IMO Practice Test — Photosynthesis in Higher Plants
6 Questions • 15 min • Olympiad level
15:00
Question 1 of 6
An experiment using heavy-oxygen-labelled water (H₂¹⁸O) shows the released O₂ is also labelled. This proves the oxygen comes from:
Water
CO₂
Glucose
Chlorophyll
Explanation: The labelled oxygen appearing in O₂ proves it originates from water, not CO₂.
Question 2 of 6
If the light reaction is blocked, the Calvin cycle soon stops because it is deprived of:
ATP and NADPH
Oxygen
Chlorophyll
Water only
Explanation: The dark reaction depends on the ATP and NADPH made by the light reaction.
Question 3 of 6
On a bright warm day with abundant light and warmth, photosynthesis may still be slow if CO₂ is low. This illustrates that:
The factor in shortest supply limits the rate
Light is always limiting
Temperature is always limiting
Nothing limits the rate
Explanation: With light and heat plentiful, low CO₂ becomes the limiting factor (Blackman's law).
Question 4 of 6
C4 plants such as sugarcane outperform C3 plants in hot climates mainly because they:
Concentrate CO₂ in bundle-sheath cells and avoid photorespiration
Do not need light
Lack chlorophyll
Use no enzymes
Explanation: The C4 pathway raises CO₂ around RuBisCO, suppressing photorespiration and boosting efficiency.
Question 5 of 6
Greenhouse growers sometimes pump extra CO₂ into the air because, up to a point, CO₂ is:
A limiting factor that increases the rate when raised
Toxic to plants
Useless to plants
Released, not used
Explanation: Raising the limiting CO₂ increases photosynthesis up to saturation, improving yield.
Question 6 of 6
During a drought, photosynthesis slows even in bright light chiefly because water shortage causes:
Stomata to close, cutting CO₂ intake
More chlorophyll to form
The Calvin cycle to speed up
Oxygen to vanish
Explanation: Closed stomata limit CO₂ entry, indirectly lowering the rate during water stress.