Digestion is the breakdown of complex insoluble food particles into smaller soluble molecules that can be easily absorbed by the body. Digestion occurs in the alimentary canal.

Alimentary canal

The alimentary canal is a long digestive tube running through the body from the mouth to the anus. The alimentary canal has many functions besides digestion, such as:

  • Ingestion – the taking in of food into the body through the mouth. Commonly known as eating.
  • Mechanical digestion – the breakdown of large food particles into smaller pieces without any chemical change to the food molecules. Commonly known as chewing.
  • Chemical digestion – the breakdown of large insoluble food molecules into small soluble molecules.
  • Absorption – the movement of digested food molecules into the body through the walls of the intestines.
  • Assimilation – the use of the absorbed food by the body.
  • Egestion – the passing out of the waste products of digestion, as faeces, through the anus.

Mechanical digestion

Mechanical digestion occurs in the mouth. This is basically a process by which food is crushed by the teeth into smaller particles. The process called is called mastication or chewing.

Chemical digestion

Chemical digestion is a chemical process that consists of breaking down insoluble food molecules to small soluble molecules. The small soluble molecules can then be absorbed through the walls of the alimentary canal into the blood vessels.

Chemical digestion is facilitated by the presence of chemical called enzymes.

Stages of digestion

The alimentary canal has many organs, each specialised for its own role in digestion.


The mouth

The food is ingested through through the mouth where it is chewed and mixed with saliva. The chewing breaks the food into pieces small enough to be swallowed. Chewing also increases the effective surface area of the food for the enzymes to work on.

Saliva helps to lubricate the food and make the small food pieces stick together. It contains any enzyme called salivary amylase, which acts on cooked starch and breaks it down into maltose.


From the mouth the food is swallowed into the gullet. In the gullet, the food is pushed along the gullet by a process called peristalsis. The alimentary canal has muscle layers that run around it (circular muscle) and the others that run along its length (longitudinal muscle).

The circular muscles contract in one region, narrowing the alimentary canal in that region. This pushes the food forward. This contraction is followed by another contraction just below it creating a wave of contractions along the canal, pushing food in front of the wave until it reaches the stomach. This process is called peristalsis.

The stomach

The stomach is a small elastic container which stretches as the food collects in it. There is a circular sphincter muscle at the bottom end of the stomach that stops the food from passing through.

The stomach has glands in its lining to produce gastric juice containing the enzymes such as pepsin for the digestion of protein. The stomach lining also produces hydrochloric acid, which provides the best pH for stomach enzymes to work efficiently. The hydrochloric acid also kills many of the bacteria in the food.

After some time, the sphincter lets through the liquid products of
digestion, a little at a time, into the upper part of the small intestine called the duodenum.

The small intestine

In the duodenum, the food is mixed with a digestive juice from the pancreas (pancreatic juice) and bile from the liver.

The pancreas is a digestive gland that produces a number of enzymes, which act on all food groups, eg:

  • Trypsin which breaks down proteins into amino acids.
  • Pancreatic amylase which breaks down starch and converts it to maltose.
  • Lipase which breaks down fats (lipids) to fatty acids and glycerol.

The mixture is then passed into the lower part of the intestine called the ileum. Absorption of digested food takes place in the ileum. The ileum is adapted for the absorption of digested food as follows:

  • It is long to present a large absorbing surface to the digested food.
  • Its internal surface are is greatly increased by circular folds with thousands of tiny projections called villi.
  • Thin walls to allow fluids to pass easily through it.
  • There is a rich network of blood capillaries.

The end products of digestion are passed into the hepatic portal vein. The end products of digestion are:

FoodEnd products
proteinsamino acids
fatsfatty acids and glycerol

The absorption of the end products of digestion is not just a matter of simple diffusion. Amino acids, sugars and salts are taken up by active transport.


Bile is a green liquid made in the liver. It is stored in the gall-bladder and poured into the duodenum via the bile duct. It contains bile salts which emulsify the fats before their digestion. This breaks them into small droplets with a large surface area for lipase to efficiently digest the fats.

Bile is slightly alkaline so it neutralises the acidic mixture of food and gastric juices as it enters the duodenum from the stomach.

The large intestine

The large intestine is made up of the colon and the rectum. The material that passes into the large intestine consists
of water and undigested matter. Absorption of water and salts takes place in the colon. The waste, called the faeces or "stool", is then passed into the rectum by peristalsis. From the rectum, the waste is expelled through the anus. The passing out of the faeces is called egestion or defecation.

Summary of digestion

Region of the alimentary canalFunction
mouthingestion of food, mechanical digestion by the teeth, chemical digestion of starch by amylase.
oesophagus (gullet)transfers food from the mouth to the stomach by peristalsis
stomachproduces gastric juice containing pepsin, for chemical digestion of protein, also hydrochloric acid to kill bacteria
duodenumupper part of the small intestine, receives pancreatic juice for chemical digestion of proteins, fats and starch as well as neutralising the acid from the stomach; receives bile to emulsify fats
ileumlower part of the small intestine, very long and has villi to increase surface area for absorption of digested food molecules
pancreasproduces pancreatic juice for chemical digestion of proteins, fats and starch
livermakes bile which contains salts to emulsify fats, assimilation of digested food such as glucose, deamination of excess amino acids
gall bladderstores bile
colonupper part of the large intestine, absorption of water from undigested food
rectumlower part of the large intestine; stores faeces anus egestion of faeces

Use of digested food

The products of digestion are transported around the body dissolved in the blood. The cells then absorb and use glucose, fats and amino acids. This use of food is called assimilation.


Glucose is used for respiration in the cells to produce energy. During respiration, glucose is oxidised to carbon dioxide and water. This produces energy to drive the many cellular processes in which result in, for example, the contraction of muscles or electrical changes in nerves.


These are assimilated into cell membranes and other cell structures. Fats can also also be used as a source of energy for cell metabolism because they can provide twice as much energy
as sugars.

Amino acids

These are absorbed by the cells and built into proteins, some of which will become plasma proteins in the blood. Others may become structures such as cell membranes.