The first stage in digestion happens in the mouth. Food is chewed and cut into smaller pieces that are able to be swallowed. This also mashes the food so that it gives a larger surface area for the digestive enzymes to work on which makes the digestion more efficient. Saliva is also added to lubricate the food for swallowing. It also contains the enzyme salivary amylase which starts to digest starch into glucose.

Digestive enzymes are responsible for the chemical breakdown of the large food molecules into ones that are small enough to be absorbed into the blood. Three types of enzymes do this:

• carbohydrases
• proteases
• lipases.

Enzymes are named according to the type of food molecule that they break down. This is fairly logical but sometimes you may find older names like amylase (carbohydrase), pepsin and trypsin (protease).

The table in question 4 summarises the different digestive enzymes.

Starch is a very large molecule that contains many sugar (glucose) molecules joined together. Carbohydrase enzymes break the bonds between sugar molecules to make a small sugar molecule called maltose. The final stage of digestion sees this molecule split into glucose for absorption.

There are several different carbohydrase enzymes that break down complex polysaccharides like starch but the main one in digestion is called amylase. This enzyme is found in saliva, which is added to the food in the mouth and is also in the digestive juices produced by the pancreas.

Fats and oils are made up of two types of molecules: fatty acids and glycerol. The enzymes lipase breaks apart the bonds that hold together them together before they are absorbed in the small intestine.

Lipase is added to the food in digestive juices that are produced by the pancreas and fats are digested in the first part of the small intestine.

Proteins are large molecules that contain a long chain of amino acids joined end-to-end. They are sometimes called polypeptides. They are digested by a group of enzymes called proteases that break the bonds between individual amino acids in the protein chain.

There are two main types of protease enzymes involved in human digestion. Pepsin is the protease that is added to food in the juices secreted by the wall of the stomach. It starts the digestion of the proteins and works best in the acid conditions found in the stomach. The second protease is Trypsin. This is added in juices from the pancreas and completes the digestion of proteins in the small intestine. It works best at neutral pH.

The stomach is a pretty harsh environment. Specialised cells in the stomach wall produce large amounts of hydrochloric acid and this means that the stomach is about pH2. This acid can sometimes cause stomach ulcers if the protective layer on the inside of the stomach wall is damaged.

The acid in the stomach helps to kill any bacteria that may be on the food. It is not foolproof though because some micro-organisms that cause food-poisoning are adapted to be able to survive in the stomach acids. The low pH provides an environment that allows the protease Pepsin to be active. This starts the digestion of proteins as the food is churned around in the stomach.

Bile is produced in the liver and stored in the gall bladder. As food leaves the stomach, bile is squirted along the bile duct and into the duodenum. It is extremely important as it helps to neutralise the acidic contents of the stomach as they enter the top of the small intestine. This prevents the acid from damaging the small intestine which is kept at a pH of about 7.4 (just slightly alkali).

Bile also helps to emulsify fats in the food. The digestive juices are all water-based and fats in the food would not normally mix with them. The bile allows the fats to mix in with the watery digestive juices and allow the enzyme lipase to digest the fats efficiently.