Explain what happens to a fat molecule during digestion, including where digestion occurs and how the products are absorbed into the body.

AQA A-Level Biology (7402) — 3.3.3 Digestion and Absorption · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Bile is released from the gallbladder into the duodenum where it emulsifies fat, breaking it into small droplets and increasing surface area.
Lipase from the pancreas then acts on these droplets in the small intestine, hydrolysing triglycerides into fatty acids and glycerol.
The fatty acids and glycerol are absorbed by the enterocytes of the villi and enter the lacteals of the lymphatic system, from which they are transported into the bloodstream.

Examiner tips

  • Mention bile and emulsification first, then lipase action, then absorption into lacteals; keep the order logical.
  • Use the exact terms 'bile', 'lipase', 'fatty acids', 'glycerol', 'lacteals', 'lymphatic system'.
  • Show the link between increased surface area and efficiency of lipase.

Common mistakes

  • Failing to state that bile emulsifies fat; only mentioning lipase.
  • Saying fat is absorbed directly into blood instead of lacteals; confusing lymphatic transport with venous uptake.
  • Using vague terms like 'break down' without specifying fatty acids and glycerol.

Mark scheme (5 marks)

  1. Bile emulsifies fat (breaks fat into smaller droplets)
  2. Lipase breaks down / digests fat in the small intestine
  3. Fat is broken down into fatty acids and glycerol
  4. Fatty acids and glycerol are absorbed into the lacteals (lymphatic system) / through the villi
  5. Emulsification increases the surface area (of fat) so lipase can act more efficiently / faster

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