Stomach

Glands in the stomach wall pour gastric juice onto the food. The juice contains protease enzymes (e.g. pepsin) that start to digest large protein molecules into smaller molecules. These enzymes work best in acid conditions and so the gastric juice contains hydrochloric acid to provide the correct pH. The acid also destroys bacteria in the food, helping to prevent infection.

Muscles in the stomach wall contract (squeeze) to mix the food with the gastric juice, speeding up digestion of proteins.

At intervals, portions of food are released into the small intestine.

Digestion in the small intestine

As food enters the small intestine three fluids are added to it:

• Pancreatic juice, made in the pancreas, travels along the pancreatic duct (a tube) into the small intestine. It contains all three types of enzyme - amylase, protease and lipase. Amylase digests starch into sugars, protease digests protein fragments into amino acids and lipase digests fats into fatty acids and glycerol.
• A fluid produced by glands in the wall of the small intestine which also contains these three types of enzyme.
• Bile is a fluid made in the liver. It is stored in the gall bladder and released onto the food through the bile duct. Bile does not contain any enzymes so it cannot digest food, but it is alkaline to provide the best pH for the enzymes in the small intestine as these require alkaline conditions to work effectively. Bile also breaks up large fat drops into smaller droplets (the bile 'emulsifies' fat) to increase their surface area. This speeds up the digestion of fat by the lipase enzymes in the small intestine.