Cellular Respiration

How your cells turn food into energy

Junior level — plain language, no maths

You eat food to get energy — but your body can't burn a sandwich in one big flame. Instead, every one of your trillions of cells runs a slow, careful, controlled version of burning called cellular respiration. It takes the sugar from your food, combines it with the oxygen you breathe, and releases the stored energy bit by bit, safely, exactly where and when the cell needs it.

The energy doesn't come out as a flame — it comes out as a tiny rechargeable battery molecule called ATP. Think of ATP as the universal energy currency of life: whenever a cell needs to do anything — flex a muscle, fire a nerve, build a protein — it spends ATP. Respiration's whole job is to keep making fresh ATP, and it does so at an astonishing rate.

Most of this happens inside tiny bean-shaped structures called mitochondria, the cell's power plants. Glucose and oxygen go in; carbon dioxide, water and lots of ATP come out. That carbon dioxide is exactly what you breathe out — so every breath out is your cells finishing the job. In the simulation below, feed a mitochondrion glucose and oxygen and watch the ATP pour out.

Glycolysis, the Krebs cycle and the electron transport chain

Student level — the core equations

Cellular respiration extracts energy from glucose in three linked stages, summarised by one deceptively simple equation: \(C_6H_{12}O_6 + 6O_2 \to 6CO_2 + 6H_2O + \text{ATP}\). First comes glycolysis, in the cytoplasm: glucose is split into two molecules of pyruvate, yielding a small net of 2 ATP and some electron-carrying NADH. Crucially, glycolysis needs no oxygen — it is the ancient core that all life shares.

If oxygen is present, pyruvate enters the mitochondrion for the Krebs cycle (citric acid cycle), which strips it apart, releasing \(CO_2\) and loading up more electron carriers — NADH and FADH₂. These carriers are the real prize: they ferry high-energy electrons to the third stage, the electron transport chain, studded across the mitochondrion's inner membrane, where the bulk of the ATP is finally made.

The tally is lopsided. Glycolysis and the Krebs cycle make only a handful of ATP directly; the electron transport chain, using oxygen as the final electron acceptor, produces the other ~90% — around 30–32 ATP per glucose in total. Take the oxygen away and only glycolysis can run: cells fall back on fermentation, salvaging a meagre 2 ATP and producing lactate (in your muscles) or ethanol and \(CO_2\) (in yeast). Oxygen is worth roughly a fifteen-fold boost in energy.

Chemiosmosis, the proton-motive force and the origin of mitochondria

Scholar level — full mathematical depth

01The idea that seemed impossible

For decades biochemists hunted for a chemical intermediate that carried energy from food to ATP, and never found one. Peter Mitchell's chemiosmotic hypothesis (1961) explained why: there isn't one. The electron transport chain instead pumps protons across the inner mitochondrial membrane, storing energy not in a molecule but in a gradient — an electrochemical battery of \(H^+\). The idea was so heterodox it was ridiculed for years before winning the 1978 Nobel Prize.

02The proton-motive force

That gradient is the proton-motive force, and it has two parts: a difference in \(H^+\) concentration (a pH gradient) and a difference in charge (a membrane voltage of about \(-150\ \text{mV}\)), combined as \(\Delta p = \Delta\psi - \tfrac{2.3RT}{F}\Delta\text{pH}\). Electrons cascading down the chain from NADH to oxygen release energy in steps, and each major complex uses it to shove protons uphill, out of the matrix — charging the membrane like a capacitor.

03A rotary motor made of protein

The stored energy is cashed in by ATP synthase, one of biology's most astonishing machines: a molecular turbine. Protons flowing back down their gradient through the enzyme physically spin a rotor at up to a hundred revolutions per second, and that rotation mechanically forces ADP and phosphate together into ATP. It is a rotary engine a few nanometres across, running on a proton current — direct, visible confirmation of Mitchell's gradient, since the shaft's spin has been filmed.

04Why oxygen, and why it matters

Oxygen's role is subtle but decisive: it is merely the final electron acceptor, sitting at the end of the chain to collect spent electrons and form water. Yet without it the whole chain backs up — the carriers stay reduced, the proton pumping stops, and ATP synthesis via oxidative phosphorylation halts within seconds. Oxygen's exceptional electron affinity is what makes the entire cascade so energetically steep, and hence so productive; it is the thermodynamic reason aerobic life out-powers everything anaerobic.

05Guests that became organs

Mitochondria have their own circular DNA, their own ribosomes, and double membranes — because they were once free-living bacteria. The endosymbiotic theory (Lynn Margulis) holds that some two billion years ago one cell engulfed an aerobic bacterium and, instead of digesting it, kept it as a power plant. Every mitochondrion in your body is a descendant of that ancient captive, and you inherit them all from your mother — which is how mitochondrial DNA traces maternal lineages back through human history.

06The double edge of the chain

The same machinery that powers you also imperils you. Electrons occasionally leak from the chain and reduce oxygen only partially, producing reactive oxygen species — free radicals implicated in ageing and disease. Cells deliberately exploit controlled leaks too: uncoupling proteins let protons flow back without making ATP, dissipating the gradient as pure heat, which is how brown fat warms newborns and hibernating animals. Respiration is not just an energy source but a finely tuned dial between making ATP and making heat.