Respiration: Spending the Sugar
Every cell runs the same reaction in reverse to get ATP — and how much it gets depends entirely on whether oxygen is available.
Builds on: 1.1 Photosynthesis: Making Food from Light
Same idea, other course:⚗️ Catalyst 5.1 — exothermic reactions and where the energy actually goes
ATP: the cell’s small change
Cells do not spend glucose directly, any more than you pay for coffee with a gold bar. Glucose is bulk storage; the working currency is ATP (adenosine triphosphate). Snapping off its third phosphate releases a usable packet of energy and leaves ADP, which gets recharged and used again.
The turnover is extraordinary. You hold only about 250 g of ATP at any moment, and you cycle through roughly your own body weight of it every day. ATP is not a battery you fill; it is a bucket brigade that never stops moving.
The reaction, backwards
Cellular respiration is photosynthesis run in reverse — and it happens in every living cell, plants included. Plants photosynthesise in the light and respire around the clock.
This is combustion, carried out carefully. Burning sugar in a flame releases the same energy in one useless burst; the cell extracts it down a staircase of small steps, each catalysed by an enzyme, capturing energy as ATP along the way instead of losing it all as heat. The reason you are warm rather than on fire is entirely one of pacing.
Note the symmetry with the previous lesson: what photosynthesis produces, respiration consumes, and vice versa. Together they form a loop through the whole biosphere.
With oxygen, and without
Aerobic respiration — with oxygen, mostly in the mitochondria — extracts around 30 ATP per glucose. Oxygen’s role is to sit at the end of the chain and accept spent electrons; without it the whole line backs up and stops.
Anaerobic respiration — without oxygen — gets only through the first stage, glycolysis, in the cytoplasm. Yield: 2 ATP per glucose. Roughly a fifteenth as much from the same sugar, because most of the energy is still locked in the partly-broken molecule.
What that leftover molecule becomes depends on the organism:
- Your muscles make lactic acid. It lets you sprint past the rate your lungs can supply, and you repay the oxygen debt afterwards — which is why you keep panting after you stop.
- Yeast makes ethanol and CO₂. This is fermentation: the CO₂ raises bread, the ethanol makes beer, and the same organism does both. Unit 6’s capstone measures it with a balloon.
The ache during hard effort is not lactic acid “building up” the way gym folklore says — lactate clears within an hour and is itself a useful fuel. The burning is the acidity that comes with it, and the soreness two days later is microscopic muscle damage, a different thing entirely.
⚗️ Lab — Aerobic vs Anaerobic
The same glucose, two routes, wildly different returns.
- Run it aerobically and count the ATP dots. Now switch off the oxygen.
- Switch the organism between human muscle and yeast — the yield is identical, the waste is not.
- Note where each route happens: mitochondrion versus bare cytoplasm.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.