How is the small intestine adapted to absorb digested food efficiently?
The absorption of the soluble products of digestion in the small intestine, the adaptations of villi and microvilli for absorption, and the role of diffusion and active transport in taking up glucose and amino acids.
A focused answer to the WJEC GCSE Biology section 1.3 topic on absorption, covering how the soluble products of digestion are absorbed in the small intestine, the adaptations of villi and microvilli for efficient absorption, and the role of diffusion and active transport.
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What this dot point is asking
WJEC wants you to describe how the products of digestion are absorbed in the small intestine, explain how villi and microvilli are adapted for absorption, and describe the roles of diffusion and active transport in taking up glucose and amino acids.
Why absorption happens in the small intestine
After digestion, large molecules have become small soluble ones: starch is now glucose, protein is now amino acids, and fat is now fatty acids and glycerol. These are small enough to pass through the gut wall into the blood. This passing of soluble food molecules into the blood is called absorption, and it happens mainly in the small intestine.
Adaptations of the small intestine
The small intestine is superbly adapted to absorb food quickly and completely. The adaptations all work to speed up the rate of diffusion and active transport.
- Very long. The small intestine is several metres long, so food spends a long time in contact with the absorbing surface.
- Villi and microvilli. The inner lining is folded and covered in millions of tiny finger-like projections called villi. Each villus is itself covered in even smaller projections called microvilli. Together these give an enormous surface area for absorption.
- Thin walls. Each villus has a wall only one cell thick, giving a very short distance for molecules to diffuse across.
- Good blood supply. Each villus contains a network of capillaries that quickly carries away the absorbed glucose and amino acids. This keeps the concentration inside the villus low, maintaining a steep concentration gradient so absorption continues.
- A lacteal. Each villus also contains a small lymph vessel called a lacteal, which absorbs the products of fat digestion (fatty acids and glycerol).
How glucose and amino acids are absorbed
Glucose and amino acids move from the small intestine into the blood in two ways.
- By diffusion when their concentration in the gut is higher than in the blood: they move down the concentration gradient.
- By active transport when their concentration in the gut is lower than in the blood: they are moved against the gradient, using energy from respiration. This means that even the last traces of glucose and amino acids can be absorbed and none are wasted.
The fatty acids and glycerol from fat digestion pass into the lacteal rather than the blood capillaries.
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 style4 marksDescribe three ways the small intestine is adapted for the efficient absorption of digested food.Show worked answer →
A 4-mark question: three adaptations, each linked to absorption, with the fourth mark for clear links throughout.
The small intestine is very long and its inner surface is folded and covered in millions of tiny projections called villi (with even smaller microvilli on them), which give a very large surface area for absorption. Each villus has a wall only one cell thick, giving a short diffusion distance. Each villus has a good blood supply (a network of capillaries) that carries absorbed glucose and amino acids away quickly, keeping a steep concentration gradient.
Markers reward: large surface area from villi and microvilli; thin (one cell thick) wall for short diffusion distance; rich blood supply to maintain the gradient. Naming villi without linking to surface area does not gain the mark.
WJEC style3 marksGlucose can still be absorbed into the blood when its concentration is already higher in the blood than in the gut. Explain how.Show worked answer →
A 3-mark question on active transport.
When glucose is more concentrated in the blood than in the gut, it cannot move in by diffusion, because diffusion only goes from high to low concentration. Instead it is absorbed by active transport, which moves glucose against the concentration gradient. This uses energy released by respiration in the cells of the villi, so that all the glucose can be absorbed.
Markers reward: diffusion cannot work against the gradient; active transport moves glucose against the gradient; using energy from respiration. Saying "it diffuses in anyway" is wrong and gains no marks.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)