Cellular Respiration
Cellular respiration is the process in which cells break down glucose with oxygen to release energy as ATP, producing carbon dioxide and water.
Formula
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}
Definition
Cellular respiration is how cells get energy from food: they combine sugar with the oxygen you breathe in, release the energy stored in the sugar, and give off carbon dioxide and water as waste. The overall equation is $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$, with the energy captured in ATP; the process begins in the cytoplasm with glycolysis and finishes in the mitochondria, and when oxygen is unavailable cells fall back on fermentation, which yields far less. Aerobic respiration has three stages: glycolysis in the cytosol (glucose to two pyruvate, net $2$ ATP and $2$ NADH), the citric acid cycle in the mitochondrial matrix (pyruvate oxidized to $CO_2$, producing NADH, FADH$_2$, and GTP), and oxidative phosphorylation on the inner mitochondrial membrane, where electrons from NADH and FADH$_2$ pass down the electron transport chain to oxygen, pumping protons that drive ATP synthase. The free energy change for glucose oxidation is about $-2{,}870\ \text{kJ/mol}$, and about $34$ percent of it is captured as ATP under cellular conditions.
Example
When you run, your muscle cells burn sugar faster and need more oxygen, so you breathe harder, and the carbon dioxide you puff out was made by cellular respiration in those muscles. One glucose fully broken down yields about $30$ to $32$ ATP, while fermenting the same glucose yields only $2$ ATP, which is why muscles tire quickly and produce lactic acid when they cannot get oxygen fast enough during a sprint. Each NADH gives roughly $2.5$ ATP and each FADH$_2$ about $1.5$ ATP through oxidative phosphorylation, and adding the $2$ ATP from glycolysis and $2$ GTP from the citric acid cycle brings the total near $30$ to $32$ ATP per glucose, depending on the shuttle used to move cytosolic NADH into the mitochondrion.
Key Insight
Every living thing does cellular respiration, including plants, which make sugar by day but must break it down for energy day and night. It is the reverse of photosynthesis, sugar and oxygen in, carbon dioxide and water out, and together the two processes cycle carbon and oxygen through the whole biosphere. Oxygen is needed only at the very last step, as the final electron acceptor; without it the electron transport chain backs up, NADH cannot be recycled, and the cell falls back on fermentation just to keep glycolysis running.