K224 / K225 / 2027

Lesson 2 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 describe named digestive organs and ingestion, digestion, absorption, assimilation and egestion.Syllabus

    Syllabus K224 / K225, Topic 4(a). Describe named digestive organs and ingestion, digestion, absorption, assimilation and egestion.

  • I can match digestive enzymes to substrates and products.Syllabus

    Syllabus K224 / K225, Topic 4(b). Match digestive enzymes to substrates and products.

  • I can state the five specified liver roles.Syllabus

    Syllabus K224 / K225, Topic 4(d). State the five specified liver roles.

About 6 min

Make a guess. You are not marked.

Bile turns large fat droplets into many tiny droplets. Has bile digested the fat?

  1. The droplets are smaller, but still fat.
  2. Smaller droplets means the fat is digested.
  3. Bile contains lipase, which splits 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: Starch to MaltoseAmylase breaks starch into maltose. Salivary and pancreatic amylase act in different parts of the canal. The shapes represent molecular units, not exact chemical structures.StarchMaltose

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 down starch into maltose. Amylase from the salivary glands starts this in the mouth, and amylase from the pancreas carries on in the small intestine. Then maltase, on the lining of the small intestine, breaks maltose into glucose, which can be absorbed.

Proteases digest proteins. A protease in the stomach starts breaking proteins into shorter chains, in acid conditions. More proteases in the small intestine finish the job, making amino acids. In short: protein is broken down to amino acids.

Lipase breaks down fat into fatty acids and glycerol. Lipase from the pancreas goes into the duodenum. Large drops of fat have only a small surface, so lipase can reach only a little of the fat at a time.

Bile is made in the liver, stored in the gallbladder and released into the duodenum. Bile emulsifies fat. This means it breaks large fat drops up into many tiny drops. Tiny drops have a larger surface area to volume ratio, so lipase can act on more fat at once, and fat is digested faster.

Notice the two different verbs. Bile breaks fat UP: the drops get smaller, but each fat molecule is still a fat molecule. Lipase breaks fat DOWN: it splits each fat molecule into fatty acids and glycerol. Examiners do not accept "bile breaks down fat".

Pancreatic juice and bile are alkaline. They help neutralise the acid coming from the stomach, giving the right conditions for the enzymes in the small intestine. Bile contains no enzymes.

Step by step
  1. 1

    Name the substrate

    Identify starch, maltose, protein or fat before choosing an enzyme.

  2. 2

    Give the correct products

    Distinguish maltose made by amylase from glucose made when maltase acts.

  3. 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?

  1. Amylase makes maltose, then maltase makes glucose.
  2. Lipase turns starch into amino acids.
  3. Bile emulsifies starch, then pancreatic lipase breaks it into glucose.
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?

  1. Lipase, mouth, glycerol and amino acids
  2. Protease, stomach, polypeptides
  3. Amylase, stomach, maltose
  4. Maltase, small intestine, starch
Show the answer

Protease, stomach, polypeptides

Pepsin, a protease, begins protein digestion in the stomach, producing shorter polypeptides.

Quick recall

Card 1 of 3. Answer in your head, then check.

Which changes break molecules, and which help enzymes reach them?

You can return to this lesson any time.