Absorption — small intestine, plus colon for water
Absorption is the movement of small, soluble food molecules — glucose, amino acids, fatty acids and glycerol — from the alimentary canal into the blood or lymph. The small intestine is the site where nutrients are absorbed, and where most water is absorbed; the colon absorbs some water, concentrating the residue into faeces. Material never broken down small enough (e.g. cellulose) is not absorbed at all — it passes on and is egested.
Villi and microvilli — surface area at two scales
(Extended) Villi are finger-like projections of the small intestine lining; the surface of each epithelial cell covering them is folded further into microscopic microvilli (the brush border). Both increase the surface area for absorption, so more molecules cross per unit time. Illustratively, a smooth tube would offer roughly 0.5 m²; villi plus microvilli raise this to around 200–300 m². Cambridge rewards the principle, not a multiplier.
Villus structure — three features, three functions
(Extended) Each feature of a villus is an adaptation. Thin wall — a single layer of epithelial cells, so a short diffusion distance into the blood. Capillary network — flowing blood removes absorbed glucose and amino acids, maintaining a steep concentration gradient, and carries them to the liver. Lacteal — a central lymph vessel taking up fatty acids and glycerol. Uptake is by diffusion, and by active transport when gut concentration falls below blood concentration.
Drawn from real examiner reports.
Water: two sites, not one
Most water is absorbed from the small intestine; some is also absorbed from the colon (large intestine). Naming only the small intestine loses the second mark; naming only the large intestine loses the "most". If the colon absorbed no water, faeces would remain liquid. This is the most consistently penalised error on 7.5 absorption questions.
Flagged w22 P12 Q16; w22 P42 Q2c
Villi ≠ microvilli
(Extended) Villi are folds of the gut lining, roughly 0.5–1 mm tall and visible under a light microscope. Microvilli are folds of the cell surface membrane of individual epithelial cells, about 1 µm tall and visible only under an electron microscope. Writing "microvilli are small villi" loses the mark: villi fold the tissue, microvilli fold the membrane.
The lacteal is not a blood vessel
(Extended) The lacteal is a lymph vessel running through the centre of the villus, and it takes up fatty acids and glycerol. Blood capillaries surround it and take up glucose and amino acids. Calling the lacteal a capillary, vein or artery — or labelling the capillary network as the lacteal — is always wrong, in prose and on a diagram.
"Capillaries carry nutrients away" gains no mark
(Extended) Asked how the capillary network adapts a villus for absorption, "capillaries transport glucose to the body" earns nothing — the mark needs the mechanism. Flowing blood continuously removes absorbed glucose and amino acids, keeping their concentration on the blood side low, so a steep concentration gradient is maintained and diffusion continues rapidly.
Absorption ≠ digestion
Digestion breaks large insoluble molecules into small soluble ones; absorption moves those small molecules across the gut wall into the blood or lymph. Physical digestion increases the surface area available for enzyme action — a separate idea from villi and microvilli increasing surface area for absorption. Do not offer one as an answer to the other.
Flagged s23 P11/21 Q14-15
The stomach absorbs no nutrients
The stomach is a site of mechanical digestion and of chemical digestion by pepsin in acidic conditions — it is not a site of nutrient absorption. Any multiple-choice option placing nutrient absorption in the stomach can be eliminated immediately: nutrients are absorbed in the small intestine.
"Thin wall" alone is too vague
(Extended) Describing the villus wall as "thin", "a membrane" or "skin" does not score. State the structural detail — a single layer of epithelial cells, one cell thick — and then give the consequence: the diffusion distance from gut lumen to blood is short, so nutrients are absorbed faster.
Use the adaptation chain
(Extended) Answer every "explain how the villus is adapted for absorption" question in four steps: name the feature → give the structural detail → say which diffusion variable it changes (area, distance or gradient) → state the effect on rate of absorption. Step 4 is the one most often dropped.
Give both sites for water
Answer any "where is water absorbed?" question with two sites and a quantifier: "most water is absorbed from the small intestine; some is absorbed from the colon (large intestine)". One site alone, or an unqualified "large intestine", costs a mark.
Label a villus diagram exactly
(Extended) Three internal labels are expected: the epithelium (single layer of cells, one cell thick), the network of blood capillaries (glucose and amino acids) and the central lacteal (fatty acids and glycerol). Microvilli on the outer surface are often omitted — add them.
Match the answer to the tier
Core questions ask only for the sites: small intestine for nutrients, small intestine (most) plus colon (some) for water. Villi, microvilli, the lacteal and gradient reasoning are Extended content — on an Extended paper, naming the site alone will not reach the higher marks.
Absorption is the movement of digested food molecules and water from the alimentary canal (gut) into the blood or lymph. It follows chemical digestion (which breaks large molecules into small, soluble ones).
Sites of absorption:
Key mark-scheme rule for water: always state BOTH sites. "Most water is absorbed from the small intestine; some water is absorbed from the colon."
State the region of the alimentary canal where digested nutrients are absorbed.
(a) State the region of the alimentary canal where digested nutrients are absorbed. (1 mark)
(b) State where water is absorbed in the alimentary canal. Include all relevant sites. (2 marks)
