Explain why the cells lining the small intestine need active transport to absorb amino acids from the intestine into the blood, even when the concentration of amino acids inside the blood is already lower than inside the intestinal fluid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
After a protein-rich meal has been digested, amino acids are present in the small intestine. The cells lining the small intestine must move these amino acids into the blood. In some situations, the concentration of amino acids in the blood is actually higher than in the intestinal fluid.
Model answer (4 marks)
Active transport moves substances against a concentration gradient, from a lower to a higher concentration.
Diffusion alone cannot move amino acids into the blood when the concentration in the blood is higher than in the intestinal fluid.
Active transport requires energy (from respiration/ATP) to move amino acids.
Active transport uses carrier proteins in the cell membrane to move amino acids into the blood.
Diffusion alone cannot move amino acids into the blood when the concentration in the blood is higher than in the intestinal fluid.
Active transport requires energy (from respiration/ATP) to move amino acids.
Active transport uses carrier proteins in the cell membrane to move amino acids into the blood.
Examiner tips
- Use the term "active transport" and explain it moves against a gradient. Show that diffusion would fail when blood concentration is higher. Mention ATP/respiration as the energy source. State that carrier proteins are required.
- common_mistakes
- :
- Confusing passive diffusion with active transport. Forgetting to mention the energy requirement or carrier proteins. Saying the blood concentration is always lower, ignoring the higher‑concentration scenario.
Mark scheme (4 marks)
- Active transport moves substances against a concentration gradient (from a lower to a higher concentration)
- Diffusion alone cannot move amino acids into the blood when the concentration in the blood is higher than in the intestinal fluid
- Active transport requires energy (from respiration / ATP) to move amino acids
- Active transport uses carrier proteins (in the cell membrane) to move amino acids into the blood
Key terms in this question
Related
- Revision notes: Transport in cells →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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