The short answer
How plants turn light into sugar — and where the oxygen you're breathing actually comes from.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Plants eat light. Photosynthesis takes photons, and water and builds glucose — releasing the oxygen you're breathing right now as a by-product. It runs in two linked stages, and most exam questions hinge on knowing which is which.
Two stages, two places
Light reactions (in the thylakoid membranes): capture light, split water, and make ATP + NADPH — releasing . Calvin cycle (in the stroma): uses that ATP + NADPH to fix into glucose. It's "light-independent" but still needs the products of the light reactions.
Where does the O₂ come from?
The oxygen released is split from water (), not from . This is a classic GAMSAT trap — if an option says the comes from carbon dioxide, it's wrong.
Light reactions vs Calvin cycle
Light reactions
- In the thylakoid membrane
- Need light directly
- Split water → release O₂
- Output: ATP + NADPH
Calvin cycle
- In the stroma
- No light needed directly
- Fix CO₂ into sugar
- Use up ATP + NADPH
Worked example
A plant is moved into complete darkness. The light reactions stop immediately. Why does the Calvin cycle also grind to a halt soon after?
Check yourself
The oxygen released during photosynthesis comes from the splitting of which molecule?
Key takeaways
- Photosynthesis: light + CO₂ + H₂O → glucose + O₂.
- Light reactions (thylakoid): make ATP + NADPH and release O₂ from splitting water.
- Calvin cycle (stroma): uses ATP + NADPH to fix CO₂ into sugar.
- The O₂ comes from water, NOT from CO₂.
- The 'light-independent' Calvin cycle still depends on the light reactions' products.
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