Why does an enzyme lose activity above about 60 degrees Celsius?

Heat disrupts the three-dimensional shape of the protein, the active site distorts and the substrate can no longer bind, an irreversible change called denaturation.

Why budget energy as percentages rather than grams?

The three nutrients supply different energy per gram, so percentages compare fairly and are then converted to grams at 17 and 39 kJ per gram.

S4 Biology: Nutrition and Digestion Paper 3 (enzymes and energy budgeting)

2026-11-03

S · 4

This paper puts digestion back onto enzymes and energy: each enzyme works in its own pH range, and every meal has to be budgeted by proportion.

Quick answer　Five pH units apart; bile has no enzymes; 4,620 kJ or 271.8 g; 340 kJ; 3.6 mg; 100 times; 60.0 g; 70.0 g, 80.3 g and 294.4 g; 944 kJ and 2,360 kJ; denaturation, not hydrolysis.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Nutrition and Digestion
Level	S4

02　Questions (10 items)

FoundationSalivary amylase works best near pH 7 and pepsin near pH 2. By how many pH units do their optima differ?
FoundationDoes bile contain a fat-digesting enzyme, and what is its main action?
FoundationA person needs 8,400 kJ a day with carbohydrate supplying 55%. How many kJ and how many grams is that at 17 kJ per gram, to one decimal place?
FoundationHow much energy does 20 g of protein release on complete oxidation at 17 kJ per gram?
IntermediateAn enzyme digests 24 mg of substrate per minute at 37 degrees Celsius and retains only 15% of that activity at 60 degrees. How many mg per minute at 60 degrees?
IntermediateFolding and villi raise the internal surface area of the small intestine from 0.3 to 30 square metres. By what factor has it increased?
IntermediateA meal has 90 g of carbohydrate, 60% of it starch and the rest glucose. After the starch is fully hydrolysed, what is the total carbohydrate mass, using 162 for the starch unit and 180 for glucose, to one decimal place?
ChallengeA person needs 9,100 kJ a day, with 30% from fat, 15% from protein and the rest from carbohydrate. How many grams of each at 39, 17 and 17 kJ per gram, to one decimal place?
ChallengeA food has, per 100 g, 8 g protein, 12 g fat and 20 g carbohydrate. How much energy does a 250 g pack supply at 17, 39 and 17 kJ per gram?
Error redoA student writes that at high temperature an enzyme is broken down into amino acids and so loses activity. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	5 units	Seven minus two gives five, so the hydrogen ion concentration in the stomach is a hundred thousand times that in the mouth.
2	It contains no enzyme; it emulsifies fat, breaking large droplets into small ones	Bile contains bile salts but no digestive enzyme; emulsification enlarges the surface area available to lipase.
3	4,620 kJ, that is 271.8 g	Fifty-five per cent of 8,400 kJ is 4,620 kJ, which divided by 17 gives 271.8 g.
4	340 kJ	Twenty times 17 gives 340 kJ, the same energy density as carbohydrate.
5	3.6 mg	Twenty-four times 0.15 gives 3.6 mg, the rest lost because some enzyme molecules have denatured.
6	100 times	Thirty divided by 0.3 gives 100, and a larger area lets more nutrient be absorbed in the same time.
7	60.0 g	Fifty-four grams of starch hydrolyses to 54 times 180 over 162, that is 60.0 g, and with the 36 g of free glucose the total is 96.0 g; this item asks only for the hydrolysed mass.
8	Fat 70.0 g, protein 80.3 g and carbohydrate 294.4 g	Fat gives 2,730 kJ, which is 70.0 g at 39 kJ per gram; protein gives 1,365 kJ, that is 80.3 g; carbohydrate gives 5,005 kJ, that is 294.4 g.
9	944 kJ per 100 g, so 2,360 kJ per pack	Eight times 17, 12 times 39 and 20 times 17 total 944 kJ per 100 g, and 250 g is 2.5 times that, giving 2,360 kJ.
10	Heat only denatures the enzyme protein; the peptide bonds are intact, so it is not hydrolysed to amino acids.	Denaturation changes only the three-dimensional shape and is not reversed on cooling, whereas hydrolysis to amino acids needs a protease or prolonged strong acid or alkali.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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