Cambridge O Level Biology · Syllabus 5090 · Human Nutrition
Small Intestine
What is Small Intestine?
The long, narrow region of the alimentary canal between the stomach and the colon, made up of the duodenum and the ileum. Chemical digestion of carbohydrate, protein and lipid is completed here using enzymes from the pancreas and the intestinal wall, and it is the main site of absorption of nutrients and of most of the water.
This definition is part of the Human Nutrition chapter in Cambridge O Level Biology.
Common mistakes with Small Intestine
- “Enzymes are made where they act.” Why it is wrong Pancreatic amylase is made in the pancreas and acts in the small intestine; bile is made in the liver and acts in the duodenum. Questions ask for both columns precisely because they differ. Write instead "Amylase is produced by the salivary glands and the pancreas, and acts in the mouth and the small intestine."
- “A villus is a cell.” Why it is wrong A villus is a projection of the intestinal wall made of many cells, containing a capillary network and a lacteal. A single cell contains neither. Write instead "A villus is a finger-like projection of the wall of the small intestine, covered by an epithelium one cell thick."
- “All the water is absorbed in the colon.” Why it is wrong Most water is absorbed in the small intestine, where the villus surface is enormous. The colon absorbs the remainder, which is why the visible change to firm faeces happens there. Write instead "Most water is absorbed in the small intestine; further water is absorbed in the colon."
Questions students ask about Small Intestine
Where is most water absorbed?
Most water is absorbed in the small intestine, where the villi provide an enormous surface area. Further water is then absorbed in the colon, which is why the contents become firmer as they pass along it. The common misconception is that all water absorption happens in the colon.
Why does the stomach need a different protease from the small intestine?
Because an enzyme's shape depends on the pH around it. The stomach is strongly acidic and the small intestine is alkaline, so an enzyme with an acidic optimum would be denatured in the intestine, and one with an alkaline optimum would be denatured in the stomach. Making two proteases with two optima is the only way to digest protein in both regions.
Is a villus a single cell?
No. A villus is a finger-like projection of the wall of the small intestine, made of many cells. Its outer covering, the epithelium, is one cell thick, and inside it are a dense network of blood capillaries and a central lacteal — none of which a single cell could contain.
What does the hepatic portal vein carry, and where?
It carries most of the molecules and ions absorbed in the ileum — glucose, amino acids, mineral ions and water — in blood, from the capillaries of the small intestine directly to the liver. It is unusual among veins because it runs from one capillary network to another instead of returning blood to the heart. The products of lipid digestion are the exception: they enter the lacteals and travel through the lymphatic system, joining the blood later.