Lesson 3 of 7 / Nutrition in humans
Digestive enzymes and bile
Which changes break molecules, and which help enzymes reach them?
I can match digestive enzymes to their substrates and products, and distinguish digestion from emulsification.
I can explain digestive organ functions and the stages from ingestion to egestion and assimilation.Syllabus
Syllabus K325, Topic 4(a). Explain digestive organ functions and the stages from ingestion to egestion and assimilation.
I can match digestive enzymes to their substrates and products.Syllabus
Syllabus K325, Topic 4(c). Match digestive enzymes to their substrates and products.
I can explain the specified nutrient-processing, bile and breakdown roles of the liver.Syllabus
Syllabus K325, Topic 4(f). Explain the specified nutrient-processing, bile and breakdown roles of the liver.
About 6 min
Make a guess. You are not marked.
Bile turns large fat droplets into many tiny droplets. Has bile digested the fat?
Show the answer
The droplets are smaller, but still fat.
Bile emulsifies fat, a physical change. Lipase then breaks fat down into fatty acids and glycerol.
The key idea
Enzymes 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. 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. Emulsification is the breaking up of large fat droplets into many small droplets. The small droplets have a larger surface area to volume ratio, so more fat is exposed to lipase at once and fat digestion is faster.
Notice the two verbs. Bile breaks up fat droplets: it is a physical change, and every fat molecule stays a fat molecule. Lipase breaks down fat: it is a chemical change that splits each fat molecule into fatty acids and glycerol. Examiners reject "bile breaks down fat" because breaking down is the job of the enzyme.
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 emulsifies fat, raising the surface area to volume ratio; lipase breaks down 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
Emulsification breaks up the fat into smaller droplets, which have a larger surface area to volume ratio. More fat is exposed to lipase at once, so digestion is 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.
Watch out for this
A student says: "Bile breaks down fat." What is wrong with this?
Show the answer
Bile breaks up fat droplets; it does not break down fat. Breaking down means splitting molecules chemically, and only lipase does that. Mark schemes reject "break down" for bile, so write "emulsifies" or "breaks up".
Check your understanding
Which sequence correctly describes starch digestion?
Show the answer
Amylase makes maltose, then maltase makes glucose.
Identify both substrates and products: starch to maltose, then maltose to glucose.
Exam-style questions
Check your understanding
Which row correctly gives an enzyme, where it acts and its product?
Show the answer
Protease, stomach, polypeptides
Pepsin, a protease, begins protein digestion in the stomach, producing shorter polypeptides.
Quick recall
Which changes break molecules, and which help enzymes reach them?