Absorption and Assimilation of Digested Food

Where absorption happens

Absorption is the movement of digested food molecules from the alimentary canal into the blood or lymph. It happens mainly in the small intestine, in a coiled section called the ileum, whose inner wall is folded and covered with millions of finger-like projections called villi (singular: villus).

The villi enormously increase the surface area available for absorption.

Adaptations of the villus

• Each villus has a wall only one cell thick, so digested food molecules have only a short distance to cross to reach the blood.
• Millions of villi, each further covered in microscopic microvilli on its epithelial cells, give the small intestine a very large total surface area for absorption.
• A dense network of blood capillaries inside each villus carries away absorbed glucose, amino acids, water and mineral salts, keeping a steep concentration gradient.
• A lacteal, a small vessel of the lymphatic system, runs through the centre of each villus to absorb fatty acids and glycerol that have been reformed into fat droplets.

What is absorbed, and how

Assimilation: what happens after absorption

Once absorbed nutrients reach the blood, they are carried first to the liver and then to cells throughout the body. Assimilation is the process by which cells take up these nutrients and use them to build new materials or to release energy.

Amino acids are assimilated into proteins and enzymes, glucose is used in respiration to release energy or stored as glycogen in the liver and muscles, and fatty acids and glycerol may be built into cell membranes or stored as fat.

How it is examined

You may be asked to label a villus diagram and explain how each feature increases the rate of absorption, to state which vessel (blood capillary or lacteal) carries a named nutrient away, or to distinguish absorption from assimilation. A common exam expectation is naming the digested food, the absorbed substance, and the assimilated product for a given nutrient.

Worked exam-style question

Question. A diagram shows a single villus from the lining of the ileum, labelled with a layer one cell thick, a central vessel, and a surrounding network of small vessels. (a) Name the central vessel and state the type of digested food it absorbs.

(b) Name the surrounding network and state two nutrients it carries away. (c) Explain how the wall being one cell thick speeds up absorption.

(d) Glucose is sometimes absorbed even when its concentration inside the villus is higher than in the gut. Name and explain the process responsible.

Model answer. (a) The central vessel is the lacteal, which absorbs the products of fat digestion, fatty acids and glycerol reassembled into fat droplets. (b) The surrounding network is the blood capillaries, which carry away glucose and amino acids (also water and mineral salts).

(c) A wall one cell thick gives a short diffusion distance, so digested molecules cross quickly into the blood, raising the rate of absorption. (d) The process is active transport: a carrier protein moves glucose against its concentration gradient using energy from ATP released in respiration, so absorption continues even when the gut concentration is lower.

Practice question

Try this. The inner surface of the small intestine is folded and covered in villi, and each epithelial cell carries microvilli. Explain how these three features together make absorption in the small intestine efficient.

Exam tip

Key terms

Revise these linked terms to answer absorption questions precisely:

• Villus, the finger-like projection that carries out most absorption.
• Diffusion, moves glucose and amino acids down a concentration gradient.
• Active transport, absorbs glucose against the gradient using ATP.
• Assimilation, how cells use the nutrients once absorbed.
• Digestion, the earlier stage that produces the small molecules to be absorbed.

Source:SRC-DSKP-EN

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