Explain how carbon dioxide and oxygen are removed from the leaves of a flowering plant as waste products of metabolic processes.

Pearson Edexcel International GCSE Biology (4BI1) — 2.12 Excretion: flowering plants · Explain · 4 marks · View as Markdown

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

Flowering plants produce waste gases as a result of photosynthesis and respiration. Unlike animals, they do not have a specialised excretory organ, yet these gases must leave the plant body.

Model answer (4 marks)

Carbon dioxide is produced in plant cells during respiration and is a waste product. Oxygen is produced in excess during photosynthesis and is also a waste product. Both gases leave the leaf by diffusion through the stomata. Diffusion occurs down a concentration gradient, from the higher concentration inside the leaf to the lower concentration outside.

Examiner tips

  • Use the exact terms ‘respiration’ and ‘photosynthesis’. Mention that CO₂ is a waste of respiration and O₂ a waste of photosynthesis. State that gases exit via stomata and that diffusion is down a concentration gradient. Keep the answer to four clear points to match the marks.

Common mistakes

  • Confusing the source of CO₂ and O₂ (e.g. saying CO₂ is from photosynthesis). Forgetting to mention stomata as the exit route. Not specifying that diffusion is down a concentration gradient.

Mark scheme (4 marks)

  1. Carbon dioxide is a waste product of respiration (in plant cells)
  2. Oxygen is a waste product of photosynthesis (produced in excess of the plant's own respiratory needs)
  3. Both gases leave the leaf by diffusion through the stomata
  4. Diffusion occurs down a concentration gradient (higher concentration inside the leaf / air spaces to lower concentration outside)

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