Nutrition and digestion · Process
Enzymes in digestion
One enzyme molecule can cut a thousand others apart and still be there at the end. Heat it by fifteen degrees and it never works again. Neither of those is what people expect.
Start here
A teaspoon of amylase digests a kilogram of starch.
Weigh the amylase before and after. It has not gone down. Filter it out at the end and it works exactly as well on the next kilogram, and the one after that. The starch is gone; the amylase is all still there.
So what is an enzyme doing?
It is a catalyst: a molecule that makes a reaction happen faster without being changed or used up by it. That is the whole definition, and it is why a small amount of enzyme is enough for an enormous amount of food.
At the bench · one tube, two dials
Run the reaction, watch the counts
not run yet
Watch all three counters, and watch what happens to the third one.
Enzyme
pH of the tube
Temperature
37 °C
Above about 50 °C the folds holding this enzyme’s shape have come apart, and the damage is already done. Cooling the tube will not bring it back — cool it first, then take a fresh tube.This enzyme was taken above 50 °C and is denatured. It is back at a temperature it would work perfectly well at, and it will not work at all. Only a fresh tube starts again.Past the optimum and falling fast. The molecule is beginning to lose its shape, and above 50 °C it will not get it back.Around body temperature — close to the fastest this enzyme goes.Cold but unharmed. Molecules collide less often, so the rate is low; warm it up and it recovers completely.Near freezing. Almost nothing happens, and nothing is damaged — this is how a fridge slows food spoiling.
starch glucoseprotein amino acidslipid fatty acids + glycerol
Rate 100% of maximum
Starch leftProtein leftLipid left
1000 units
Glucose madeAmino acids madeFatty acids and glycerol made
0 units
Enzyme molecules present
40 — unchanged
Nothing was digested and nothing will be. All forty enzyme molecules are still in the tube — they have not been used up, they have been ruined. Turn the temperature back down and run it again: still nothing. That is the difference between denatured and merely cold.Nothing was digested at all. The enzyme is intact — it has not been ruined, and the counter still reads forty — but in these conditions it is not working. Move the pH towards one this enzyme works at, or the temperature back towards 37 °C, and run it again.A little product, slowly. The enzyme is intact and these are not the conditions it works best in — move the pH towards one this enzyme works at, or the temperature towards 37 °C, and run it again to see the difference.Substrate down, product up, enzyme unchanged. Forty molecules converted hundreds of units and are all still there, ready to do it again.
The three you must know
One enzyme, one substrate, one place it works best.
Carbohydrase (amylase)
Made in the salivary glands and pancreas starch glucose · Starts in the mouth, stops in the acid of the stomach, and starts again in the small intestine when pancreatic alkali has neutralised the acid. Best at pH 7 (neutral)
Protease
Made in the stomach and pancreas protein amino acids · The stomach version is built to work in acid, which is unusual for a protein — most would denature at pH 2. It is a genuinely specialised molecule. Best at pH 2 in the stomach, 8 in the small intestine
Lipase
Made in the pancreas lipid fatty acids + glycerol · Works on the surface of fat droplets, so bile’s emulsifying job multiplies its rate rather than helping it chemically. Best at pH 8 (slightly alkaline)
Key fact
An enzyme is a biological catalyst: it speeds a reaction up and is not used up doing it. Each one has a shape that fits one substrate, and above about 50 °C that shape is destroyed permanently.
Think again
“Enzymes are killed by heat.”
An enzyme cannot be killed, because it was never alive. It is a single protein molecule — no cell, no membrane, no respiration, nothing that could die. What heat does is shake the molecule until the folds holding its shape come apart, and since the shape is the whole point, a molecule with the wrong shape no longer fits its substrate. The word is denatured, and examiners take the difference seriously. It matters practically too: “killed” suggests something that could be replaced by growing more, whereas a denatured enzyme is a permanently ruined tool. Cool it back to 37 °C and nothing recovers — which is exactly what you saw on the bench above, and it is why the change is not simply the reverse of warming up.
“The enzyme gets used up as the food is digested.”
Watch the third counter on the bench. Substrate falls, product rises, enzyme does not move. A single amylase molecule binds a starch chain, cuts it, releases the pieces and is immediately free to bind the next one — thousands of times a second in some cases. This is what the word catalyst means, and it explains a fact that is otherwise baffling: your pancreas makes a few grams of enzyme a day and it handles a kilogram of food. If enzymes were consumed by the reactions they catalyse, digestion would be limited by how fast you could manufacture them, and no animal could eat a large meal.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · What a catalyst is
After digesting a large amount of starch, how much of the original amylase is left?
Rung 2 · The one that catches people
Amylase at 70 °C digests no starch. It is cooled back to 37 °C and still digests no starch. Why not?
Rung 3 · Explain the pH switch
Amylase from saliva works well in the mouth, stops working in the stomach, and then starch digestion resumes in the small intestine. Explain all three stages, and say what the pancreas contributes besides enzymes.
Rung 4 · Take it somewhere new
A washing powder claims to work at any temperature. Using what you know about enzymes, say what you would expect to happen at 20 °C, at 40 °C and at 90 °C, and design a fair test of the claim.
Key note
Enzymes are biological catalysts. Carbohydrase breaks starch to glucose; protease breaks protein to amino acids; lipase breaks lipid to fatty acids and glycerol. Each works fastest at about 37 °C and at the pH of the organ that makes it. Above about 50 °C the enzyme is denatured and the change is permanent.
Going further
Biological washing powder contains protease and lipase, which is why it removes blood and grease and why the box tells you to wash at 40 °C rather than 60 °C — a hot wash denatures the enzymes you paid for. The awkward consequence is that a wash cool enough to keep the enzymes working is also cool enough to leave some bacteria alive, so the choice between a 40 °C enzyme wash and a 60 °C sterilising wash is a genuine trade-off rather than a mistake. Enzymes are also why you cannot make jelly with fresh pineapple: it contains a protease that cuts the gelatin protein apart faster than it can set. Tinned pineapple works perfectly, because canning heats it and denatures the enzyme.
Before this lesson
Connects to
At GCSE this becomes
- The lock-and-key model, active sites, and rate-against-temperature and rate-against-pH curves.
Where to next
Ask Mr Badmus AI
Want to talk through why denaturing cannot be undone?
The bench is a simplified model: rate is shown against temperature and pH only, and one substrate at a time. Real digestion runs several enzymes at once on a mixture, and the curves are smoother than the numbers here suggest.
Lesson content © MrBadmusAI.