Our teeth break food down into small pieces when we chew. This is only a start to the process of digestion, as chewed pieces of food are still too large to be absorbed by the body. Food has to be broken down chemically into really small particles before it can be absorbed. Enzymes are the biological catalysts needed to make this happen quickly enough to be useful.
Enzymes are not living things. They are just special proteins that can break large molecules into small molecules. Different types of enzymes can break down different nutrients:
Carbohydrates are digested in the mouth, stomach and small intestine. Carbohydrase enzymes break down starch into sugars.
The saliva in your mouth contains amylase, which is another starch digesting enzyme. If you chew a piece of bread for long enough, the starch it contains is digested to sugar, and it begins to taste sweet.
Proteins are digested in the stomach and small intestine. Protease enzymes break down proteins into amino acids. Digestion of proteins in the stomach is helped by stomach acid, which is strong hydrochloric acid. This also kills harmful microorganisms that may be in the food.
Lipase enzymes break down fat into fatty acids and glycerol. Digestion of fat in the small intestine is helped by bile, made in the liver. Bile breaks the fat into small droplets that are easier for the lipase enzymes to work on. Bile is not an enzyme.
Minerals, vitamins and water are already small enough to be absorbed by the body without being broken down, so they are not digested.
Digestive enzymes cannot break down dietary fibre, which is why the body cannot absorb it.