IGCSE Biology 0610: Structure and Adaptations of Villi for Absorption

The wall of the small intestine is folded into millions of tiny, finger-like villi, and the surface of each villus is further covered in even smaller microvilli.

(a) Name two structures found inside each villus that absorb the small, soluble products of digestion, and state which digested nutrients each structure mainly absorbs. [4]

(b) State one feature of the wall of a villus that allows absorbed substances to diffuse into these internal structures over a very short distance. [1]

(d) Explain why having millions of villi, rather than just a small number of larger folds, further helps the small intestine absorb nutrients efficiently. [1]

Part (a): Internal structures of a villus

Each villus contains:

Blood capillaries, which absorb the small, soluble products of carbohydrate and protein digestion, glucose and amino acids, into the blood.
A lacteal, a small vessel of the lymphatic system, which absorbs the small, soluble products of fat digestion, fatty acids and glycerol.

Part (b): The thin wall of a villus

The wall of a villus is only one cell thick. This gives a very short diffusion pathway, so absorbed substances can move quickly from inside the small intestine into the blood capillaries and lacteal.

Part (c): The role of microvilli

Microvilli are tiny, finger-like folds of the cell surface membrane on the cells covering each villus. Because each cell’s membrane is folded into many microvilli, the total surface area of the villus that is in contact with digested food is greatly increased, without needing to increase the villus’s overall size. A larger surface area allows more nutrient molecules to diffuse across into the blood and lacteal per unit time, increasing the rate of absorption.

Part (d): Why millions of villi matter

A single villus, however well adapted, can only absorb a limited amount of nutrient at a time. Because the lining of the small intestine is folded into millions of villi rather than just a few larger folds, the individual surface areas of all these villi add together to give the small intestine an enormously large total internal surface area, allowing far more nutrients to be absorbed overall.

Final answers

(a) Blood capillaries absorb glucose and amino acids; the lacteal absorbs fatty acids and glycerol.
(b) The wall of a villus is one cell thick, giving a short diffusion pathway.
(c) Microvilli are folds of the cell surface membrane that further increase the surface area for absorption.
(d) Millions of villi together give a far greater total surface area than a small number of larger folds.