MYP Biology: The Carbon & Nitrogen Cycles — FAQ

The carbon cycle tracks how carbon moves between the atmosphere, living organisms and the ocean via photosynthesis, respiration, decomposition and combustion. The nitrogen cycle tracks how nitrogen moves through fixation, nitrification, assimilation and denitrification. Both are studied in MYP Biology as examples of matter cycling through ecosystems, never disappearing.

The carbon cycle moves carbon mainly through photosynthesis and respiration, with carbon dioxide as the key gas. The nitrogen cycle moves nitrogen through bacteria-driven steps — fixation, nitrification, denitrification — because plants can't use atmospheric nitrogen (N2) directly. That single fact is the whole reason bacteria matter so much in one cycle and not the other.

Decomposers — bacteria and fungi — break down dead organisms and waste, releasing carbon (as CO2, through their own respiration) and nitrogen (as ammonium ions) back into the soil and air. Without them, carbon and nitrogen would stay locked inside dead matter and both cycles would grind to a halt.

Nitrogen fixation is the conversion of unusable atmospheric nitrogen gas (N2) into ammonia or ammonium, mostly by bacteria such as Rhizobium living in root nodules of legumes. It matters because plants can only absorb nitrogen in this fixed, soluble form — not as the inert gas that makes up 78% of the atmosphere.

Assessment typically falls under Criterion A (Knowing and Understanding) for labelling diagrams and explaining processes, and Criterion D (Thinking Critically) for evaluating human impacts like fertiliser runoff or deforestation. Unit tests often use a labelled diagram plus a short-answer or data-response section rather than one long essay.

The most common command terms are describe (state the process step by step), explain (give reasons, using "because"), and evaluate (weigh up human impacts, like the pros and cons of nitrogen fertiliser use). Mixing these up is the single biggest reason students lose marks on this topic.

You need to confidently label a carbon cycle diagram showing photosynthesis, respiration, combustion, decomposition and ocean exchange, and a nitrogen cycle diagram showing fixation, nitrification, assimilation, ammonification and denitrification. Examiners in MYP unit tests almost always include at least one blank or partially-labelled diagram question.

Learn the five named processes as a fixed sequence — fixation, nitrification, assimilation, ammonification, denitrification — and attach one bacterium or condition to each step rather than memorising isolated facts. Students who tie each word to a concrete example (Rhizobium, lightning, decomposer bacteria) retain it far longer than those who just read the diagram.

The three I see most often: confusing nitrification with nitrogen fixation, forgetting that respiration happens in both plants and animals (not just animals), and describing decomposition vaguely instead of naming the molecule released. Each one is an easy fix once you notice you're doing it.

Human activity disrupts the natural balance of the carbon cycle mainly through burning fossil fuels, which releases carbon that took millions of years to accumulate underground, and through deforestation, which removes the photosynthesis that would otherwise absorb it. This imbalance is the mechanism behind the enhanced greenhouse effect, a topic that often follows directly on from this unit.

This MYP unit builds the foundation for DP Biology's Topic 4 (Ecology), where the same cycles return with more depth — quantitative data on carbon flux, eutrophication case studies and biogeochemical cycling. According to the IB, first exams for the current DP Biology guide were 2025, and Topic 4 explicitly assumes familiarity with cycle diagrams learned earlier.

Not conceptually hard, but it's dense — there are five named nitrogen processes and several carbon pathways to keep straight, and the terminology (nitrification vs denitrification) trips students up more than the actual biology does. Most students who struggle are memorising labels without understanding why each step happens.

Look for resources that combine a clearly labelled diagram, a step-by-step process list, and practice questions using real command terms (describe, explain, evaluate) — not just a textbook summary. On RevisionPrep, the MYP Biology Revision Notes and Topical Worksheets are built around exactly this structure, with practice questions marked against the actual MYP criteria.