Digestive system: chemical digestion

Chewing makes food smaller, but food is still made of huge molecules that cannot pass into your blood. Chemical digestion breaks them into small pieces, and it depends on enzymes. An enzyme is a protein that speeds up a chemical reaction without being used up. Each enzyme fits one type of molecule, like a key that fits one lock, so the body needs many different enzymes.

Food contains three main kinds of large molecules, and each breaks down into different pieces. Carbohydrates, such as starch, are broken into simple sugars like glucose. Proteins are broken into amino acids. Fats are broken into fatty acids and glycerol. These small pieces are the nutrients that your body can absorb.

Chemical digestion starts in the mouth. Saliva contains an enzyme called amylase, which begins breaking starch into sugars. If you hold a cracker in your mouth for a few minutes, it starts to taste sweet. In the stomach, strong acid and an enzyme called pepsin start breaking down proteins. The acid also kills many germs. Pepsin works best in acid, which is one reason the stomach is so acidic.

Most chemical digestion happens in the small intestine. Food arrives there as an acidic liquid. The pancreas sends in a mix of enzymes that digest carbohydrates, proteins, and fats, along with a base that neutralizes the stomach acid, because those enzymes work best when it is not acidic. The liver makes bile, which the gallbladder stores and releases into the small intestine. Bile is not an enzyme. It acts like dish soap, breaking large fat droplets into tiny ones so the fat-digesting enzyme can reach more of the fat. This is called emulsification. The wall of the small intestine adds its own enzymes to finish the job.

Enzymes depend on conditions such as temperature and acidity. That is why each part of the tract provides the conditions its enzymes need.

Common mistakes: thinking that bile is an enzyme, and thinking that chemical digestion uses up the enzymes. Enzymes are reused. Also, the stomach is not where most digestion ends; it is mostly where protein digestion begins. Next lesson: how nutrients get into the blood.