4.1 Digestion and nutrition (Part 4)

Some of these organs make up the alimentary canal, through which food passes directly: the mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus. The liver, gallbladder, salivary glands and pancreas are accessory organs that add secretions.

Food passes through five stages in the alimentary canal (the gut): ingestion (taking food into the mouth), digestion (physical and chemical breakdown), absorption (movement of small molecules into the blood or lymph), assimilation (use of the molecules by body cells) and egestion (passing out undigested food as faeces through the anus).

The role of the digestive system is to break down large insoluble molecules into smaller, soluble food molecules that provide the body with nutrients.

Physical (mechanical) digestion is the breakdown of food into smaller pieces without any chemical change to the food molecules. It increases the surface area of the food so that enzymes can act on it during chemical digestion.

It is carried out mainly by three processes: the chewing action of the teeth, the churning action of the stomach, and the emulsification of fats by bile in the duodenum.

Teeth are embedded in the gums and in the bone of the jaws. They chew food to increase its surface area. The structure of a human tooth includes enamel, the hard outer layer that protects the tooth, and dentine, the layer beneath the enamel that protects the nerves in the pulp cavity at the centre.

The stomach is one organ of the digestive system.

Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, which changes the structure of the molecules. For example, starch (large, insoluble) is broken down into simple sugars (small, soluble) by the enzyme amylase.

It is needed because large insoluble molecules cannot be absorbed through the wall of the small intestine, whereas the small soluble molecules produced are easily absorbed into the surrounding capillaries. Make sure you can state the distinction clearly: physical digestion changes the size of the pieces, chemical digestion changes the molecules themselves.

The enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach.

Cells in the liver make bile, which is stored in the gallbladder and released into the duodenum. Bile is alkaline, so it neutralises the hydrochloric acid arriving from the stomach and provides a pH in which the enzymes of the small intestine work well.

Bile also breaks large drops of fat into many smaller droplets. This is emulsification, a physical change. The larger surface area of the droplets lets lipase chemically break down the lipid into glycerol and fatty acids faster.