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Respiration

Cellular respiration is the set of chemical pathways by which cells break down food molecules such as glucose to release energy in a usable form, ATP. With oxygen, it can be summarised as C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O and runs through glycolysis, the citric acid cycle and the electron transport chain; about 90 percent of the ATP comes from chemiosmosis in mitochondria. Without enough oxygen, cells fall back on fermentation, which yields far less ATP.

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Chemiosmotic coupling

The electron transport chain — four complexes in the inner mitochondrial membrane plus mobile carriers — transfers electrons from NADH and FADH₂ to O₂, the final acceptor, producing water. Free energy released along the chain pumps protons into the intermembrane space; protons return through ATP synthase, which phosphorylates ADP. This chemiosmosis produces about 90 percent of the ATP from aerobic glucose catabolism.

  • FADH₂ delivers electrons to complex II, bypassing complex I, so fewer protons are pumped and fewer ATP made per FADH₂.
  • The pathways extract about 34 percent of glucose's energy; the remainder is released as heat.
  • Mitchell proposed chemi-osmotic coupling in Nature in 1961 and received the 1978 Nobel Prize in Chemistry.
Common misconception: ATP is not made by the electron carriers handing over energy directly; the coupling runs through the proton electrochemical gradient across the membrane.
Full explanation — the complete reference version every reading depth is based on

What respiration is

In biology, respiration usually means cellular respiration: the chemistry inside cells that releases energy from food. It is not the same thing as breathing — breathing is how animals bring oxygen into the body and get rid of carbon dioxide, while cellular respiration is what uses that oxygen inside every cell. All living things, plants included, break down carbohydrates and other carbon-rich molecules for energy.

C6H12O6+6 O2→6 CO2+6 H2O+energy\begin{aligned} &\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \\ &\quad \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} + \text{energy} \end{aligned}

Summary of aerobic respiration — the reverse of the overall photosynthesis reaction.

The released energy is captured in ATP, the cell's 'energy currency', which powers jobs such as muscle contraction and pumping ions across membranes.

Three stages

  1. Glycolysis (cytoplasm, no oxygen needed): one glucose is split into two pyruvate, with a net gain of 2 ATP and 2 NADH.
  2. Citric acid (Krebs) cycle (mitochondrial matrix): pyruvate is converted to acetyl CoA and its carbons are released as carbon dioxide, loading electron carriers NADH and FADH₂.
  3. Electron transport chain and chemiosmosis (inner mitochondrial membrane): electrons pass along to oxygen, which forms water; the pumped protons flow back through ATP synthase, making about 90% of the ATP.

Worked example

A cell breaks down 10 glucose molecules by glycolysis alone, as mature red blood cells must because they have no mitochondria. Each glucose gives a net 2 ATP, so the cell gains 10 × 2 = 20 ATP and makes 20 pyruvate. With oxygen and mitochondria, most of the remaining energy is extracted later by the citric acid cycle and chemiosmosis. (Our arithmetic from the per-glucose figures.)

Info: Respiration is not perfectly efficient: one textbook gives about 34 percent of the energy in glucose captured, with the rest released as heat. The number of ATP made per glucose varies — proton pumping differs between species, electron shuttles differ between tissues, and some intermediates are used to build other molecules — so textbooks quote different totals.

Without oxygen: fermentation

If aerobic respiration cannot continue, glycolysis can keep going only if NAD⁺ is regenerated. Fermentation does this. Animals use lactic acid fermentation — in skeletal muscle short of oxygen and in red blood cells. Yeast uses alcohol fermentation, turning pyruvate into ethanol and releasing carbon dioxide.

Common misconception: A common misconception is that lactic acid is what makes muscles sore after exercise. That was once widely believed, but more recent research disputes it.
Common misconception: Another misconception is that plants photosynthesise instead of respiring. Plants make sugar by photosynthesis and also break sugar down by cellular respiration, like all living things.

Where this connects

Respiration happens in cells, mostly in mitochondria, and is a transformation of energy from chemical bonds into ATP and heat. It mirrors photosynthesis: the carbon dioxide respiration releases is the raw material photosynthesis uses.

Assumptions and limits

  • The summary equation is a net balance; the cell runs it as many enzyme-controlled steps across the cytoplasm and mitochondria.
  • 'About 90 percent of the ATP from chemiosmosis' and 'about 34 percent of the energy captured' are textbook figures; because the yield per glucose varies, no single total is quoted here.
  • Without oxygen, some microbes use anaerobic respiration with another final electron acceptor, such as sulfate, instead of fermentation.

How we know

Hans Krebs worked out the steps of the citric acid cycle in the 1930s using pigeon flight muscle. In 1961 Peter Mitchell proposed in Nature that ATP production is coupled to electron transfer by a chemi-osmotic mechanism; his chemiosmotic theory earned the 1978 Nobel Prize in Chemistry.

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Content status: published 1 October 2026.

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