Cellular respiration is the sum of the various biochemical means that eukaryotic organisms employ to extract energy from food, specifically glucose molecules.
The cellular respiration process includes four basic stages or steps: Glycolysis, which occurs in all organisms, prokaryotic and eukaryotic; the bridge reaction, which stets the stage for aerobic respiration; and the Krebs cycle and the electron transport chain, oxygen-dependent pathways that occur in sequence in the mitochondria.
The steps of cellular respiration do not happen at the same speed, and the same set of reactions may proceed at different rates in the same organism at different times. For example, the rate of glycolysis in muscle cells would be expected to greatly increase during intense anaerobic exercise, which incurs an "oxygen debt," but the steps of aerobic respiration do not speed up appreciably unless exercise is performed at an aerobic, "pay-as-you-go" intensity level.
Cellular Respiration Equation
The complete cellular respiration formula looks slightly different from source to source, depending on what the authors choose to include as meaningful reactants and products. For example, many sources omit the electron carriers NAD+/NADH and FAD2+/FADH2 from the biochemical balance sheet.
Overall, the six-carbon sugar molecule glucose is converted to carbon dioxide and water in the presence of oxygen to yield 36 to 38 molecules of ATP (adenosine triphosphate, the nature-wide "energy currency" of cells). This chemical equation is represented by the following equation:
C6H12O6 + 6 O2 → 6 CO2 + 12 H2O + 36 ATP
The first stage of cellular respiration is glycolysis, which is a set of ten reactions that do not require oxygen and hence occurs in every living cell. Prokaryotes (from the domains Bacteria and the Archaea, formerly called "archaebacteria") utilize glycolysis almost exclusively, whereas eukaryotes (animals, fungi, protists and plants) use it chiefly as a table-setter for the more energetically lucrative reactions of aerobic respiration.
Glycolysis takes place in the cytoplasm. In the "investment phase" of the process, two ATP are consumed as two phosphates are added to the glucose derivative before it is split into two three-carbon compounds. These are transformed into two molecules of pyruvate, 2 NADH and four ATP for a net gain of two ATP.
The Bridge Reaction
The second stage of cellular respiration, the transition or bridge reaction, gets less attention than the rest of cellular respiration. As the name implies, however, there would be no way to get from glycolysis to the aerobic reactions beyond without it.
In this reaction, which occurs in the mitochondria, the two pyruvate molecules from glycolysis are converted into two molecules of acetyl coenzyme A (acetyl CoA), with two molecules of CO2 produced as metabolic waste. No ATP is produced.
The Krebs Cycle
The Krebs cycle does not generate much energy (two ATP), but by combining the two-carbon molecule acetyl CoA with the four-carbon molecule oxaloacetate, and cycling the resulting product through a series of transitions that trim the molecule back to oxaloacetate, it generates eight NADH and two FADH2, another electron carrier (four NADH and one FADH2 per glucose molecule entering cellular respiration at glycolysis).
These molecules are needed for the electron transport chain, and in the course of their synthesis, four more CO2 molecules are shed from the cell as waste.
The Electron Transport Chain
The fourth and final stage of cellular respiration is where the major energy "creation" is done. The electrons carried by NADH and FADH2 are pulled from these molecules by enzymes in the mitochondrial membrane and used to drive a process called oxidative phosphorylation, wherein an electrochemical gradient driven by the released of the aforementioned electrons powers the addition of phosphate molecules to ADP to produce ATP.
Oxygen is required for this step, as it is the final electron acceptor in the chain. This creates H2O, so this step is where the water in the cellular respiration equation comes from.
In all, 32 to 34 molecules of ATP are generated in this step, depending on how the energy yield is summed. Thus cellular respiration yields a total of 36 to 38 ATP: 2 + 2 + (32 or 34).