4cCycles within ecosystems

Carbon is constantly recycled between the atmosphere, living organisms and fossil fuels. Only four processes are named in the specification — learn them as the four "engines" of the cycle.

The carbon store in the air

Carbon is present in the air as carbon dioxide. Everything in the cycle either removes CO₂ from the air or releases CO₂ into the air.

The stages, in order

1. Photosynthesis — plants absorb carbon dioxide from the air and use light energy to convert it into glucose. The carbon is now locked into carbohydrates, fats and proteins in the plant.
2. Feeding — animals eat plants (or eat other animals), so carbon compounds are passed along the food chain.
3. Respiration — plants, animals and microorganisms break down glucose and release carbon dioxide back into the air. This happens all the time, day and night.
4. Death and waste — dead organisms, faeces and urine contain carbon compounds.
5. Decomposition — decomposers (bacteria and fungi) feed on the dead material and waste. As they respire, they release CO₂ back into the atmosphere.
6. Fossilisation — if dead organisms are not decomposed (for example in airless, waterlogged conditions), over millions of years their remains are compressed to form fossil fuels such as coal, oil and natural gas. Carbon is locked away here for a very long time.
7. Combustion — burning fossil fuels or wood releases the stored carbon as carbon dioxide.

Getting a description question right

If you are asked to describe the carbon cycle, name the process, name the organism doing it, and name the substance moved. "Plants take in carbon" is too vague. "Plants remove carbon dioxide from the air by photosynthesis and convert it into glucose" scores.

Common mistake: writing about photosynthesis and forgetting to say that it removes carbon dioxide from the atmosphere. Examiners repeatedly report answers that mention photosynthesis but never state the removal of CO₂ — that is where the mark sits.

Examiner tip. Two errors lose marks here every year. (1) Confusing the gases: photosynthesis takes in carbon dioxide and gives out oxygen; respiration takes in oxygen and gives out carbon dioxide. Never write that a leaf 'needs oxygen for photosynthesis', and never say respiration needs light. (2) In data or comprehension questions, don't just repeat phrases from the passage — say explicitly which process removes CO₂ and which releases it.

Separate Biology only — not in Science (Double Award).

About 78% of the air is nitrogen gas (N₂), but it is very unreactive — plants and animals cannot use nitrogen gas directly. Organisms need nitrogen to make amino acids, and therefore proteins (and DNA). Bacteria do almost all of the converting.

You do not need the names of any species of bacteria — only their roles.

The four groups of microorganisms

The stages, in order

1. Nitrogen fixation — nitrogen fixing bacteria convert nitrogen gas from the air into nitrogen compounds the plant can use. Lightning also fixes a small amount of nitrogen, forming nitrates that wash into the soil.
2. Absorption — plants take up nitrates from the soil through their roots (by active transport).
3. Protein synthesis — plants use the nitrates to make amino acids, which are built into proteins.
4. Feeding — animals eat plants and use the plant protein to make their own protein.
5. Death and excretion — dead plants and animals, plus urine and faeces, return nitrogen compounds to the soil.
6. Decomposition — decomposers break these down, releasing ammonium compounds.
7. Nitrification — nitrifying bacteria oxidise ammonium compounds to nitrites and then to nitrates, which plants can absorb again.
8. Denitrification — in waterlogged soil, denitrifying bacteria convert nitrates back to nitrogen gas, returning it to the atmosphere.

Why farmers plough and drain soil

Ploughing and draining add oxygen to the soil. This favours nitrifying bacteria (which need oxygen) and reduces the activity of denitrifying bacteria (which work without oxygen), so more nitrate stays in the soil.

Growing legumes raises soil nitrate because of the nitrogen fixing bacteria in their root nodules.

Examiner tip. The single most common error on this topic is mixing up the bacteria. Fix the names by their first syllable: nitrogen FIXing = takes N₂ from the air and fixes it into the soil; NITRIFying = makes nitrates from ammonium compounds; DE-nitrifying = removes nitrates, back to N₂ gas. If a question asks about a waterlogged field, it wants denitrifying bacteria; if it asks how ammonium compounds become usable by plants, it wants nitrifying bacteria.

Both cycles are examined in the same few ways. Getting the technique right is worth as much as knowing the content.

Which cycle applies to you? The carbon cycle is for everyone. The nitrogen cycle is 4.11B — separate Biology only — not in Science (Double Award), so if you are taking Double Award you can use the technique below but skip the nitrogen examples.

Note. This is a technique page, not a content page. Learn the stages of the cycles themselves from the carbon cycle and nitrogen cycle notes, then come back here for how to write the answers.

Describe vs explain

Examiners report that these two command words are confused more than any others.

If the question says explain, a list of stages scores nothing. If it says describe, you don't need reasons — but you do need the correct technical terms.

Use the diagram or data you are given

Many questions give a diagram of a cycle with unlabelled arrows, or data from an ecosystem. Examiners note that candidates lose marks by ignoring the diagram and writing a general answer they have learnt.

• Check which way each arrow points: into the atmosphere = respiration, decomposition or combustion; out of the atmosphere = photosynthesis (or, on a nitrogen cycle diagram, nitrogen fixation — separate Biology only — not in Science (Double Award)).
• If the diagram shows dead material building up, say less decomposition and therefore fewer mineral ions released — but only if the diagram actually shows that.

Precision that earns marks

Carbon cycle:

• Say carbon dioxide, not "carbon", when you mean the gas.
• Say respiration of decomposers releases carbon dioxide — decomposition on its own is not a gas-producing magic step.
• Name the organism: plants, animals or decomposers (bacteria and fungi).

Nitrogen cycle (separate Biology only — not in Science (Double Award)):

• Say nitrates, not "nitrogen", when you mean what a plant absorbs from the soil.
• Say nitrogen gas or N₂ when you mean the atmospheric form.
• Name the type of bacterium the question is about — nitrogen-fixing, nitrifying or denitrifying (the specific species names are never required).

Temperature and decay questions

Both cycles rely on decomposers, so questions often ask why decay is faster in warm conditions.

Say: warmer temperatures increase the rate of respiration and enzyme activity in decomposers, so they break down dead material faster. At very high temperatures, enzymes are denatured and the bacteria are killed.

Common mistake: writing that high temperature "kills enzymes" or "denatures bacteria". Enzymes are denatured; organisms are killed. Use the words the right way round.

Examiner tip. Read the question stem twice and answer the question actually asked. Examiners note that most lost marks come from answering a different question — describing a pattern instead of explaining it, or describing an effect on the whole ecosystem when only one process was asked about. Underline the command word and the named process before you start writing.