What is cellular respiration? Definition, equation and the four stages

What is cellular respiration?

Cellular respiration is the controlled breakdown of organic molecules in which the released energy is captured as ATP. The control is the point: the same glucose that burns away in a second in a flame is dismantled in the cell across some thirty small steps, so the energy can be captured piece by piece rather than lost as heat.

ATP is the cell's energy currency because the bond between its second and third phosphate group can be broken on demand to release a useful quantity of energy. Cells store very little of it – a human turns over roughly their own body weight in ATP per day, recycling the same molecules over and over.

What is the equation for cellular respiration?

The overall equation is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy. One glucose molecule and six oxygen molecules yield six carbon dioxide molecules, six water molecules and energy captured as ATP.

Note that the equation mirrors photosynthesis but is not its reverse reaction. The steps, enzymes, organelles and electron carriers are all different – the resemblance exists only in the accounting.

What happens in glycolysis?

Glycolysis splits one glucose molecule into two molecules of pyruvate, with a net yield of 2 ATP and 2 NADH. It happens in the cytoplasm, requires no oxygen, and occurs in virtually all life, which is usually read as evidence that it is evolutionarily ancient.

The pathway first invests 2 ATP to make the sugar reactive and then recovers 4, giving the net of 2. That detail is worth remembering, because it is the most common source of error in exam questions on ATP yield.

What happens in the Krebs cycle?

The Krebs cycle oxidises carbon fully to carbon dioxide and loads the electron carriers NADH and FADH₂. Before the cycle, the link reaction converts pyruvate into acetyl-CoA, releasing one carbon dioxide. The acetyl group then joins oxaloacetate and travels the cycle, which per turn releases 2 CO₂ and produces 3 NADH, 1 FADH₂ and 1 GTP or ATP. Since each glucose yields two pyruvates, the cycle turns twice per glucose molecule.

The carbon dioxide you exhale comes from here and from the link reaction – not from the final stage, as many assume.

What does the electron transport chain do?

The electron transport chain uses electrons from NADH and FADH₂ to pump protons out of the matrix, creating a gradient across the inner mitochondrial membrane, and ATP synthase then lets the protons flow back, making ATP. The mechanism is called chemiosmosis and was described by Peter Mitchell (1961), who received the Nobel Prize in Chemistry for it in 1978.

Oxygen accepts the electrons at the end and forms water with protons. That is oxygen's entire role in the process. Without it the chain backs up with electrons, NAD⁺ is not regenerated, and the Krebs cycle stops too.

How much ATP does one glucose molecule yield?

Older textbooks state 36 to 38 ATP. That figure rests on a rounded estimate of how many protons ATP synthase needs; more recent measurements give 30 to 32. State which model your course uses if you calculate a yield in an exam.

What happens without oxygen?

Without oxygen the cell switches to fermentation, which regenerates NAD⁺ so glycolysis can continue but produces no extra ATP. In lactic acid fermentation pyruvate becomes lactate, which is what happens in muscle cells under hard effort. In alcoholic fermentation the products are ethanol and carbon dioxide, which is what yeast does and what makes bread and beer possible.

The yield is 2 ATP per glucose against roughly 30 for full respiration. That is why endurance work requires oxygen and why a sprint cannot be sustained.

How does respiration differ from photosynthesis?

Respiration releases energy from glucose and runs in mitochondria in virtually every eukaryotic cell, while photosynthesis stores energy in glucose and runs only in the chloroplasts of plants, algae and cyanobacteria. Respiration continues around the clock; photosynthesis requires light. Plant cells have both organelles and run both processes simultaneously in daylight.

How should I revise the stages?

Learn the pathway as a ledger of three things per stage: where it happens, what goes in, and what comes out. Then draw the mitochondrion from memory and place the four stages inside it. Write out the ATP table from memory three times a few days apart rather than three times in one evening – that is the spacing effect described in our article on spaced repetition, and Memmo's flashcards schedule it for you with FSRS from your own chapter. Then practise on past papers; our article on studying from past exams covers how to do that systematically.