Explain how carbon moves from atmospheric carbon dioxide into the organic molecules of a primary consumer.

IB DP Biology Standard Level (2023 syllabus) — C4.2 Transfers of energy and matter · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Plants absorb atmospheric CO₂ during photosynthesis and fix it into organic molecules such as glucose. The primary consumer then feeds on the plant, ingesting these carbon‑containing molecules. In the consumer’s digestive system the plant carbohydrates are broken down, and the resulting carbon atoms are assimilated into the consumer’s own organic molecules through biosynthetic pathways.

Examiner tips

Use the term ‘photosynthesis’ and ‘fix CO₂’ to show understanding of the source of carbon.
Mention that the consumer obtains carbon by feeding on the producer and that digestion and assimilation occur.
Show the flow from atmospheric CO₂ → plant organic molecules → consumer organic molecules.
Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

Failing to mention photosynthesis as the source of carbon.
Confusing the consumer’s digestion with the plant’s photosynthesis.
Using vague terms like ‘food’ instead of ‘organic molecules’ or ‘carbohydrates’.

Mark scheme (4 marks)

Plants (producers) absorb/fix atmospheric CO₂ during photosynthesis
Carbon is incorporated into organic/carbon-containing molecules (e.g. glucose/sugars/carbohydrates) in the producer
The primary consumer obtains carbon by feeding on / consuming the producer
Carbon-containing molecules from the producer are digested and assimilated into the consumer's own organic molecules / used in biosynthesis

Key terms in this question

organic molecules · primary consumer