Cellular respiration
Cellular respiration is the set of metabolic pathways cells use to transfer energy from food molecules into ATP, the small molecule that powers much of cell work.
What cellular respiration is
Cellular respiration is a group of chemical pathways that transfer energy from food molecules into ATP. ATP acts like a short-term energy currency: cells spend it to move molecules, build structures, power movement, maintain ion gradients, and run many other forms of work.
The basic energy problem
Glucose and other fuels contain stored chemical energy, but cells cannot use that energy safely by burning it all at once. Respiration releases energy through controlled enzyme-catalyzed steps. Electron carriers such as NADH and FADH2 help move high-energy electrons from fuel breakdown toward ATP-making machinery.
Glycolysis
Glycolysis happens in the cytoplasm and begins the breakdown of glucose. One six-carbon glucose molecule is split into two three-carbon pyruvate molecules. The pathway produces a small net gain of ATP and reduces NAD+ to NADH. Glycolysis does not require oxygen directly, so it can operate in both aerobic and anaerobic settings.
Pyruvate and the citric acid cycle
When oxygen-dependent respiration is available in eukaryotic cells, pyruvate moves into mitochondria and is converted into acetyl-CoA. The citric acid cycle then strips more electrons from carbon compounds, releases carbon dioxide, and produces NADH, FADH2, and a small amount of ATP or a similar energy carrier.
Oxidative phosphorylation
Oxidative phosphorylation produces much of the ATP associated with aerobic respiration. Electrons move through the electron transport chain, and that movement helps pump protons across the inner mitochondrial membrane. As protons flow back through ATP synthase, the enzyme uses the gradient to make ATP from ADP and phosphate.
Oxygen and fermentation
Oxygen is not used in glycolysis, but it is central to the aerobic electron transport chain. Without oxygen or another suitable electron acceptor, cells must regenerate NAD+ another way. Fermentation keeps glycolysis running by recycling NADH to NAD+, but it yields far less ATP from each glucose molecule than aerobic respiration.
Regulation and fuel flexibility
Respiration is regulated because cells need to match ATP production with demand. Enzymes respond to signals such as ATP, ADP, NADH, citrate, and other pathway molecules. Although glucose is the familiar example, cells can also feed parts of the respiration system with products from fats, amino acids, and other organic molecules.
Why it matters
Cellular respiration connects food, oxygen, carbon dioxide, heat, and biological work. It explains why organisms need fuel, why many cells need oxygen, why mitochondria are central in eukaryotic energy metabolism, and why disruptions in energy flow can affect everything from muscle contraction to growth and survival.