11.18: Digestive System Enzymes

What happens if you suck on a piece of white bread?

If you kept a bite of white bread in your mouth for a long period of time, it would start to get really mushy. Then it would start tasting sweet. That's because you have enzymes in your saliva. The enzymes break down the complex carbohydrates in the bread into simple sugars.

The Role of Enzymes in the Digestive System

Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that speeds up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down large food molecules into small molecules.

Did you ever use a wrench to tighten a bolt? You could tighten a bolt with your fingers, but it would be difficult and slow. If you use a wrench, you can tighten a bolt much more easily and quickly. Enzymes are like wrenches. They make it much easier and quicker for chemical reactions to take place. Like a wrench, enzymes can also be used over and over again. But you need the appropriate size and shape of the wrench to efficiently tighten the bolt, just like each enzyme is specific for the reaction it helps.

Digestive enzymes are released, or secreted, by the organs of the digestive system. These enzymes include proteases that digest proteins, and nucleases that digest nucleic acids. Examples of digestive enzymes are:

• Amylase, produced in the mouth. It helps break down large starch molecules into smaller sugar molecules.
• Pepsin, produced in the stomach. Pepsin helps break down proteins into amino acids.
• Trypsin, produced in the pancreas. Trypsin also breaks down proteins.
• Pancreatic lipase, produced in the pancreas. It is used to break apart fats.
• Deoxyribonuclease and ribonuclease, produced in the pancreas. They are enzymes that break bonds in nucleic acids like DNA and RNA.

Bile salts are bile acids that help to break down fat. Bile acids are made in the liver. When you eat a meal, bile is secreted into the intestine, where it breaks down the fats (Figure below).

Figure \(\PageIndex{1}\): Bile is made in the liver, stored in the gallbladder, and then secreted into the intestine. It helps break down fats.

Hormones and Digestion

If you are a typical teenager, you like to eat. For your body to break down, absorb and spread the nutrients from your food throughout your body, your digestive system and endocrine system need to work together. The endocrine system sends hormones around your body to communicate between cells. Essentially, hormones are chemical messenger molecules.

Digestive hormones are made by cells lining the stomach and small intestine. These hormones cross into the blood where they can affect other parts of the digestive system. Some of these hormones are listed below.

• Gastrin, which signals the secretion of gastric acid.
• Cholecystokinin, which signals the secretion of pancreatic enzymes.
• Secretin, which signals secretion of water and bicarbonate from the pancreas.
• Ghrelin, which signals when you are hungry.
• Gastric inhibitory polypeptide, which stops or decreases gastric secretion. It also causes the release of insulin in response to high blood glucose levels.

Science Friday: Stained Glass Conservation

Stained glass from the Middle Ages is often hundreds of years old. Unfortunately, many of these relics are in need of cleaning and maintenance. In this video by Science Friday, conservator Mary Higgins discusses the methods used to protect the stained glass.

Summary

• Digestive enzymes speed up the reactions of chemical digestion.
• Hormones, chemical messengers used to communicate between cells, are important in regulating digestion.

Review

1. What is an enzyme?
2. Explain the role of enzymes in digestion. Give an example to illustrate your answer.
3. Give an example of how a hormone affects digestion.