Why does a white patch on a variegated leaf stay brown?

No chlorophyll means no light reaction, so no starch forms and iodine shows no blue-black colour.

How is a limiting factor identified?

Change one factor at a time; if the rate rises with it, that factor was limiting.

S5 Biology: Photosynthesis (Paper 2)

2026-11-01

The second paper covers the starch test, the leaf-disc experiment and limiting factors, matching each result to the design of the experiment.

Direct answer　Dark storage clears old starch before testing; areas without chlorophyll never turn blue-black; the lowest of light, carbon dioxide and temperature limits the rate.

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)

FoundationWhat colour shows a positive starch test with iodine solution on a leaf?
FoundationA variegated leaf is lit and iodine-tested. What happens in the region with no chlorophyll?
FoundationBesides counting oxygen bubbles, which measurement can show the rate of photosynthesis?
FoundationWhy is a plant kept in the dark for a day before the leaf starch test?
IntermediateRaising carbon dioxide from 0.04 to 0.1 per cent lifts the rate from 30 to 45 units an hour. What is the percentage rise?
IntermediateRaising temperature from 35 to 45 °C drops the rate from 48 to 10. Why?
IntermediateA plant sealed in a clear container keeps the carbon dioxide level steady in the light. What does that mean?
ChallengeEight of ten leaf discs float after five minutes of light. How many of thirty discs should float in the same time?
ChallengeA 200 m² greenhouse needs 2.5 g of carbon dioxide per square metre per hour over ten lit hours. What mass is needed in a day?
Error redoA student says the oxygen released comes from carbon dioxide. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Blue-black	Starch turns blue-black with iodine, and the depth of colour hints at how much is present.
2	It stays brown, with no blue-black colour	Without chlorophyll no light reaction occurs, so no starch forms and the area stays brown.
3	The gain in the dry mass of the leaf	Photosynthesis adds organic matter, so comparing dry mass before and after gives a rate.
4	So that existing starch is used up or exported	Clearing old starch first means any starch found must come from the experiment itself.
5	50 per cent	Forty-five minus thirty is fifteen, which is half of thirty, so the rate rises by 50 per cent.
6	The enzymes of both stages are denatured	Enzymes lose their shape and activity at high temperature, so the rate collapses.
7	Uptake by photosynthesis equals release by respiration	The two processes cancel out, giving zero net photosynthesis at the compensation point.
8	Twenty-four discs	Eight tenths of thirty is twenty-four, keeping the same proportion.
9	5 kg	Two hundred square metres times 2.5 g times ten hours is 5 000 g, or 5 kg.
10	The oxygen comes from the photolysis of water	Photolysis splits water into oxygen, protons and electrons, while carbon dioxide only supplies carbon.

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.

Book an S5 Biology tutor Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.