How should the equation be remembered?

Six carbon dioxide plus six water, with light and chlorophyll, gives one glucose and six oxygen, 372 units on each side.

Why does the oxygen not come from carbon dioxide?

Photolysis splits water and releases its oxygen; carbon dioxide supplies carbon for the sugar.

S5 Biology: Photosynthesis (Paper 1)

2026-11-01

Tutor Prep Guide · Primary English

The first paper starts from the overall equation, mass balance and the absorption spectrum of chlorophyll, then scales uptake from leaf area to a whole day.

Direct answer　Photosynthesis happens in the chloroplast: the light reaction on the thylakoid membranes makes ATP and NADPH and releases oxygen, while the Calvin cycle in the stroma reduces carbon dioxide into glucose.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Biology: Photosynthesis
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationHow many carbon dioxide molecules are needed to build one glucose molecule?
FoundationWhich raw material supplies the oxygen released in photosynthesis?
FoundationWhich two regions of the spectrum does chlorophyll absorb most strongly?
FoundationSix carbon dioxide molecules (Mr 44) and six water molecules (18) form one glucose (180) and six oxygen molecules (32). What is the total mass on each side?
IntermediateWhere in the chloroplast do the light-dependent reactions take place?
IntermediateWhere in the chloroplast does the Calvin cycle run?
IntermediateLight intensity doubles from 400 to 800 units yet bubbles stay at 48 per minute. What does this show?
ChallengeA 10 cm² leaf takes up 6 cm³ of carbon dioxide per hour. How much does 250 cm² of leaf take up in the same time?
ChallengeA plant fixes 0.5 g of carbon dioxide an hour over 12 hours of light. How much glucose could that carbon make?
Error redoA student says photosynthesis happens only by day and respiration only at night. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Six molecules	Glucose holds six carbon atoms, so six carbon dioxide molecules must supply them.
2	Water, through photolysis	Labelling water with a heavy oxygen isotope showed the released gas comes from water, not carbon dioxide.
3	Red light and blue-violet light	Chlorophyll absorbs red and blue-violet light best and reflects most green light, which is why leaves look green.
4	372 units on both sides	Reactants give 264 plus 108, or 372; products give 180 plus 192, also 372.
5	On the thylakoid membranes	The thylakoid membranes carry the photosystems and electron transport chain where light energy becomes ATP and NADPH.
6	In the stroma	The stroma holds the Calvin cycle enzymes that use ATP and NADPH to reduce carbon dioxide into sugar.
7	The light saturation point is reached and light is no longer limiting	Carbon dioxide concentration or temperature has become limiting, so further light gives no gain.
8	150 cm³	Six divided by ten gives 0.6 per square centimetre, times 250 is 150 cubic centimetres.
9	About 4.09 g	Twelve hours at 0.5 g gives 6 g of carbon dioxide, about 0.136 mol; a sixth of that is 0.0227 mol of glucose, or 4.09 g.
10	Respiration never stops, while photosynthesis needs light	Both run together in daylight, and net photosynthesis is gross photosynthesis minus respiration, so at night the plant only consumes.

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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