The short answer
How a single glucose is banked into ~30+ ATP across glycolysis, the Krebs cycle and the electron transport chain.
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.
Every cell runs on ATP, and cellular respiration is how it converts the energy in glucose into that usable currency. The overall story is simple — — but the marks are in knowing the three stages, where each happens, and which one makes most of the ATP.
Three stages, two locations
Glycolysis happens in the cytoplasm (splits glucose into 2 pyruvate, nets 2 ATP). The Krebs (citric acid) cycle and the electron transport chain (ETC) happen in the mitochondria. The ETC, via oxidative phosphorylation, makes the vast majority of the ATP.
Following one glucose
Glycolysis (cytoplasm)
Glucose → 2 pyruvate. Net 2 ATP + 2 NADH. No oxygen needed yet.
Link reaction + Krebs (mitochondrial matrix)
Pyruvate → acetyl-CoA → cycle. Releases and loads electron carriers (NADH, FADH₂); a little ATP.
Electron transport chain (inner membrane)
NADH/FADH₂ drop electrons down the chain, pumping H⁺ to build a gradient. ATP synthase rides that gradient to make most of the ATP (~26–28).
Oxygen's job
is the final electron acceptor at the end of the chain, forming water. No → the chain backs up and stops.
Where the ATP really comes from
Glycolysis and Krebs directly make only a handful of ATP. Most ATP comes from the ETC (oxidative phosphorylation), powered by the electrons NADH and FADH₂ deliver. If a question asks where most ATP is generated, the answer is the electron transport chain, not glycolysis.
Aerobic vs anaerobic
Aerobic (with O₂)
- All three stages run
- O₂ is the final electron acceptor
- ~30+ ATP per glucose
- Products: CO₂ + H₂O
Anaerobic (no O₂)
- Only glycolysis runs
- Pyruvate → lactate (or ethanol) by fermentation
- Just 2 ATP per glucose
- Fermentation regenerates NAD⁺ so glycolysis can continue
Worked example
A muscle cell is suddenly deprived of oxygen during intense exercise. Why does its ATP output plummet, and what lets glycolysis keep going at all?
Check yourself
In aerobic respiration, the largest amount of ATP is produced during which stage, and what is oxygen's role?
Key takeaways
- Respiration: glucose + O₂ → CO₂ + H₂O + ATP, in three stages.
- Glycolysis (cytoplasm) → Krebs cycle + ETC (mitochondria).
- Most ATP is made by the electron transport chain (oxidative phosphorylation).
- O₂ is the FINAL electron acceptor; no O₂ ⇒ only glycolysis (2 ATP) + fermentation.
- Fermentation's job is to regenerate NAD⁺ so glycolysis can keep going.
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