Ch 5 · Absorption · O Level Biology 5090
Topic 5 of 5 in Human Nutrition — O Level Biology 5090 — 38 lessons.
Introduction
Digestion produces glucose, amino acids, fatty acids and glycerol, but they are useless until they are inside the blood. Absorption moves them across the intestine wall, and the small intestine is superbly adapted for this, with a lining folded into millions of finger-like villi. This lesson covers where absorption happens, the structure of a villus and the jobs of its capillaries and lacteal, and, for candidates, why surface area matters and how absorbed molecules are carried away.
Key ideas — Where absorption happens
The small intestine is the region where the products of digestion (glucose, amino acids, fatty acids and glycerol), together with vitamins and mineral ions, are absorbed. Most of this happens in the ileum, after digestion has been completed in the duodenum.
🧑🏫 Teacher's explanation
Imagine you are trying to soak up water using a sponge. If you use a flat, smooth sponge, it won't pick up much water quickly. But if that sponge is covered in millions of tiny, thin threads, it has a massive surface area and can soak up water incredibly fast. The small intestine works exactly like that sponge.
📖 Definitions
🔍 Worked example — Easy
A student observes a diagram of a villus. Identify the two main types of transport used to move nutrients from the lumen into the epithelial cells.
🔍 Worked example — Standard
Explain why the presence of villi and microvilli is essential for efficient nutrition.
🔍 Worked example — Exam-style
A patient has a condition where the villi in their small intestine are flattened (atrophy). Predict the effect on their nutritional status and explain why.
⚠️ Common errors
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🎯 Exam question
Key ideas — Structure of a villus
The inner surface of the small intestine is covered with millions of tiny projections about one millimetre long called villi (singular villus).
🧑🏫 Teacher's explanation
Imagine you are trying to soak up water using a towel. If you use a flat, smooth sheet of plastic, very little water gets absorbed. But if you use a thick, fluffy towel with millions of tiny loops, the surface area increases massively, and it absorbs water much faster. The villi are like those loops, turning the smooth wall of your intestine into a massive, fuzzy sponge.
📖 Definitions
🔍 Worked example — Easy
Identify the specific component within a villus responsible for absorbing glucose and amino acids.
🔍 Worked example — Standard
Explain why the epithelium of a villus is only one cell thick.
🔍 Worked example — Exam-style
A student observes a cross-section of a villus. They note that the concentration of glucose in the intestinal lumen is 5.0 mmol/L and the concentration in the capillary is 1.0 mmol/L. Explain the significance of the blood flow in maintaining this gradient.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Surface area and transport of nutrients
villi and microvilli. The rate of absorption depends on the surface area across which molecules can pass. The small intestine is long (about 5 to 6 metres) and its lining is folded. On those folds sit millions of villi, and each epithelial cell carries thousands of microvilli. Together these multiply the internal surface area hundreds of times compared with a smooth tube, so far more molecules can be absorbed per second. The one-cell-thick epithelium and rich blood supply add to this by keeping the diffusion distance short and the gradient steep.
🧑🏫 Teacher's explanation
Imagine you are trying to dry a wet sponge. If you leave a large, flat sponge on a table, it dries slowly. But if you shred that sponge into thousands of tiny pieces and spread them out, the total surface area increases massively, and the water evaporates much faster. This is exactly how your small intestine works.
📖 Definitions
🔍 Worked example — Easy
Identify the two main components of a villus that facilitate the rapid transport of nutrients away from the site of absorption.
🔍 Worked example — Standard
Explain why the epithelium of the villus is only one cell thick.
🔍 Worked example — Exam-style
A student is studying the absorption of different nutrients. Compare the pathway of glucose and the pathway of fatty acids from the lumen of the small intestine to the circulatory system.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Explain how the structure of a villus makes it well adapted for the absorption of digested food. [5]
🔍 Worked example — Easy
Identify the two main types of transported substances and their corresponding vessels within a villus.
🔍 Worked example — Standard
Calculate the percentage increase in surface area if a flat intestine has an area of 2 m^2 and the presence of villi increases it to 30 m^2.
🔍 Worked example — Exam-style
Explain why a one-cell-thick epithelium is essential for efficient absorption. [2 marks]
⚠️ Common errors
Common mistakes
Practice
Answers (a) Small intestine (ileum) [B1]; (b) small intestine [B1]; (c) colon [B1]. (a) Glucose and amino acids (also mineral ions, water, water-soluble vitamins) [B1]; (b) fatty acids and glycerol [B1]. Short distance for molecules to diffuse or be actively transported [B1]; so absorption is faster [B1]. Glucose enters a blood capillary in the villus [B1]; capillaries join to form the hepatic portal vein, which carries blood to the liver [B1]. Microvilli are folds of the epithelial cell membrane that greatly increase its surface area [B1]; more molecules can cross per second, so absorption is faster [B1].