Edexcel IGCSE Biology · Spec 2.32
Absorption
How the villi in the small intestine are adapted for the fast, efficient absorption of digested food.
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Absorption
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Explained
The villus, and the four adaptations that make it work
Digestion breaks food into small soluble molecules. Absorption is the separate step of getting those molecules out of the gut and into the blood, and it happens almost entirely in the ileum, the second part of the small intestine.
The lining of the ileum is covered in millions of finger like projections called villi, and each villus is itself covered in much smaller microvilli. Every feature of a villus exists to make diffusion faster.
The four adaptations
- A large surface area, because there are millions of villi and each has microvilli on top of that. More surface means more diffusion at once.
- A wall one cell thick, giving a very short diffusion distance between the gut contents and the blood.
- A dense network of capillaries, which carries the absorbed glucose and amino acids away continuously. That keeps the concentration in the blood low, so the concentration gradient stays steep and diffusion keeps going.
- A lacteal running up the centre, which is a lymph vessel that absorbs the products of fat digestion, the fatty acids and glycerol.
Notice that the capillary network and the lacteal do different jobs. Sugars and amino acids go into the blood; fat products go into the lacteal. Questions ask which goes where.
Diffusion is not the whole story
Most absorption is by diffusion, because the concentration in the gut is higher than in the blood. Towards the end, though, most of the glucose has already gone, so the gradient runs the wrong way.
Villus cells finish the job by active transport, moving the last of the glucose and amino acids against the gradient using energy from respiration. That is why villus cells contain a great many mitochondria, and a diagram showing unusually many mitochondria is usually pointing at this.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for an exchange surface question lists the credited points as villi, a large surface area, blood or capillaries maintaining or making a steep gradient, a thin or short distance between the two sides, and diffusion or active transport. Its guidance then says that a short diffusion path is accepted for two marks, because the phrase covers both the thinness and its consequence.
On a different question about cells with many mitochondria, the mark scheme credits that villus cells carry out active transport and so need a lot of energy. The mitochondria are the observation; the active transport is the explanation.
One instruction from a related adaptation question sets out the general rule for this whole style of question. The note beside it reads: adaptation and how it helps. Naming the feature is half of it, and only half.
Where the absorbed food goes
Glucose and amino acids enter the capillaries, which drain into the hepatic portal vein and travel to the liver before reaching the rest of the body. The liver regulates glucose and processes excess amino acids, so nothing absorbed goes straight into general circulation.
Water is absorbed too, mostly in the small intestine but with the remainder taken up in the large intestine, which is why the material reaching the rectum is solid.
Answering an adaptation question
Give the feature, then what it increases or decreases, then why that speeds up absorption. Villi increase the surface area, so more molecules can be absorbed at once. The wall is one cell thick, so the diffusion distance is short, so diffusion is faster.
Three of those, and a four mark question is answered.
Spec 2.32
What you need to know
- Recall where digested food is absorbed
- Describe the structure of a villus
- Explain how villi are adapted for absorption
Active recall
Quick check
Answer each question before opening the answer.
Where are the products of digestion absorbed?
In the small intestine (ileum), through the villi.
Give two adaptations of a villus for absorption.
Any two of: large surface area, thin (one-cell) wall, rich blood supply, a lacteal for absorbing fats.
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