Absorption

Where absorption happens

Absorption is the movement of small food molecules and ions through the wall of the intestine into the blood or lymph. The small intestine is the main site where the products of digestion are absorbed. Most water is also absorbed there, rather than further along in the gut, with only someof what remains being absorbed in the colon. That precise split — most in the small intestine, some in the colon — is worth stating exactly, since the loose version, “water is absorbed in the large intestine,” is an oversimplification that costs marks.

Absorption is not the same process as digestion, and it is not the same as assimilation either: digestion, covered in chemical digestion, produces the small soluble molecules in the first place; absorption moves them into the blood or lymph; assimilation, covered in the digestive system, is what happens afterwards, once a cell actually takes up and uses what has been absorbed.

Villi and microvilli — two levels of folding

Because absorption happens across a surface, the amount that can be absorbed at any moment depends directly on how much surface is available — and the small intestine is folded at two separate scales to maximise exactly that. Its lining carries many finger-like projections called villi — singular villus — which greatly increase the internal surface area on their own. The epithelial cells covering each villus then carry much smaller projections of their own, called microvilli, increasing the surface area further still. A villus is a tissue-level structure made of many cells; a microvillus is a far smaller projection of a single cell’s own membrane — related terms, but not interchangeable ones, and neither word is simply the plural of the other.

It is worth being clear about what this increased surface area is actually for: it increases the rate and quantityof absorption, not the size of the molecules being absorbed. Making molecules smaller is digestion’s job, not absorption’s — see chemical digestion.

Inside a villus

A villus combines that large surface area with a short distance for molecules to travel and an effective way of carrying absorbed nutrients away, and the three work together as a single mechanism rather than as three unrelated facts. Its epithelium is commonly described as one cell thick, giving absorbed molecules only a short distance to cross. Inside, a villus contains a network of blood capillaries, which carry away absorbed water-soluble nutrients such as glucose and amino acids, and a lacteal, part of the lymphatic system, which receives the products of fat digestion — fatty acids and glycerol.

Keep those two transport routes apart: fatty acids and glycerol do not pass into the blood capillaries here — they are taken up into the lacteal instead, and sending fat into the capillaries is the standard error in this section. Rapid removal of absorbed molecules by both routes also does real work of its own: by keeping the concentration inside the villus low, it maintains the concentration gradient across the wall that absorption depends on, so absorption can continue rather than stalling once the villus fills up. A strong answer on villus adaptations pairs the large surface area with this short diffusion distance and the transport that keeps the gradient going — surface area alone is not the complete explanation.

Because this unit also carries Paper 6, be ready to identify capillaries, the lacteal and the epithelial surface in an unfamiliar diagram of a villus, rather than only in one familiar, memorised drawing.