Uses of digestion products

Digestion breaks large food molecules into small, soluble molecules that can be absorbed into the bloodstream. The blood carries these products to cells, where they can be used to build molecules or release energy.

Different food molecules produce different digestion products:

• Carbohydrates are broken down by carbohydrases into simple sugars, including glucose.
• Proteins are broken down into amino acids.
• Lipids are broken down into glycerol and fatty acids.

Cells use absorbed simple sugars to build new carbohydrates, amino acids to build new proteins, and glycerol and fatty acids to build new lipids. These new molecules can then be used by the body.

Some absorbed glucose is also used in respiration, which releases energy for cellular activities.

Bile in digestion

Bile is a fluid that helps lipase break down fat in the small intestine. It is made in the liver, stored in the gall bladder and released into the small intestine.

How bile helps lipase

Food arriving from the stomach contains hydrochloric acid. Bile is alkaline, so it neutralises this acid. This provides alkaline conditions in the small intestine that increase lipase activity.

Bile also emulsifies fat: it breaks large amounts of fat into small droplets. The smaller droplets have a greater total surface area, giving lipase more surface to act on. This increases the rate of fat breakdown.

Bile is not an enzyme. It helps lipase work by changing the conditions and dispersing fat; lipase is the enzyme that breaks down the fat.

Lipases in digestion

Lipases are digestive enzymes that break down lipids, including fats, into glycerol and fatty acids. The products are small molecules that can be absorbed through the wall of the small intestine.

Lipase is produced by the pancreas and the small intestine. It acts in the small intestine, where it digests lipids in food.

The breakdown can be represented by a word equation:

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.

Enzyme action

Enzymes speed up chemical reactions in living organisms. They are biological catalysts: they increase the rate of reactions without being used up. Most enzymes are proteins.

How an enzyme acts on a substrate

Each enzyme has an active site, a region with a particular shape. A molecule called the substrate binds to the active site if its shape is complementary to it.

At the active site, the substrate is converted into a product or products. The products leave the active site, and the enzyme can be used again to catalyse the same reaction.

The shape of the active site helps explain enzyme specificity. Only substrates with a complementary shape can bind to the active site in the required way, so an enzyme catalyses a specific reaction.

The lock and key model

The lock and key model is a simplified way of representing enzyme specificity. It compares the fit between a substrate and an enzyme’s active site to the fit between a key and a lock. A substrate with a complementary shape fits the active site; one with a different shape does not fit in the required way.

Enzymes and metabolism

Metabolism is the sum of all the chemical reactions in an organism. Enzymes catalyse metabolic reactions that build larger molecules as well as reactions that break molecules down.

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.