The reason you eat is to ultimately create a molecule called ATP (adenosine triphosphate) so that your cells have the means to power themselves, and therefore you, along. And not incidentally, the reason you breathe is that oxygen is needed in order to get the maximal amount of cell energy from the precursors of the glucose molecules in that food.

The process human cells use to generate ATP is called cellular respiration. It results in the creation of 36 to 38 ATP per molecule of glucose. It consists of a series of stages, beginning in the cell cytoplasm and moving to the mitochondria, the "power plants" of eukaryotic cells. The two ATP-producing processes can be viewed as glycolysis (the anaerobic part) followed by aerobic respiration (the oxygen-requiring part).

Chemically, ATP is a nucleotide. Nucleotides are also the building blocks of DNA. All nucleotides consist of a five-carbon sugar portion, a nitrogenous base and one to three phosphate groups. The base can be either adenine (A), cytosine (C), guanine (G), thymine (T) or uracil (U). As you can discern from its name, the base in ATP is adenine, and it contains three phosphate groups.

When ATP is "built," its immediate precursor is ADP (adenosine diphosphate), which itself comes from AMP (adenosine monophosphate). The only difference between the two is the third phosphate group attached to the phosphate-phosphate "chain" in ADP. The enzyme responsible is called ATP synthase.

When ATP is "spent" by the cell, The ATP to ADP reaction name is hydrolysis, as water is used to break the bond between the two terminal phosphate groups. A simple equation for reforming ATP from its nucleotide relatives is ADP + P_i, or even AMP + 2 P_i. where P_i is inorganic (that is, not attached to a molecule containing carbon) phosphate.

Cellular respiration occurs only in eukaryotes, which are nature's many-celled, larger and more complex answer to the single-celled prokaryotes. Humans are among the former, while bacteria populate the latter. The process unfolds in four stages: glycolysis, which also occurs in prokaryotes and does not require oxygen; the bridge reaction; and the two reaction sets of aerobic respiration, the Krebs cycle and the electron transport chain.

To start glycolysis, a glucose molecule that has diffused into the cell across the plasma membrane has a phosphate attached to one of its carbon atoms. It is then rearranged into a fructose molecule, at which point a second phosphate group is attached to a different carbon atom. The resulting doubly phosphorylated six-carbon molecule is split into two three-carbon molecules. This phase costs two ATP.

The second part of glycolysis proceeds with the three-carbon molecules being rearranged in a series of steps into pyruvate, while in the meantime, two phosphates are added and then all four are removed and added to ADP to form ATP. This phase produces four ATP, making the net yield of glycolysis two ATP.

The bridge reaction in the mitochondria gets the pyruvate molecule ready for action by removing one of its carbons and two oxygens to yield acetate, which is then appended to coenzyme A to form acetyl CoA.

The two-carbon acetyl CoA is added to a four-carbon molecule, oxaloacetate, to get the reactions going. The resulting six-carbon molecule is eventually reduced to oxaloacetate (hence "cycle" in the title; a reactant is also a product). In the process, two ATP and 10 molecules known as electron carriers (eight NADH and two FADH₂) are produced.

In the final phase of cellular respiration, and the second aerobic phase, the various high-energy electron carriers are put to use. Their electrons are stripped off by enzymes embedded in the mitochondrial membrane, and their energy is used to power the addition of phosphate groups to ADP to form ATP, a process called oxidative phosphorylation. Oxygen is the final electron acceptor in the end.

The result is 32 to 34 ATP, meaning that, adding two ATP each from glycolysis and the Krebs cycle, cellular respiration produces 36 to 38 ATP per glucose molecule.