A level Biology exam revision resources written by A level Examiners


The small intestine is divided into sections. The first section is called the duodenum and is responsible for the majority of the digestion of food. The next main section is called the ileum and is responsible for the majority of absorption of the products of digestion.

The diagram below shows the duodenum and the ducts that allow digestive secretions to enter it.

Figure 2 : The Duodenum
The Duodenum

The digestive secretions entering the duodenum can be summarised as

Table 1 : Summary of digestive secretions entering the duodenum
Summary of digestive secretions entering the duodenum

The wall of the small intestine is specialised in several ways. The figure(s) below show the general structure of the small intestine, and how it contains clear adaptations. These figure(s) show:

  • how the entire inner surface is folded
  • how the cells of the surface are arranged in villi
  • how the cells which make up the villi themselves have microvilli.

These specialisation's increase the surface area of the small intestine greatly, which enables more absorption to take place.

Figure 3 : Cross-section of Small Intestine
small intestine

Figure 4 : A villus (enlarged view)
small intestine

Figure 5 : Columnar cell
Cross-section of Villi

The role of the liver in digestion

The liver does not produce any digestive enzymes. It produces bile.

Bile is a biological detergent, which is produced in the liver. Bile reduces the surface tension of the contents of the gut, which causes them to emulsify (this increases the surface area). The increased surface area allows the enzyme lipase to act on a larger volume of material in a shorter time, ensuring that enzymes operate at their optimum rate. Bile also neutralises stomach acid, and provides the optimum pH for pancreatic digestive enzymes to work.

The role of the pancreas in digestion

The pancreas produces a wide range of digestive enzymes:

  • Lipase - for the digestion of lipids. Lipase hydrolyses triglycerides to fatty acids and glycerol, which can be absorbed.
  • The endopeptidase trypsin - for the digestion of proteins. Trypsin hydrolyses proteins to polypeptides.
  • Amylase - for the digestion of starch. Amylase hydrolyses starch to maltose.

The role of the epithelial cells in digestion

There are enzyme molecules in the surface membrane of the epithelial cells of the small intestine.

  • Exopeptidase - for the digestion of peptides. Exopeptidases act on the peptides created by the endopeptidases and hydrolyses them to amino acids, which can be absorbed.
  • Maltase - for the hydrolysis of maltose. Maltase acts on the disaccharide sugar maltose and hydrolyses the glycoside bonds between the units of glucose. The sugar is broken down to its simplest form glucose, and can then be absorbed.

The structure of the wall of the small intestine (especially the ileum) has the following adaptations, which increase it's effectiveness at absorbing the products of digestion.

  • Moist surface
  • Thin (epithelial) surface provides a short absorption pathway
  • Folds of the inner wall, villi and microvilli increase surface area
  • The length of the small intestine (6.5 metres) provides a large surface area for absorption.
  • Lacteals in villi allow for absorption into the lymph system
  • There is an extensive capillary network in villi providing a good blood supply
  • Epithelial cells have many mitochondria to supply ATP which is used as a source of energy for active transport
  • Epithelial cells have many carrier proteins in their membranes for the uptake of the products of digestion

The products of digestion are absorbed by diffusion, facilitated diffusion and active transport.

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