Absorption and assimilation

Biology · Human nutrition. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Absorption is the movement of digested nutrients from the gut into the cells lining it, and then into the blood. It happens in the small intestine, mainly the ileum, by diffusion, osmosis and active transport.

The wall of the ileum is covered with millions of tiny finger-like villi, and the cells on each villus have microvilli. Together they give a very large surface area. Each villus has a wall one cell thick, a network of blood capillaries and a central lacteal.

Blood from the ileum goes to the liver first, in the hepatic portal vein. Assimilation is the next step: body cells take nutrients from the blood and use them.

Rules to remember

Absorption: nutrients move from the gut into the lining cells and then into the blood (small intestine)
Assimilation: uptake and use of nutrients from the blood by body cells
Capillaries in a villus absorb glucose and amino acids; the lacteal absorbs fatty acids and glycerol
Villi and microvilli increase the surface area; the one-cell-thick wall gives a short diffusion distance; blood flow keeps the concentration gradient steep
Active transport (needs energy from respiration) takes up glucose when its concentration in the gut is lower than in the cells
Water is absorbed by osmosis in the small intestine and the colon; most is absorbed in the small intestine
Hepatic portal vein: carries absorbed nutrients from the ileum to the liver

Common mistake

Many students say most water is absorbed in the colon. The colon absorbs some, but most water is absorbed in the small intestine.

Worked example

A poison stops the cells lining the villi from releasing energy by respiration. Which type of absorption stops, and what is the effect on glucose uptake?

Diffusion and osmosis need no energy from the cell, so they continue.
Active transport needs energy from respiration, so it stops.
Glucose can now enter the cells only by diffusion, which works only while the glucose concentration in the gut is higher than in the cells.
When the concentrations become equal, uptake stops and the rest of the glucose stays in the gut.

Answer: Active transport stops, so less glucose is absorbed: none can be taken up against the concentration gradient.

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