What is cellular respiration?
Cellular respiration is the set of chemical reactions that transfers energy from glucose and other food molecules to ATP. It happens continuously in almost every living cell, in plants as well as animals. It is not the same as breathing: breathing moves air in and out of the lungs to supply oxygen and remove carbon dioxide, while respiration is the chemistry inside cells that uses that oxygen.
ATP: the cell’s energy currency
Glucose holds a lot of energy, but cells cannot use it directly. Instead, the energy is passed in small amounts toadenosine triphosphate (ATP). When ATP is broken down to ADP and phosphate, it releases energy exactly where it is needed: to contract muscles, pump ions across membranes, build proteins and DNA, and send nerve impulses. A cell recycles its ATP thousands of times a day, so respiration must never stop.
Where it happens
The first stage, glycolysis, takes place in the cytoplasm. The later stages take place in the mitochondria. A mitochondrion has a smooth outer membrane and a highly folded inner membrane; the folds, called cristae, provide a large surface for the proteins of the electron transport chain. The fluid inside is the matrix. Active cells such as muscle and liver cells contain hundreds or thousands of mitochondria.
The four stages
- Glycolysis (cytoplasm): glucose (6 carbons) is split into two pyruvate (3 carbons each). It uses 2 ATP and makes 4, a net gain of 2 ATP, plus 2 NADH.
- Link reaction (matrix): each pyruvate loses a carbon as CO₂, and the remaining acetyl group joins coenzyme A to form acetyl-CoA. NADH is made.
- Krebs cycle (matrix): acetyl-CoA joins a 4-carbon molecule to make citrate. As the cycle turns, CO₂ is released and hydrogen is passed to the carriers NAD⁺ and FAD. It turns twice per glucose, making 2 ATP in total.
- Electron transport chain (inner membrane): NADH and FADH₂ deliver electrons to a chain of proteins. Their energy pumps H⁺ ions into the intermembrane space. The ions flow back through ATP synthase, which turns like a tiny motor and makes most of the cell’s ATP.
What oxygen does
Oxygen is needed only at the very end. It is the final electron acceptor: it combines with electrons and H⁺ ions to form water. Without oxygen the chain backs up, NADH cannot be turned back into NAD⁺, and the link reaction and Krebs cycle stop as well. This is why oxygen is so important even though it is used in just one step.
Anaerobic respiration
When oxygen is short, cells can still run glycolysis, but they need a way to regenerate NAD⁺. Inlactic acid fermentation, used by animal muscle cells during intense exercise, pyruvate is turned into lactate. In alcoholic fermentation, used by yeast, it becomes ethanol and carbon dioxide. Both give only 2 ATP per glucose, so cells must use glucose much faster. After exercise, extra oxygen is used to deal with the lactate, which is one reason you keep breathing hard after you stop.
Equations and ATP yield
- Aerobic: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O, about 30–32 ATP per glucose.
- Lactic acid fermentation: C₆H₁₂O₆ → 2 C₃H₆O₃ (lactic acid; lactate in cells), 2 ATP.
- Alcoholic fermentation: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂, 2 ATP.
Older textbooks give 36–38 ATP per glucose. Modern measurements give lower values because the H⁺ gradient leaks slightly and moving molecules into the mitochondria costs energy. The yield is an average, not a fixed whole number.
Key terms
- ATP
- Adenosine triphosphate, the molecule cells use to transfer energy.
- Pyruvate
- The 3-carbon product of glycolysis.
- NAD⁺ / NADH
- A carrier that picks up hydrogen and electrons (NADH) and delivers them to the electron transport chain.
- Mitochondrion
- The organelle where aerobic respiration is completed.
- Fermentation
- Anaerobic pathways that regenerate NAD⁺ so glycolysis can continue.
Real-world applications
Athletes train to deliver oxygen to muscles faster and to tolerate lactate. Bakers rely on yeast fermentation to make bread rise, and brewers to make alcohol. Some poisons, such as cyanide, block the electron transport chain, which is why they are so dangerous. Many inherited conditions affect mitochondria, especially in tissues with high energy needs like muscle and brain.
Common misconceptions
- “Respiration means breathing.” Breathing supplies oxygen; respiration is the chemistry in cells.
- “Plants photosynthesise instead of respiring.” Plants respire all the time, day and night.
- “Respiration makes energy.” Energy cannot be created; respiration transfers it from glucose to ATP.
- “Lactate causes the muscle soreness felt days later.” Lactate is cleared within hours; later soreness has other causes.
Key takeaways
- Respiration transfers energy from glucose to ATP in every living cell.
- Glycolysis happens in the cytoplasm; the link reaction, Krebs cycle and electron transport chain happen in mitochondria.
- Oxygen is the final electron acceptor; without it only glycolysis can continue.
- Aerobic respiration gives about 30–32 ATP per glucose; fermentation gives only 2.
Model note: the simulator is a simplified teaching model. Real respiration involves around 20 enzyme-catalysed reactions, several shuttles and transport proteins, and tight regulation. Rates, oxygen and glucose levels, and lactate are shown in relative units, not measured values, and ATP per glucose uses rounded textbook figures.