Why convert to kJ before comparing energy rather than comparing grams?

Each nutrient supplies a different energy per gram, and fat at 39 kJ per gram is more than twice carbohydrate or protein, so gram counts alone understate fat.

How do the length and the lining of the small intestine affect absorption?

About seven metres of gut with circular folds and villi enlarge the surface area a hundredfold, so length and folding together set the rate of absorption.

S4 Biology: Nutrition and Digestion Paper 4 (proportions, absorption and energy needs)

2026-11-03

S · 4

This paper works on proportion and demand: tooth counts and gut length become percentages, and a daily energy requirement is split into grams of each nutrient.

Quick answer　60.0 per cent; 77.8 per cent; 425 kJ; 40 times; 47.9 per cent; 67.3 g; 4.5 mg; 52.3 g, 60.0 g and 220.0 g; 1,291.5 kJ and 15.4 per cent; the colon absorbs water.

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)

FoundationAn adult has 32 permanent teeth while a child has 20 milk teeth. By what percentage is the permanent set larger, to one decimal place?
FoundationThe adult alimentary canal is about 9 m long and the small intestine takes about 7 m of it. What percentage of the total is that, to one decimal place?
FoundationHow much energy does 25 g of protein release on complete oxidation at 17 kJ per gram?
FoundationBile salts emulsify one oil droplet 2 mm across into droplets 0.05 mm across with the volume unchanged. By what factor does the total surface area increase?
IntermediateA meal contains 12 g of fat and 30 g of carbohydrate. What percentage of the meal energy comes from fat at 39 and 17 kJ per gram, to one decimal place?
IntermediateA person needs 10,500 kJ a day with 25 per cent from fat. How many grams of fat is that at 39 kJ per gram, to one decimal place?
IntermediateAn enzyme digests 36 mg of substrate per minute at 40 degrees Celsius and keeps only 12.5 per cent of that at 70 degrees. How many mg per minute at 70 degrees?
ChallengeA person needs 6,800 kJ a day with 15 per cent from protein, 30 per cent from fat 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 6 g protein, 9 g fat and 24 g carbohydrate. How much energy does a 150 g pack supply, and what percentage of 8,400 kJ a day is that at 17, 39 and 17 kJ per gram, to one decimal place?
Error redoA student writes that the small intestine absorbs everything and the colon absorbs nothing at all. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	60.0 per cent	The difference is 12 teeth, and 12 over 20 times 100 gives 60.0 per cent.
2	77.8 per cent	Seven over nine times 100 gives 77.8 per cent, and most absorption area lies in the small intestine.
3	425 kJ	Twenty-five times 17 gives 425 kJ, the same as the same mass of carbohydrate.
4	40 times	At fixed total volume the surface area varies inversely with diameter, and 2 over 0.05 gives 40.
5	47.9 per cent	Fat supplies 468 kJ and carbohydrate 510 kJ for a total of 978 kJ, and 468 over 978 gives 47.9 per cent.
6	67.3 g	Twenty-five per cent of 10,500 kJ is 2,625 kJ, which divided by 39 gives 67.3 g.
7	4.5 mg	Thirty-six times 0.125 gives 4.5 mg, the rest lost to denaturation.
8	Fat 52.3 g, protein 60.0 g and carbohydrate 220.0 g	Fat gives 2,040 kJ or 52.3 g, protein gives 1,020 kJ or 60.0 g, and carbohydrate gives 3,740 kJ or 220.0 g.
9	1,291.5 kJ, which is 15.4 per cent	One hundred grams gives 861 kJ, and 150 g is 1.5 times that, so 1,291.5 kJ, which over 8,400 is 15.4 per cent.
10	The colon does absorb water and some mineral salts, so it is not inert.	The colon reclaims one to two litres of water a day, which turns liquid chyme into semi-solid faeces, so the small intestine is not the only absorbing surface.

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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