Digestive enzymes are proteins that speed up the chemical breakdown of food into small molecules the body can absorb. They matter because starches, proteins, and fats are too large to pass directly into the bloodstream. Enzymes work in specific regions of the digestive tract, including the mouth, stomach, pancreas, and small intestine.
Each enzyme has a target molecule and a preferred pH range where it works best.
Digestion begins in the mouth when salivary amylase starts breaking starch into smaller sugars. In the stomach, acidic conditions help pepsin break proteins into shorter peptide chains. In the small intestine, pancreatic enzymes and intestinal enzymes complete most chemical digestion, while bile helps lipase access fats by emulsifying them.
The final products, such as glucose, amino acids, fatty acids, and glycerol, are absorbed mainly through the villi of the small intestine.
Key Facts
- Amylase breaks starch into maltose and smaller sugars: starch + amylase -> maltose
- Proteases break proteins into peptides and amino acids: protein + protease -> amino acids
- Lipase breaks fats into fatty acids and glycerol: triglyceride + lipase -> fatty acids + glycerol
- Pepsin works best in the acidic stomach at about pH 2
- Pancreatic enzymes work best in the small intestine at about pH 7 to 8
- Enzymes are specific because their active sites fit particular substrates
Vocabulary
- Enzyme
- An enzyme is a biological catalyst, usually a protein, that speeds up a chemical reaction without being used up.
- Substrate
- A substrate is the molecule an enzyme binds to and acts on during a reaction.
- Amylase
- Amylase is a digestive enzyme that breaks starch into smaller sugar molecules.
- Protease
- A protease is a digestive enzyme that breaks proteins into peptides or amino acids.
- Lipase
- Lipase is a digestive enzyme that breaks fats into fatty acids and glycerol.
Common Mistakes to Avoid
- Thinking all enzymes work in every part of the digestive system. This is wrong because each enzyme has a preferred pH and location, such as pepsin in the acidic stomach and pancreatic amylase in the small intestine.
- Saying bile is an enzyme. Bile is not an enzyme because it does not chemically break bonds, but it emulsifies fats into smaller droplets so lipase can work more effectively.
- Mixing up substrates and products. Starch, protein, and fat are substrates, while sugars, amino acids, fatty acids, and glycerol are products of digestion.
- Assuming high temperature or extreme pH always makes enzymes faster. Extreme conditions can denature enzymes, changing their active sites so substrates no longer fit properly.
Practice Questions
- 1 A meal contains 60 g of starch. If amylase breaks down 75% of the starch before it leaves the small intestine, how many grams of starch remain undigested?
- 2 A sample of stomach fluid has pH 2 and a sample of small intestine fluid has pH 8. Which sample would allow pepsin to work better, and which would allow pancreatic enzymes to work better?
- 3 A person has reduced bile production. Explain how this would affect fat digestion even if lipase is still being made normally.