Why is the small intestine folded and covered with villi?

Folds, villi and microvilli multiply the inner surface area hundreds of times, so absorption is faster and more complete.

Why does the mass rise when starch is hydrolysed to glucose?

Each glucose unit gains a water molecule on hydrolysis, so the total mass exceeds that of the original starch.

S4 Biology: Nutrition and Digestion Paper 2 (milk teeth, absorption and emulsification)

2026-11-03

S · 4

This paper focuses on absorption, from tooth counts to villi, showing how much nutrient actually reaches the blood.

Quick answer　12 teeth; 1,215 kJ; 58.3 g; 27.5 g; 11.1%; amino acids; villi enlarge surface area; 100 times; 1,020 kJ; the stomach is not the main absorption site.

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 and a child has 20 milk teeth. How many more teeth does an adult have?
FoundationA meal has 45 g carbohydrate, 15 g protein and 5 g fat. Using 17 kJ per gram for carbohydrate and protein and 39 kJ per gram for fat, what is the total energy?
FoundationAfter pepsin and intestinal peptidases act, into which class of molecules is protein finally broken down for absorption?
FoundationWhat is the main role of the villi in the small intestine?
IntermediateA person needs 9,100 kJ a day with fat supplying one quarter of the energy. How many grams of fat is that, to one decimal place?
IntermediateHow much energy comes from 12 g of fat, and how many grams of carbohydrate would supply the same energy at 39 and 17 kJ per gram respectively, to one decimal place?
IntermediateForty grams of starch is fully hydrolysed to glucose. Using 162 for the C6H10O5 unit and 180 for glucose, by what percentage does the total mass increase, to one decimal place?
ChallengeBile breaks a 2 mm oil drop into droplets of 20 micrometres. Assuming constant total volume, by what factor does the total surface area increase?
ChallengeA meal contains 80 g of protein and a quarter is not absorbed. What energy does the absorbed part give at 17 kJ per gram?
Error redoA student writes that the stomach is the main site of absorption and most nutrients are absorbed there. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	12 teeth	Subtracting 20 from 32 gives 12, the premolars and molars added when milk teeth are replaced.
2	1,215 kJ	Forty-five grams of carbohydrate gives 765 kJ, 15 g protein 255 kJ and 5 g fat 195 kJ, totalling 1,215 kJ.
3	Amino acids	Proteinases first cut protein into peptides, and peptidases then release amino acids small enough to cross the intestinal lining.
4	It greatly increases the surface area for absorption	Villi multiply the surface area and, with dense capillaries and lacteals, speed nutrients into blood and lymph.
5	58.3 g	A quarter of 9,100 kJ is 2,275 kJ, which divided by 39 gives 58.3 g.
6	468 kJ; 27.5 g of carbohydrate	Twelve grams times 39 kJ gives 468 kJ, and 468 divided by 17 gives 27.5 g of carbohydrate.
7	11.1%	Each unit grows from 162 to 180, so 180 over 162 is 1.111, an 11.1% gain because one water molecule is added per unit.
8	100 times	Two millimetres is 2,000 micrometres, a hundredfold reduction in diameter, so the number of droplets rises a millionfold and total area rises a hundredfold.
9	1,020 kJ	Three quarters of 80 g is 60 g, and 60 times 17 gives 1,020 kJ.
10	The stomach absorbs only some water, alcohol and certain drugs; most nutrients are absorbed in the small intestine.	The stomach has no villi and little surface area, so absorption is mainly done by the villi of the small intestine.

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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