Chemical digestion

Some food molecules are too large and insoluble to be absorbed from the digestive system. Chemical digestion breaks these molecules into smaller, soluble molecules. Digestive enzymes speed up the chemical reactions that break the large molecules down.

The small products can dissolve in the fluid in the digestive system and pass through its lining into the body. This transfer is called absorption. Digestion therefore makes the molecules small and soluble enough to be absorbed.

Amylase in digestion

Amylase is a digestive enzyme that breaks down starch, a large carbohydrate in food, into smaller sugars. These include maltose, a reducing sugar.

The salivary glands secrete amylase into the mouth. As food is chewed and mixed with saliva, the amylase begins to break down its starch.

The pancreas also secretes amylase into the small intestine. There, it continues to break down starch that remains in the food.

Proteases in digestion

Proteins are made of chains of amino acids. Proteases are digestive enzymes that break these chains down into amino acids.

Protein digestion begins in the stomach, where proteases are secreted and act on proteins. Digestion continues in the small intestine. The pancreas secretes proteases into the small intestine, and the small intestine also secretes proteases. These enzymes act in the small intestine to continue breaking down proteins into amino acids.

Lipase in digestion

Lipase is a digestive enzyme that breaks down fats and oils into smaller molecules: fatty acids and glycerol.

Lipase acts in the small intestine. The pancreas secretes lipase into the small intestine, and the small intestine also secretes lipase.

Hydrochloric acid in gastric juice

The stomach contains gastric juice, a digestive fluid that includes hydrochloric acid. The acid gives gastric juice a low pH, making the stomach contents acidic.

This acidity has two functions:

• It kills harmful microorganisms that may enter the stomach with food.
• It provides the acidic pH at which digestive enzymes in gastric juice work best, allowing them to digest food effectively.

Maltase in starch digestion

Starch is digested in stages. First, the enzyme amylase breaks starch down into the sugar maltose. Maltose is then broken down by a different enzyme, maltase, to produce glucose.

Maltase acts on the membranes of the epithelial cells that line the small intestine. There, it completes the conversion of starch into glucose, a small, soluble sugar that can be absorbed.

Pepsin and trypsin in protein digestion

Proteins are large molecules that must be broken down during digestion. Pepsin and trypsin are proteases: enzymes that break proteins into shorter chains called peptides.

Pepsin acts in the stomach, where conditions are acidic. Its activity is suited to this low pH, so it can digest protein in the stomach.

Trypsin acts in the small intestine, where conditions are alkaline. It digests protein there and is suited to the higher pH of the small intestine.

Each protease works best under different pH conditions. The pH at each site therefore suits the protease that acts there.

Bile neutralisation in the duodenum

Food leaves the stomach mixed with acidic gastric juices. When this mixture enters the duodenum, the first part of the small intestine, it meets bile.

Bile is an alkaline mixture. It neutralises the acid in the food mixture, raising its pH. This produces a pH suitable for digestive enzymes to act in the small intestine.