Respiration is a conversion of chemical energy of organic molecules into metabolically usable energy within the cells. Respiration is a series of exergonic (heat-generating) reactions. It is a kind of slow burning of food, in which the bonds of molecules are broken and the locked-in energy becomes available for metabolic work. In this sense, the energy from the carbon-carbon bond is removed bit by bit, bond by bond. Some of the energy released is packaged into a new chemical energy, and some escape as heat. You will recall that during photosynthesis, the chemicals were formed and the energy from sunlight was trapped. Respiration is a catabolic process, but photosynthesis is anabolic.
Respiration consists of three main correlated events. First, there is a removal of hydrogen from the food or fuel. This is called dehydrogenation. Removal of hydrogen from a compound is also called oxidation. Second, freed hydrogen is attached to appropriate acceptors. This step may be called hydrogen transfer. Oxygen is the ultimate acceptor of hydrogen to form water. Third, removal of hydrogen from the fuel causes its breakdown with attendant release of energy. Some of this energy escapes as heat but part of it is harvested to from ATP. This phase is called energy transfer.
Let us now see how energy is extracted from glucose. This molecule is broken down gradually in small steps in two distinct overall stages. The first stage is called glycolysis or anaerobic which takes place in the cytoplasm, outside mitochondria. The second stage is the aerobic respiration, which takes place inside mitochondria, enzyme-catalysed reactions popularly known as krebs cycle, named after a reputed biochemist Sir Hans krebs.
Partial oxidation of food in the absence of oxygen, resulting in the release of some amount of energy, is called anaerobic respiration. To put it simply, respiration in the absence of oxygen is called anaerobic respiration. We have already seen that respiration involves three major steps. In the first step, there is a removal of hydrogen from the organic compound, say, glucose. If you look at the molecular structure of glucose, you find that there are six carbon atoms joined to each other by covalent bonds. To these carbon atoms are also attached hydrogen and oxygen atoms. Removal of hydrogen atoms from the molecules is also termed oxidation of that molecule. In other words, glucose is oxidized when hydrogen is removed from it.
What happens to the removed hydrogen? This is received by another compound, called hydrogen acceptor. Once this acceptor accepts hydrogen, glucose becomes reduced. Most biological oxidations are actually dehydrogenations. But coupled with this oxidation is another processes, there is no direct participation of oxygen. The compound which accepts hydrogen does not keep this hydrogen for long. It donates this hydrogen to another acceptor molecule, which in turn will transfer to still another acceptor. In this way hydrogen atoms are transported along a chain of acceptors. Finally, this hydrogen combines with the oxygen that we have inhaled from the atmosphere to form water molecule.
Hydrogen atoms removed from glucose, are accepted by an acceptor molecule called nicotine-adenine dinucleotide (NAD). Glucose is finally broken down into two molecules of pyruvic acid. The breakdown of glucose in a series of reactions, leading to the formation of pyruvic acid is called glycolysis. In the presence of sufficient oxygen pyruvic acid undergoes further decomposition to liberate carbon dioxide and energy. When oxygen is not available, the transfer of removed hydromgens from one acceptor to another is blocked. Under the circumstances, pyruvic acid which had donated hydrogen to and acceptor, takes the paired hydrogen back and by doing so, it becomes lactic acid. This completes anaerobic respiration. Accumulation of excess lactic acid in muscles causes pain. In yeast cells, pyruvic acid is converted into ethanol in the absence of oxygen. Whether pyruvic acid is converted into lactic acid in animal cell or it is converted into ethanol in yeast cells, it happens in the absence of oxygen. Anaerobic respiration carried up to this point may also be called fermentation. In the case of yeast cells, it is referred to as alcoholic fermentation.
Complete oxidation of food yielding only carbon dioxide water and energy in the presence of oxygen is called aerobic respiration. Aerobic respiration means respiration in the presence of oxygen. Earlier we have seen the formation of two molecules of pyruvic acid from one molecule of glucose. Pyruvic acid has three carbon atoms. Pyruvic acid loses one carbon atom in the form of carbon dioxide and becomes Acetyl-coenzyme. A which entered into the mitochondria, interacts with water and oxaloacetic acid which has four carbon atoms. The result is the formation of a compound with six carbon atms.
This compound is called citric. The lattrer loses hydrogen and carbon dioxide to become ketoghutaric acid, which has 5 carbon atoms. Ketoglutaric acid loses carbon dioxide and becomes a four-carbon compound, succinic acid. The latter becomes rearranged into oxaloacetic acid which has four-carbon atoms. Oxaloacetic acid combines with acetyl coenzyme. A to become citric acid and the whole process starts again. The whole sequence described just above is known as the citric acid cycle or Krebs cycle. You might have noticed that in this cycle too, there was no direct involvement of molecular oxygen. Oxygen is needed only after the hydrogen atoms (or electrons) have been transferred through an additional series of oxidation-reduction reactions, to the end-point where hydrogens are accepted by molecular oxygen to form water molecule.