S4 Biology: Nutrition, Digestion and Absorption prep guide | Energy sums, enzymes and surface area

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can recite 17 and 39 kilojoules per gram but freeze when a label asks for a percentage, and they answer the long small intestine question with the word time only. This lesson turns both into arithmetic.

Objectives — Students can: (1) compute the energy and percentage contribution of a meal from its macronutrient masses; (2) name the enzyme, the substrate and the product at each stage of digestion; (3) estimate the absorptive surface area from gut length and villi magnification and explain why it matters.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Energy units	How many kilojoules in one kilocalorie	4.184 kJ
Enzyme specificity	Can a protease digest starch	No, enzymes are specific
Gut anatomy	Where most absorption happens	Jejunum and ileum

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Labels first	Two food labels side by side, ask which carries more energy	Read the mass column
8-22 min	Energy sum	60 times 17 plus 25 times 17 plus 20 times 39	2225 kJ
22-34 min	Percentages	Protein 425 of 2225 kJ, and the meal share of 8400 kJ	19.1 percent, 26.5 percent
34-46 min	Enzyme map	Mouth, stomach and small intestine compared	Draw the hydrolysis chain
46-54 min	Surface area	pi times 0.025 m times 6 m, then times 600	283 m2
54-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Energy first, health claims second

The energy values are 17, 17 and 39 kilojoules per gram. The carbohydrate gives 1020 kJ, the protein 425 kJ and the fat 780 kJ, so the meal totals 2225 kJ, which is 532 kilocalories.
Percentages are 45.8, 19.1 and 35.1. The fat is only 19.0 percent of the mass but 35.1 percent of the energy, and that gap is the whole arithmetic behind the warning about oil.

Task: double the protein to 50 g and halve the fat, then state the new total in kilojoules.

Scene 2: Digestion is a production line with fixed stations

In the mouth, salivary amylase turns starch into maltose; starch digestion is never finished there. The stomach secretes pepsin working near pH 1.5 to 2, breaking protein into peptides, and it absorbs almost nothing. In the small intestine pancreatic amylase, protease and lipase take over, while bile from the liver and gall bladder emulsifies fat into small droplets, which is a physical change and not an enzyme action.
Monosaccharides and amino acids cross the villi into blood; fatty acids and glycerol enter lymph first.

Task: rewrite the sentence bile digests fat so that it is chemically correct.

Scene 3: Surface area sets the ceiling on absorption rate

Treating the gut as a cylinder gives pi times 0.025 m times 6 m, which is only 0.471 m2. Villi and microvilli multiply that by about 600, giving 283 m2, roughly a tennis court. The length also raises the time food spends against the epithelium, so absorption depends on area and time together.
In diarrhoea the villi flatten and transit speeds up, so both factors collapse at once.

Warn: an answer that mentions only length misses half the marks, because length alone does not create area.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Energy of a meal	Sum of mass times energy value	2225 kJ
Calculation	Percentage contribution	Nutrient energy divided by total	19.1 percent
Explanation	Enzyme to site to product	Name, substrate, product and pH	Salivary amylase, starch to maltose
Calculation	Absorptive area	pi d L times magnification	283 m2

Sanity check (teach this): The three percentages must sum to 100, and 45.8 plus 19.1 plus 35.1 does. The meal is 26.5 percent of a 8400 kJ day, so any figure above 100 means kilojoules and kilocalories were mixed. Fat carries 2.3 times the energy of carbohydrate per gram, so its energy share must exceed its mass share.

05 Independent practice (3 questions)

Question	Answer	What to watch
Same meal with 50 g protein and 10 g fat	2260 kJ	Scale each term first
Body mass index at 60 kg and 1.65 m	22.0	Mass over height squared
Type of change caused by bile	Physical emulsification	No enzyme involved
Cylinder area of the gut alone	0.471 m2	pi d L

06 Common mistakes and what to say

Mistake	Student writes	Say this
Mixing kilocalories with kilojoules	Reporting 532 kJ as kilocalories	“One kilocalorie is 4.184 kilojoules.”
Dividing by the wrong total	Dividing protein energy by total mass	“The denominator is total energy, not total grams.”
Claiming bile digests fat	Bile breaks fat into fatty acids	“Bile only emulsifies; lipase does the splitting.”
Forgetting the magnification	Stopping at 0.47 m2	“The cylinder is step one, microvilli multiply it by 600.”

07 Homework (paper, section and question numbers)

Secondary biology chapter index — All chapters
DSE biology practice paper — Human physiology, questions 3 to 7

08 Exit check (last 5 minutes)

Question	Answer	Decision
Total energy of 60, 25 and 20 g	2225 kJ	All correct: move to respiration
Energy share of fat	35.1 percent	Wrong: redo the percentages
Enzyme acting on protein in the stomach	Pepsin near pH 1.5 to 2	Two wrong: restart act two

09 Tutor FAQ

Must the energy values be memorised

Yes, learn 17, 17 and 39, and always write the unit as kilojoules per gram.

Why is the fat share higher than its mass share

Fat gives 39 kilojoules per gram, more than twice carbohydrate, so the same mass contributes double energy.

Is a long gut enough on its own

No, the length gives a base area and the microvilli multiply it about 600 times.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.