to ammonium → nitrifying bacteria make nitrate → denitrifying bacteria make . Name each group, the compound it acts on, and its product.
Describe vs explain in cycle answers
"Describe" asks what happens; "explain" asks what happens AND why. A cycle explanation must name the process or organism responsible — not just "the nitrogen goes back into the soil". Say which compound moves where, driven by which group, so the mechanism is clear.
This topic covers two nutrient cycles assessed on Edexcel 4BI1: the carbon cycle (spec statement 4.10, both papers) and the nitrogen cycle (spec statement 4.11B, Paper 2B only). Both cycles involve living organisms transforming compounds between forms that can and cannot be used by plants and animals.
Carbon (C) exists in the atmosphere as carbon dioxide (). Four processes cycle it:
| Process | Direction of CO₂ | Organisms involved |
|---|---|---|
| Photosynthesis | Atmosphere → organisms (removed) | Green plants and algae |
| Respiration | Organisms → atmosphere (released) | ALL living organisms |
| Decomposition | Dead matter → atmosphere (released) | Decomposers (bacteria, fungi) |
| Combustion | Fossil fuels / wood → atmosphere (released) | Non-biological (burning) |
Two-element definition of photosynthesis (mark-scheme form): Photosynthesis is the process by which green plants use light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen.
Word equation: carbon dioxide + water glucose + oxygen (requires light energy / chlorophyll)
Balanced chemical equation:
Two-element definition of respiration (mark-scheme form): Respiration is the chemical release of energy from glucose in living cells — not "breathing". It occurs in every cell of every organism.
Word equation (aerobic): glucose + oxygen carbon dioxide + water (+ energy released)
Nitrogen gas () makes up about 78% of the atmosphere but plants and animals cannot use it directly. It must be converted into usable compounds (ammonium ions or nitrate ions) first.
The four types of bacteria and their roles:
| Bacterial group | What they do | Where they live |
|---|---|---|
| Nitrogen-fixing bacteria | Convert gas → ammonium ions | Free in soil OR in root nodules of legumes (peas, beans, clover) |
| Decomposers (putrefying bacteria + fungi) | Break down proteins / urea in dead matter → ammonium ions | Soil, dead organic material |
| Nitrifying bacteria | Convert ammonium ions → nitrate ions (via nitrite) | Soil (aerobic conditions) |
| Denitrifying bacteria | Convert nitrate ions → gas | Waterlogged / anaerobic soils |
Flow through the nitrogen cycle:
Plants absorb nitrate ions through their roots and use them (with glucose from photosynthesis) to make proteins and other nitrogen-containing molecules. Animals obtain nitrogen by eating plants or other animals.
Define photosynthesis (mark-scheme form, two elements).
A carbon-cycle diagram shows four labelled arrows:
Name the process represented by each arrow. (4 marks)
