Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 16% of your exam marks.
stable
Very High
Stable16%
Enzymes in digestion and the role of digestive structures appear regularly across both papers.
Once digestion is complete, the small soluble products (glucose, amino acids, fatty acids and glycerol) need to be absorbed into the blood. This happens in the ileum, the long second part of the small intestine.
Adaptations of the small intestine for absorption
The ileum is brilliantly adapted to take in the maximum amount of dissolved food in the shortest time:
- Very long (around 6 metres in an adult): the food spends a long time in contact with the absorbing surface
- Inner surface folded into millions of tiny finger-like projections called villi: each villus is about 1 mm long
- The cells covering each villus have their surfaces folded into thousands of even tinier microvilli
- Combined, the villi and microvilli increase the inner surface area to roughly 250 m², about the size of a tennis court
Structure of a villus
Each villus has:
- A thin wall just one cell thick (a single layer of epithelial cells), so the diffusion distance for absorbed molecules is very short
- A dense network of blood capillaries running just inside the wall, which carries away the glucose and amino acids as soon as they are absorbed. This keeps the concentration gradient between the gut and the villus steep, maintaining fast absorption
- A lacteal (a small lymph vessel) running up the centre, which absorbs the fatty acids and glycerol and carries them into the lymphatic system
Most digested food is absorbed by diffusion (down its concentration gradient into the blood). When the gradient runs out, active transport is used to absorb the last molecules against the gradient.
