Did you know that our saliva contains something called enzymes? And did you know that without these enzymes digestion wouldn't be able to occur? So what are these enzymes anyway, and how do we use them in digestion?
Enzymes are proteins. They are biological catalysts that speed up a chemical reaction without being used up in the reaction. Enzymes have a complex 3D shape. The specific shape that allows molecules to fit in is called the active site. It's here that large insoluble food substances are broken into smaller soluble molecules. Smaller molecules are easily absorbed by the blood. These smaller molecules might be used for different processes, for example, glucose can be used for respiration. Some smaller molecules will be used to build and make new products. Proteins can be broken down into peptides, which are short chains of amino acids, and then further broken down into amino acids. These amino acids can be used to build new proteins needed for growth and repair.
The lock and key theory
The shape of the active site of an enzyme matches the shape of the food molecule (substrate). A bit like a key is specific to the lock it opens, a substrate will only match a specific enzyme. This is called the lock and key theory.
Different types of food are broken down by different enzymes. Digestive enzymes are classified by the type of food that they affect. There are three main types:
carbohydrase – breaks carbohydrate into smaller sugars
protease – breaks protein into amino acids
lipase – breaks fat into fatty acids and glycerol
Carbohydrates are chains of identical sugar molecules. The digestive enzyme called carbohydrase breaks the chemical bonds between the sugar molecules in each carbohydrate chain. An example is amylase. Amylase is found in saliva and pancreatic juices. Amylase breaks starch down into small sugar molecules called maltose. Maltose can be further converted to glucose by another enzyme called maltase in the small intestine.
Proteins are made up of amino acids. There are over 20 different types of amino acids. Proteins are digested by digestive enzymes called proteases. These enzymes break proteins into smaller peptides and individual amino acids. Amino acids can be built up to make new proteins for growth and repair. Proteases can be found in the stomach, pancreas and small intestine.
Fats are digested in two stages:
Firstly, bile (made by the liver) emulsifies fat into smaller droplets called an emulsion. This increases the surface area of the fat.
Secondly, the digestive enzyme lipase breaks each fat molecule into the smaller glycerol and fatty acid molecules. These can now be absorbed by the blood and can be used by the body for energy storage, and building cell structures like cell membranes. Bile is alkaline and helps to neutralise the acidic food leaving the stomach.
Lipases are found in the pancreas and small intestine.
In the following activity, you will compare the different enzymes in the digestive system.