How is the small intestine adapted to absorb digested food efficiently?
Absorption of the soluble products of digestion in the small intestine, and how the villi are adapted for efficient absorption.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on absorption, covering how the soluble products of digestion are absorbed in the small intestine and how the villi are adapted for efficient absorption by diffusion and active transport.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to describe how the soluble products of digestion are absorbed in the small intestine, and explain how the villi are adapted for efficient absorption.
What is absorbed
After chemical digestion, food has been broken into small soluble molecules:
- Carbohydrates to glucose and other sugars,
- Proteins to amino acids,
- Fats to fatty acids and glycerol.
These are absorbed through the wall of the small intestine into the blood (sugars and amino acids) or into the lacteal (fatty acids and glycerol). Large insoluble molecules cannot be absorbed, which is why digestion must happen first.
How the villi are adapted
The villi are adapted for efficient absorption:
- Large surface area: millions of villi, each covered in microvilli, hugely increase the area for absorption.
- Thin walls (one cell thick): the distance for molecules to diffuse into the blood is very short.
- Good blood supply (network of capillaries): absorbed glucose and amino acids are carried away quickly, keeping a steep concentration gradient.
- A lacteal: a small lymph vessel that absorbs the products of fat digestion (fatty acids and glycerol).
- Long small intestine: gives plenty of time and area for absorption as food passes along.
Diffusion and active transport in absorption
Most food is absorbed by diffusion, moving from a high concentration in the gut to a lower concentration in the blood. But after a meal has been mostly absorbed, the concentration in the gut can fall below that in the blood. Glucose and amino acids can still be absorbed against this gradient by active transport, using energy from respiration, so that as much food as possible is taken up.
Where the absorbed food goes
Once absorbed, the food does not stay in the gut wall. Glucose and amino acids enter the blood capillaries of each villus and are carried first to the liver and then around the body. Fatty acids and glycerol enter the lacteal (a lymph vessel) and join the blood later. The blood then delivers glucose to cells for respiration, amino acids for building new proteins, and stores any excess. This is why a good blood supply in the villi matters: it carries food away quickly and keeps the concentration gradient steep for more absorption.
Absorbing water in the large intestine
The small intestine absorbs the digested food and most of the water. The material that is left passes into the large intestine, where the remaining water is absorbed back into the body, leaving the solid waste (faeces) to be removed. If too little water is absorbed (for example during illness), the waste stays watery, causing diarrhoea, which is why rehydration matters. Absorbing water is just as important as absorbing food, and it is a common point in data questions.
Try this
Q1. Name the small finger-like projections that line the small intestine. [1 mark]
- Cue. Villi.
Q2. State one way a villus is adapted to absorb food quickly. [1 mark]
- Cue. Any one of: large surface area, thin (one-cell-thick) walls, good blood supply, or a lacteal.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style5 marksDescribe three ways the villi in the small intestine are adapted for efficient absorption, explaining how each adaptation helps.Show worked answer →
A Unit 1 extended question worth 5 marks. Reward up to: a large surface area (millions of villi, and microvilli) so more food is absorbed (1); thin walls (one cell thick) so the diffusion distance is short (1); a good blood supply (capillaries) to carry absorbed food away and keep a steep gradient (1); a lacteal to absorb fatty acids and glycerol (1); and being long to give time for absorption (1). Markers credit three adaptations each linked to faster or more complete absorption. A common error is to list features without explaining how they help.
WJEC style3 marksExplain why glucose can be absorbed by both diffusion and active transport in the small intestine.Show worked answer →
A Unit 1 explain question. When the concentration of glucose in the gut is higher than in the blood, glucose moves into the blood by diffusion, down the concentration gradient (1 mark). When the concentration in the gut becomes lower than in the blood, glucose can still be absorbed by active transport, which moves it against the gradient using energy from respiration (1 mark), so that as much glucose as possible is absorbed (1 mark). Markers reward diffusion down the gradient, active transport against the gradient, and the reason. A common error is to say only diffusion is used.
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