Cellular respiration

Cellular respiration is the set of chemical reactions a cell uses to release the energy stored in glucose and transfer it into ATP, the molecule the cell can actually spend. The energy was already there, locked in glucose's bonds. Respiration does not create that energy — it transforms it from one form (chemical bonds in food) into another (ATP, plus heat). Energy is conserved throughout; none is made from nothing and none is destroyed.

Notice the word “cellular.” Respiration is a chemical process that happens inside cells — in a eukaryotic cell, in the cytoplasm and at the mitochondria. It is not the same thing as breathing. Breathing (ventilation) moves air in and out of your lungs; cellular respiration is the biochemistry that burns glucose inside the cells themselves. Breathing supplies the O₂ that respiration eventually uses, but the two are different levels of the story.

And it is very nearly everywhere. Glycolysis is essentially universal — virtually every cell runs it, yours right now and a plant's too, day and night. What varies is what happens next: not every cell even has mitochondria (prokaryotes don't, and neither do your mature red blood cells), and not every cell is continuously running aerobic respiration. Keep three questions in mind for the rest of the topic: where each stage happens, how much ATP it yields, and whether oxygen is available. Those three decide everything.

In a eukaryotic cell with oxygen available, aerobic respiration runs glucose through three stages in order. Watch the location of each — the compartment matters as much as the chemistry. (These are eukaryotic addresses — cells without mitochondria are handled at the end of this section.)

1. Glycolysis — in the cytoplasm. Glucose (a 6-carbon sugar) is split into two molecules of pyruvate (3 carbons each). This happens in the cytoplasm, outside the mitochondria, and needs no oxygen. It yields a small net amount of ATP (about 2) and some high-energy electrons carried by NADH.
2. Pyruvate oxidation — entering the mitochondrion. Each pyruvate moves into the mitochondrial matrix, loses a carbon as CO₂, and is attached to coenzyme A to form acetyl-CoA. More NADH is made. This is the bridge that connects glycolysis to the Krebs cycle.
3. Krebs (citric acid) cycle — in the mitochondrial matrix. Acetyl-CoA is fed into a cycle that releases the remaining carbons as CO₂ and loads up the electron carriers NADH and FADH₂. Only a little ATP is made directly here; the real payoff is those loaded electron carriers.
4. Electron transport chain — on the inner mitochondrial membrane. NADH and FADH₂ drop their electrons into a chain of proteins embedded in the inner membrane. The energy released pumps H⁺ across the membrane, building a gradient that drives ATP synthase. Oxygen is the final electron acceptor, forming water. This stage — oxidative phosphorylation — makes the great majority of the ATP.
5. The tally. Add it up and aerobic respiration yields roughly 30+ ATP per glucose — and the electron transport chain / oxidative phosphorylation is responsible for most of it. Glycolysis and the Krebs cycle contribute only a handful of ATP directly; their main job is delivering electrons to the chain.

Through-line: energy moves from glucose → electron carriers → the proton gradient → ATP. In a eukaryote each stage has a fixed address — cytoplasm, then matrix, then inner membrane — and the inner membrane is where the bulk of the ATP is finally made.

Cells without mitochondria. Those addresses are eukaryotic, not universal. Prokaryotes have no mitochondria at all: they run glycolysis and the Krebs cycle in the cytoplasm and hang their electron transport chain on the plasma membrane, pumping H⁺ across it instead of across an inner mitochondrial membrane. Same logic, different real estate. Closer to home, your mature red blood cells also lack mitochondria — they get their ATP from glycolysis alone. The compartment is a detail of the cell; the chemistry is the constant.

Quick reference card. For each term, read where it happens and what it contributes — location and ATP yield are the whole game.

Everything above assumes oxygen is available. That case — aerobic respiration — runs all three stages, sends electrons all the way down the transport chain to oxygen, and captures roughly 30+ ATP per glucose. The electron transport chain is the big earner precisely because O₂ at the end pulls electrons through and keeps the whole chain running.

Without oxygen, the chain stalls. If there is no O₂ to accept the electrons, the electron transport chain backs up: NADH has nowhere to unload, so it can't be recycled back to NAD⁺. Glycolysis needs NAD⁺ to keep going, so the whole process would grind to a halt — unless the cell has another way to regenerate it.

Fermentation is that workaround. Fermentation is an anaerobic pathway that regenerates NAD⁺ so glycolysis can continue. In your muscle cells pyruvate is converted to lactate; in yeast it becomes ethanol and CO₂. Crucially, fermentation itself makes no additional ATP — its only job is to recycle NAD⁺. All the ATP comes from glycolysis, so the total is just the ~2 ATP from that first stage.

So the gap is huge, not small. Aerobic respiration generally yields much more ATP per glucose than fermentation. Roughly fifteen times more (~30+ ATP versus ~2). Fermentation is not “almost as good” — it is a low-yield pathway that lets a cell keep making a trickle of ATP without ever sending electrons to oxygen.

Why it exists at all. A little ATP fast is better than none. Sprinting muscles and oxygen-starved tissues fall back on fermentation to survive short bursts, and plenty of microbes live on it as their primary pathway. And the yield rule is a tendency, not a law about what cells will do: some cells ferment even when oxygen is plentiful, trading ATP per glucose for speed. Yield is one pressure on a cell, not the only one.