6.1 Essential ideas

6.1.1 Digestion and absorption

Figure 6.1.1a shows the important structures of the human digestive system. Scroll over the labels to review their functions.

Figure 6.1.1a – The human digestive system

Skill: Draw an annotated diagram of the digestive system

  • You should be able to draw an annotated diagram like the one in Figure 6.1.1a.
  • The locations of the structures relative to each other must be clear.
  • The connections between these structures must also be clear:
    • stomach and oesophagus
    • liver and gall bladder
    • gall bladder and pancreas with small intestine
    • small and large intestine

Digestion in the small intestine

  • There are layers of tissue in the wall of the small intestine, as shown in Figure 6.1.1b. Moving from the outside and working inwards, these are the serosa, the longitudinal and circular muscle, the sub-mucosa, and the mucosa or epithelium.
  • Wave-like contractions of the circular and longitudinal muscle, called peristalsis, move food along the lumen of the gut.
  • As food travels down the lumen, it is mixed with enzymes secreted by the pancreas.
  • Different enzymes catalyse the hydrolysis of a specific macromolecule into monomers, as shown in the table below:
Enzyme secreted by pancreas Macromolecule polymer (substrate) Monomers (products)
Amylase Carbohydrates (polysaccharides like starch) Dissacharides (e.g. maltose)
Lipase Lipids (triglycerides) Monoglyceride (i.e. fatty acid + glycerol)
Endopeptidase (trypsin) Proteins Amino acids
Nuclease Nucleic acids Nucleotides

  • Different parts of the small intestine digest and absorb different types of macromolecule depending on the structure of membrane proteins and their relative location (i.e. duodenum, jejunum, ileum).
  • Some macromolecules, particularly cellulose, cannot be digested at all because humans lack the necessary enzymes.
  • Cellulose continues through the system and is excreted without having been digested at all.

Absorption in the small intestine

absorption in the small intestineFigure 6.1.1b – (A) Tissue layers in the small intestine (B) Villus detail (C) Epithelium cell

  • The small intestine is a highly convoluted and narrow tube, so it has a very large surface area for nutrient absorption.
  • The surface area of the innermost layer of epithelial tissue is increased further by villi. Figure 6.1.1b shows the structure of a villus. Villi are the sites of nutrient absorption.
  • Each of the epithelial cells on a villus has tiny protrusions, called microvilli, which increase the surface area for absorption even more.
  • Nutrient monomers as well as mineral ions and vitamins travel across the plasma membrane of the epithelial cells and are absorbed into the bloodstream.

Skill: Identifying tissue layers in the small intestine

Examine Figure 6.1.1e, a micrograph showing details of the small intestine of a mouse.

  • Can you identify the tissue layers? (Click the image for answers)
  • Why does it appear that the some of the villi are suspended in the lumen? (These areas are indicated by the white circles)
  • Is this a longitudinal or a transverse section? How do you know?

Activity: The digestion of starch

Put the following steps in order to make a summary of the processes involved when starch is digested:

  1. Blood carrying glucose is carried away from the villus to the hepatic portal vein.
  2. Pancreatic juice contains amylase, an enzyme that hydrolyses 1,4- glycosidic bonds in starch to form maltose (a dissacharide) in the lumen of the small intestine. The 1,6-glycosidic bonds of amylopectin cannot be digested by amylase.
  3. There are two types of starch, amylopectin and amylose. Both are polysaccharides, but one is more highly branched than the other (Figure 6.1.1f).
  4. The liver converts glucose into glycogen, a highly branched polysaccharide. It is stored both in the liver and in the muscles.
  5. Glucose is transported from the lumen into the epithelium cell by facilitated diffusion with a sodium symport protein.
  6. Enzymes located in the plasma membranes of epithelia – dextrinase, maltase and glucosidase – continue to hydrolyse glycosidic bonds to form glucose monomers.

Key concept

The structure of the wall of the small intestine allows it to move, digest and absorb food.

Language help

  • The mucosa tissue layer is made of columnar epithelial cells and goblet cells.
  • Columnar cells are sometimes called enterocytes. These absorb nutrients using all the mechanisms that are explained in Page 6.2.1.
  • Goblet cells secrete mucin, which moistens the absorptive surface. This is why the tissue layer is called ‘mucosa’.

goblet cellFigure 6.1.1c – Goblet cell
The mucosa tissue layer is named after the lubricating mucin secreted by goblet cells in the epithelium.

Concept help

  • Why are two types of muscle (circular and longitudinal) necessary to produce waves of contractions? Where else in the body does peristalsis occur?
  • Which molecules are the unique substrates for each of these enzymes? Lactase, protease, phospholipase, maltase, dextrinase, glucosidase – what do the names of most enzymes have in common?
  • Suggest a name for an enzyme that builds glycogen from glucose molecules. Google it. Were you right? 

Did you know?

‘Dietary fibre’ is actually cellulose. Humans do not digest cellulose but it is a necessary bulking agent that makes faeces solid, and is necessary for a healthy colon. Some mammals digest cellulose with the help of symbiotic bacteria in the gut. One clue that they are able to do this is their very liquid stools!

goatFigure 6.1.1d – Goat
It’s true. Goats can digest cardboard boxes because of the symbiotic bacteria in their gut.

Nature of Science

Using models: Dialysis tubing can be used to model absorption in the small and large intestine.

issues layers of mouse small intestineFigure 6.1.1e – Tissues layers of mouse small intestine

PolysaccharidesFigure 6.1.1f – Polysaccharides
Different polysaccharides are made of the same monosaccharides, but their arrangements vary.