Cambridge O Level Biology · Syllabus 5090 · Human Nutrition
Absorption
What is Absorption?
The movement of digested food molecules through the wall of the intestine into the blood or the lymphatic system. It happens mainly in the ileum, and different substances cross by different processes: glucose and amino acids by diffusion and by active transport, water by osmosis, and the products of lipid digestion into the lacteal.
This definition is part of the Human Nutrition chapter in Cambridge O Level Biology.
Absorption in context
Human nutrition is the study of how food is taken in, broken down and used. A balanced diet supplies every required nutrient, plus fibre, water and energy, in suitable proportions. Digestion converts large, usually insoluble food molecules into small soluble ones. Absorption moves those small molecules through the wall of the intestine into the blood or lymph. Assimilation is what the body's cells then do with them. Material that was never digested leaves the body unchanged as faeces — that is egestion, and it is not excretion.
Common mistakes with Absorption
- “Digestion and absorption are the same thing.” Why it is wrong Digestion is a chemical and physical breakdown that happens in the cavity of the gut. Absorption is a movement across the gut wall. Different events, different places. Write instead "Digestion breaks large molecules into small soluble ones; absorption then moves those molecules through the intestinal wall into the blood."
- “Absorption and assimilation are two words for the same thing.” Why it is wrong Absorption is entry into the blood or lymph across the gut wall. Assimilation is what a cell then does with the molecule. A glucose molecule in the plasma has been absorbed and not yet assimilated. Write instead "Absorption is movement into the blood; assimilation is uptake and use by cells."
Questions students ask about Absorption
What is the difference between digestion and absorption?
Digestion is the breakdown of large, usually insoluble food molecules into small soluble molecules, and it happens in the cavity of the alimentary canal. Absorption is the movement of those small molecules through the wall of the intestine into the blood or the lymphatic system. Digestion prepares the molecules; absorption moves them.
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.