Lesson overview
Absorption: How does the small intestine absorb so much so quickly?
Focus a slide of the ileum from ×40 to ×400 and lay a string along the folded edge of its villi (tiny finger-like folds of the lining) to find how much longer it is than a straight edge. Then look inside one villus: its thin lining, the microvilli and where absorbed food goes.
Suggested sequence
- Lesson 1 · Where food is absorbed. Where digested food and water are absorbed: the small intestine and the colon. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Villi under the microscope. Villi under the microscope: how much the folded lining adds to the surface. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Inside a villus. Inside a villus: the lining, microvilli, capillaries and the lacteal. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 6 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 7 questions, 15 min, lesson closed.
Learning goals
Part 1: Where food is absorbed
- State that the small intestine is the region where nutrients are absorbed (7.5.1)
- State that most water is absorbed from the small intestine but that some is also absorbed from the colon (7.5.2)
Part 2: Villi under the microscope
- Supplement: explain the significance of villi and microvilli in increasing the internal surface area of the small intestine (S7.5.3)
Part 3: Inside a villus
- Supplement: describe the structure of a villus (S7.5.4)
- Supplement: describe the roles of capillaries and lacteals in villi (S7.5.5)
Misconceptions to listen for
- “E, the stomach, absorbs the food.” — The stomach churns food and starts to digest protein, but very little is absorbed there.
- “H, the colon, absorbs the nutrients.” — The colon absorbs some water. By the time food reaches it, the nutrients have already been absorbed in the small intestine.
- “F, the pancreas, absorbs the nutrients.” — The pancreas makes a digestive juice that flows into the small intestine. Food never passes through the pancreas, so nothing is absorbed there.
- “Absorption is taking food into the mouth.” — That is ingestion. Absorption is the movement of nutrients from the intestines into the blood.
- “Nutrients are absorbed in the stomach.” — The stomach absorbs very little. Nutrients are absorbed in the small intestine.
- “Most water is absorbed from the colon.” — The colon does absorb water, but by the time food reaches it most of the water has already been absorbed from the small intestine.
- “Most water is absorbed from the stomach.” — Very little is absorbed through the wall of the stomach. Most water is absorbed from the small intestine.
- “No water is absorbed from the colon.” — The colon does absorb some water. That is why faeces are much drier than the contents of the small intestine.
- “Water moving into the blood is not absorption, because water is not food.” — Absorption is movement from the intestines into the blood. Water moving from the colon into the blood is absorption.
- “Maltase making glucose is absorption, because glucose is what gets absorbed.” — Making glucose from maltose breaks a molecule down: that is digestion. The glucose moving into the blood afterwards is absorption.
Scope
Absorption (7.5.1–7.5.5): the small intestine as the region where nutrients are absorbed, where water is absorbed (small intestine and colon), and, for the Supplement, villi and microvilli, the structure of a villus and the roles of its capillaries and lacteal. Digestion itself is in the physical and chemical digestion modules; assimilation and the hepatic portal vein are not covered.