Describe how the small intestine is adapted for absorption and explain assimilation

What this dot point is asking

SEAB wants you to describe how the small intestine is adapted to absorb the products of digestion, naming the villi and their features, and to explain assimilation, the use of absorbed molecules by the body. You should be able to say what happens to glucose, amino acids, and fatty acids and glycerol after absorption, and distinguish absorption from assimilation.

The answer

Absorption in the small intestine

Absorption is the movement of digested food molecules from the small intestine into the blood (and lymph). The small intestine is highly adapted for this:

• It is very long, giving plenty of time and area for absorption.
• Its inner surface is folded and covered in millions of tiny finger-like projections called villi (and the cells have even smaller microvilli), giving a very large surface area.
• Each villus has a thin wall, one cell thick, giving a short diffusion distance.
• Each villus has a rich blood supply (a network of capillaries) that carries absorbed glucose and amino acids away, keeping the concentration gradient steep.
• Each villus has a lacteal, a small lymph vessel, which absorbs fatty acids and glycerol.

Glucose and amino acids pass into the blood capillaries (by diffusion and active transport); fatty acids and glycerol enter the lacteal.

Assimilation

Assimilation is the process by which absorbed food molecules are taken into the cells of the body and used to become part of the body or to release energy. In other words, absorption gets the molecules into the blood, and assimilation puts them to use in the cells.

What happens to each:

• Glucose is carried to cells and used in respiration to release energy; excess is stored as glycogen in the liver and muscles.
• Amino acids are used to build new proteins for growth and repair; excess amino acids are broken down in the liver (deamination), since they cannot be stored.
• Fatty acids and glycerol are used to build cell membranes and may be stored as fat.

Absorption versus assimilation

Absorption is moving molecules from the gut into the blood. Assimilation is using those molecules inside the body's cells. They are two separate stages.

Examples in context

Example 1. Glucose to the liver. After a meal, glucose absorbed from the villi travels in the blood to the liver, which stores any excess as glycogen. This keeps the blood glucose level steady and links absorption to the control of blood glucose.

Example 2. Building muscle after exercise. Amino acids absorbed from a protein meal are assimilated by muscle cells, which use them to build new muscle protein for growth and repair. This is assimilation in action.

Try this

Q1. Name the projections that line the small intestine. [1 mark]

• Cue. Villi (singular: villus).

Q2. State two features of a villus that aid absorption. [2 marks]

• Cue. Any two of: a large surface area (villi and microvilli), a thin one-cell-thick wall, a rich blood supply, a lacteal for fats.

Q3. Define assimilation. [2 marks]

• Cue. The process by which absorbed food molecules are taken into the cells of the body and used to build the body or release energy.