Exam Tips: The Carbon Cycle

Plants both absorb and release CO2: One of the most common errors is stating that plants only take in CO2. Plants carry out photosynthesis (absorbs CO2) AND respiration (releases CO2) simultaneously when there is light. In the dark, only respiration occurs. On balance, healthy growing plants are net absorbers of CO2, but they are not one-way CO2 sponges. In exam answers, say plants are net absorbers during the day, not that they never release CO2.

Deforestation has two effects on CO2: When a forest is cleared, CO2 rises for TWO reasons: (1) the trees that previously removed CO2 through photosynthesis are gone — so less CO2 is absorbed; (2) burning or decomposing the felled trees releases CO2. Always give both reasons in a deforestation question for full marks.

Know why decomposition is slow in bogs: Peat bogs are waterlogged and oxygen-poor (anaerobic). Most decomposers are aerobic — they need oxygen for respiration. Without oxygen, decomposition is very slow and carbon accumulates as peat. Draining the bog allows oxygen in, enabling aerobic decomposition and CO2 release. This chain of reasoning is worth 3-4 marks in extended response questions.

Only photosynthesis removes CO2 from the atmosphere: This is a one-mark, guaranteed exam question. No other process removes CO2 from the air into living organisms. Decomposers, animals, and non-photosynthetic bacteria all only release CO2. When asked "how is carbon removed from the atmosphere?", the answer is always photosynthesis.