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Primary 6 Science: Photosynthesis & Respiration Enrichment Practice
Real Primary 6 Science questions on Photosynthesis & Respiration Enrichment, pulled directly from the Olibits question bank — the same questions a child would actually see, not separately written for this page.
MOE strand: Discovery Science
Curriculum alignment
Science Teaching & Learning Syllabus, Primary Three to Six (MOE, 2023 syllabus)
Science is not a taught subject at P1/P2 under MOE's own syllabus — our P1/P2 Science content is offered as enrichment beyond the official syllabus, not as a claim of syllabus coverage at those grades.
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Real Photosynthesis & Respiration Enrichment practice questions
A plant appears to give out oxygen and take in carbon dioxide during a sunny day, but gives out carbon dioxide at night. What best explains this?
During the day, the rate of photosynthesis is higher than the rate of respiration, so more oxygen is produced than used; at night only respiration occurs ✓
Plants stop respiring completely whenever there is any sunlight at all
The plant randomly swaps between only photosynthesising and only respiring
Plants only respire during the day and only photosynthesise at night
In daylight, photosynthesis (which uses CO2 and releases O2) happens faster than respiration (which uses O2 and releases CO2), so the plant is a net oxygen releaser. At night, only respiration occurs, so it releases carbon dioxide.
A leaf is boiled in water to soften it, then treated to remove its green colour, and finally tested with iodine solution - it turns blue-black. What does this result show?
The leaf contains starch, showing that photosynthesis has taken place ✓
The leaf is dead and can no longer photosynthesise
The leaf contains no starch and has not photosynthesised
The leaf has too little chlorophyll to ever photosynthesise
Iodine solution turns blue-black in the presence of starch. Since glucose made in photosynthesis is often stored as starch, a positive result shows photosynthesis has occurred.
Before a starch test, a plant is often kept in a dark cupboard for a day or two ('destarched'). Why is this step important?
To kill the leaf completely so it can be safely handled
To make the leaf turn a brighter green before testing
To remove any starch already present in the leaf, so results only show starch made during the test period ✓
To add extra chlorophyll into the leaf before the test
Destarching removes leftover starch from before the experiment, ensuring that any starch detected afterwards is genuinely due to photosynthesis occurring during the actual test - making it a fair test.
A variegated leaf (with green and white patches) is exposed to light for several hours, then tested with iodine solution. What result would you expect, and why?
Only the white parts turn blue-black, showing chlorophyll is not needed
The whole leaf turns blue-black equally, showing colour makes no difference
Only the green parts turn blue-black, showing that chlorophyll is needed for photosynthesis ✓
Neither part turns blue-black, since variegated leaves cannot photosynthesise at all
Only the green, chlorophyll-containing parts can carry out photosynthesis and make starch, so only those parts turn blue-black with iodine, showing chlorophyll is necessary for photosynthesis.