In the natural world, substances change their chemical structures apparently spontaneously. When a metal oxidized or underwent oxidation it was supposed this was related to oxygen or dioxide as it is sometimes termed because of its O2 structure. Like most things in science the word for the original concept is retained, even when the concept is found to be an insufficient explanation of the mechanism. It is now known that the process called "oxidation" does not necessarily involve oxygen, although in most biological systems it does.
When it was realized that the process is not merely an addition of oxygen to the substance in question, the chemists defined the alteration in electrochemical terms: oxidation is the loss of electrons, whereby an oxidation state is created. Since there are two sides to an equation in which electrons are moved, the other side of the equation is the reduction state in which there is a gain in electrons (the word reduction, as in people, was originally employed to record a loss of weight in the process). Together, the reduction-oxidation equation is contracted for convenience to redox. In really sophisticated analysis, it is found that sometimes the transfer of electrons doesn't happen, and there is a metathesis, but for biological purposes one should know it's a possibility and then put it out of mind.
There has to be an intermediary agent to cause this process to happen; such agents are (of course) given various names, but those easiest to use are oxidants and reductants; in biological systems, oxygen is the prominent oxidant and hydrogen the prominent reductant.
We usually think of respiration as an action taking place in the lungs when oxygen is taken in (inhaled) and carbon dioxide (CO2) is given out (exhaled), or methane (CH4) if you happen to be a cow, but the biochemist thinks of the action at the opposite end of the chemical chain and talks of cellular respiration. In this redox process, glucose, the essential energy foodstuff of the body, is oxidized to CO2 and oxygen is reduced to water. The chemical equation is easily written in words as: One glucose molecule plus 6 oxygen molecules is converted to 6 molecules each of water and carbon dioxide.
Or in chemical terms: C6H12O6 + 6 O2 → 6 CO2 + 6 H20
The purpose of the reaction of course is not to produce water, it's to use the glucose to liberate energy stored in chemical bonds.
The body, a factory of physical chemistry activity, is the site of an enormous number of simultaneous, non-stop processes; the single cell bacteria E coli has more than a thousand at a time, so how many have we? In our society electricity is generated in power stations by transfer of energy from running water, wind, coal, nuclear structure, etc. The equivalent in the human body to this power station are the cytoplasmic mitochondria. In structure, the minute power-house mitochondria are 2μm in length and 0.5μm in diameter. They have an outer membrane and an inner membrane which is highly folded into internal ridges named cristae. The double system of membranes creates two compartments: intermembrane and matrix. The inner membrane surrounds the matrix. Oxidative phosphorylation occurs in the inner membrane; the matrix is where fatty acid oxidation and the citric acid cycle occurs.
Think of an old fashioned market where people come from the countryside to sell their products. One sells vegetables, another bread, a third the potatoes he grew, a fourth the wood he cut up. Each exchanges his goods for the common currency which can be transferred to whatever other purpose they choose.
In biochemistry this common currency is ATP – adenosine triphosphate. It is the intermediary, the medium, through which energy is transferred from the breakdown process of metabolism of fat, protein and carbohydrate to the energy expending activities the body undertakes. ATP is composed of adenine, ribose and three phosphate radicals, two of which have high energy bonds, readily broken down to transfer their energy to other mitochondrial activities. The oxidation of all three foodstuff types, carbohydrate, fat and protein, occurs in the mitochondria, those minute factories, all are converted to the intermediary substance, acetyl-CoA. In the ensuing process carbon dioxide is formed, hydrogen atoms are liberated and combine with oxygen to liberate the store of energy (think hydrogen bomb!).
As iron ore is used to make steel which is used to make a car, energy in the form of man's muscles, heat and electricity is expended at every step. As proteins are formed from amino-acids, energy is expended, and that energy comes from the body's energy currency, its ATP, giving up thousands of high energy ATP phosphate bonds in the course of making one large protein molecule.
Currency must remain in circulation, must be available to be used or trade comes to a halt. ATP must be available, and must therefore be built back up as quickly as possible.
Glucose surplus to the body's requirements undergoes the process of glycogenesis and is stored in liver and muscle as glycogen. When the body needs energy for the muscles' action, beyond the immediately available glucose, the glycogen undergoes glycogenolysis back to glucose, a process directed by the enzyme phosphorylase and known as phosphorylation. The glucose molecule is then broken down through a series of ten changes, each with its own enzyme system, to two molecules of pyruvic acid, which are then converted, molecule for molecule, to coenzyme A. Along the way some ATP is re-formed.
The continuing process of metabolism in the mitochondria at this point is variously termed the citric acid, Krebs ,or tricarboxylic acid cycle, all of which are the same and describe the process in which acetyl co-enzyme A acts on oxaloacetic acid, continuing through numerous stages, of which citric acid is only one, with an end result that each molecule of glucose is reduced with added water to 4 molecules of carbon dioxide, 16 atoms of hydrogen, and 2 atoms of ATP; the oxaloacetic acid is reformed in the process, hence the expression cycle.
To a biochemist the citric acid cycle is doubtless very interesting, but to someone looking to see where the energy is coming from, its production of ATP is described as "pitifully small," so obviously that plentiful supply of ATP is coming from elsewhere, and that elsewhere is in the mitochondria, by oxidation, making use of the liberated atoms of hydrogen, in the process termed oxidative phosphorylation. The real liberation of energy depends on the hydrogen ion which is transported through a series of proteins in the mitochondrial wall, eventually combining with oxygen to form water but in the process giving up its energy for the formation of ATP. The net result is for each molecule of glucose 38 molecules of ATP are formed.