Lesson 2 of 7 / Nutrition in humans
Digestive enzymes and bile
Which changes break molecules, and which help enzymes reach them?
In this lesson: Match digestive enzymes to their substrates and products, and distinguish digestion from emulsification.
About 6 min
The key ideaEnzymes chemically digest food molecules. Chewing and bile emulsification increase surface area so enzymes can act more effectively.
Physical digestion makes pieces or droplets smaller without changing their molecules. Chemical digestion breaks large molecules into smaller soluble products.
Explore the idea
Which enzyme changes this molecule?
Amylase breaks starch into maltose. Salivary and pancreatic amylase act in different parts of the canal.
Original models, not to scale. The protein diagram summarises several digestion stages. Droplet circles illustrate exposure and do not calculate three-dimensional surface area.
Explanation
Amylase breaks starch down to maltose. Salivary amylase begins this in the mouth, and pancreatic amylase acts in the small intestine. Maltase at the small intestine lining breaks maltose into glucose, which can be absorbed. Starch does not become glucose in a single step performed by amylase.
Proteases catalyse protein digestion. A stomach protease begins breaking proteins into shorter peptides in acidic conditions. Further proteases in the small intestine complete digestion to amino acids. When summarising the overall route, write protein to amino acids, while recognising that several stages are involved.
Lipase acts on fats to produce fatty acids and glycerol at this syllabus level. Pancreatic lipase is delivered into the duodenum. Large fat droplets offer relatively little surface to the watery digestive fluid, limiting enzyme access.
Bile is produced by the liver, stored and concentrated in the gallbladder, and released into the duodenum. Its bile salts emulsify fat: they divide large droplets into smaller droplets. This increases the surface area available for lipase. The fat molecules themselves remain fat until lipase acts.
Alkaline digestive secretions, especially pancreatic fluid and also bile, help neutralise acidic stomach contents entering the duodenum. This provides suitable conditions for intestinal enzymes. Bile has no digestive enzyme, and stomach acid itself is not an enzyme.
Step by step
- 1
Name the substrate
Identify starch, maltose, protein or fat before choosing an enzyme.
- 2
Give the correct products
Distinguish maltose made by amylase from glucose made when maltase acts.
- 3
Separate helping from digesting
Bile increases the surface available; lipase changes the fat molecules chemically.
Worked example
Two equal samples of oil
Both samples contain equal amounts of fat and lipase at the same pH and temperature. One sample is emulsified into smaller droplets. Explain why digestion can be faster in that sample.
One way to explain it
Smaller droplets provide a larger total surface area for the same amount of fat. More fat is exposed to lipase at once, so digestion can occur faster. Emulsification itself does not produce fatty acids or glycerol.
Why this answer works
- Keep the total amount of fat constant.
- Link droplet size to total surface area and enzyme access.
- Assign chemical breakdown to lipase, not to bile.
Is this true? "Bile breaks large fat molecules into smaller fat molecules."
Bile breaks up droplets containing many fat molecules. The molecules remain chemically unchanged during emulsification. Lipase produces the smaller digestion products.