Cellular respiration is a set of metabolic processes through which cells obtain usable energy by oxidizing chemical fuels and transferring electrons to a terminal electron acceptor. Much of the released energy supports the synthesis of adenosine triphosphate (ATP), which powers cellular work. Aerobic respiration uses oxygen as the terminal acceptor; anaerobic respiration uses other substances. Although commonly explained through the breakdown of glucose, respiration can draw on several fuels and is distinct from fermentation, which does not require a respiratory electron transport chain. (openstax.org)
Chemical basis and cellular organization
Respiration involves coordinated oxidation–reduction reactions. Fuel molecules lose electrons, while carriers such as NAD⁺ accept them, forming NADH. These carriers subsequently deliver electrons to a membrane-associated transport system. Rather than releasing all available energy in one reaction, cells capture it through successive, enzyme-catalyzed steps. Some energy is conserved in ATP, while some is dissipated as heat. (ncbi.nlm.nih.gov)
The overall oxidation of glucose in aerobic respiration is often represented by:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
This equation summarizes the conversion of glucose and oxygen into carbon dioxide and water; it does not show the intermediate reactions or specify a fixed ATP yield. (ncbi.nlm.nih.gov)
In eukaryotic cells, glycolysis occurs in the cytosol, whereas pyruvate oxidation and most citric acid cycle reactions occur in the matrix of the mitochondrion. Respiratory electron transport occurs in its inner membrane. Respiratory prokaryotes lack mitochondria and instead place their electron transport machinery in the cell membrane. (ncbi.nlm.nih.gov)
Glycolysis
Glycolysis converts one six-carbon glucose molecule into two three-carbon pyruvate molecules through ten reactions. An initial investment phase consumes two ATP molecules; the subsequent payoff phase produces four ATP and two NADH, giving a net yield of two ATP per glucose. Oxygen is not directly required by these reactions. (openstax.org)
ATP is formed here by substrate-level phosphorylation: an enzyme transfers a phosphate group directly from a metabolic intermediate to ADP. Glycolysis also requires a continuing supply of NAD⁺. If NADH cannot be reoxidized, the pathway slows or stops. Respiratory electron transport or fermentation can regenerate this oxidized carrier, depending on the organism and conditions. (openstax.org)
Pyruvate oxidation and the citric acid cycle
During aerobic glucose oxidation, pyruvate is converted into acetyl coenzyme A, or acetyl-CoA. Each pyruvate releases one carbon dioxide molecule and produces one NADH. Thus, this transition step produces two acetyl-CoA, two carbon dioxide, and two NADH per glucose. (openstax.org)
The citric acid cycle, also called the Krebs cycle or tricarboxylic acid cycle, begins when a two-carbon acetyl group combines with four-carbon oxaloacetate to form citrate. Subsequent reactions regenerate oxaloacetate, allowing another turn. Each turn yields two carbon dioxide, three NADH, one FADH₂, and one ATP or GTP, depending on the enzyme system. Two turns correspond to the two acetyl groups supplied by one glucose molecule. The carbon atoms entering in an acetyl group are not necessarily released during that same turn. (openstax.org)
The cycle also connects energy production with biosynthesis. Its intermediates can supply materials for making amino acids and other compounds, so it functions in both breakdown and synthesis rather than serving solely as a fuel-disposal pathway. (openstax.org)
Electron transport and oxidative phosphorylation
Most ATP from aerobic glucose oxidation is produced by oxidative phosphorylation. NADH and FADH₂ supply electrons to the electron transport chain, whose carriers include membrane-associated protein complexes. Electron transfer is coupled to the movement of hydrogen ions across the membrane, establishing an electrochemical gradient. In mitochondria, these ions accumulate in the intermembrane space relative to the matrix. (openstax.org)
Hydrogen ions return through ATP synthase, which couples their movement to ATP formation from ADP and inorganic phosphate. This coupling mechanism is called chemiosmosis. Oxygen accepts electrons at the end of the aerobic chain and, together with hydrogen ions, forms water. Without an available terminal acceptor, sustained electron transport cannot continue. (openstax.org)
Complete aerobic glucose oxidation can yield about 30 ATP in animal cells, but the yield is not universal. It depends on electron-shuttle systems, membrane coupling, transport requirements, and diversion of intermediates into other pathways. The ATP produced directly during glycolysis and the cycle accounts for only a small part of the total. (ncbi.nlm.nih.gov)
Anaerobic respiration and fermentation
Some bacteria and archaea carry out anaerobic respiration using terminal electron acceptors other than oxygen, including nitrate or sulfate. They retain respiratory electron transport and membrane-linked energy conservation, although their pathways and energy yields differ from those of aerobic organisms. (openstax.org)
Fermentation instead regenerates NAD⁺ by transferring electrons to organic compounds without a respiratory electron transport chain. In familiar pathways, pyruvate is reduced to lactate or converted into ethanol and carbon dioxide. When these pathways accompany glycolysis, the net ATP yield remains two per glucose; fermentation enables glycolysis to continue rather than providing the large additional yield of oxidative phosphorylation. (openstax.org)
Regulation
Respiratory activity is adjusted to cellular energy demand and biosynthetic requirements. ATP, ADP, AMP, NADH, and metabolic intermediates influence key enzymes. For example, high ATP availability inhibits important glycolytic reactions, while signals of lower energy availability can increase their activity. Regulation also controls whether intermediates continue through energy-producing pathways or are redirected toward synthesis of cellular materials. (openstax.org)