The glucose molecule, as manufactured by plants during photosynthesis, is fundamental to life as we know it. Both plants and animals use the glucose molecule as a basic, high-energy food source. Plants, animals and microbes have evolved a complex set of chemical pathways that make use of oxygen in order to extract the maximum amount of concentrated energy packed into the glucose molecule.
One of these pathways is called glycolysis. Glycolysis alone, though, does not yield the full energy potential from the glucose molecule. Another set of chemical reactions, called aerobic respiration, carry the process to completion. To do this, cells use a sort of energy currency known chemically as adenosine triphosphate (ATP). The energy comes from the glucose molecule but cannot be used directly in that form. Therefore, the cell must manufacture this ATP.
The "problem" with glycolysis is that it generates very little ATP, leaving most of the energy available from the original glucose molecule still in an unusable form. Biochemically, the reason for this is that glycolysis does not make use of oxygen. So there is a secondary set of reactions known as the electron transport system. The electron transport system is the centerpiece of aerobic respiration. This system is active in structures within the cell known as mitochondria. Mitochondria are, in a sense, the little power plants of the cell.
Even the simplest explanation of these metabolic pathways can be very complicated, involving many different chemical reactions. But oxygen is the chemical key to this whole process. It functions by accepting the electrons as a whole series of biochemical reactions that take place, beginning with glucose. When oxygen accepts these electrons, it is then chemically situated to combine with hydrogen present within the cell. When this combination happens, the result is simply water as a waste product.
Again, the ATP molecule is the energy currency of the cell. In a sense, the glucose molecule is the energy bank. Think of oxygen as providing the account number and the means to extract the currency from the bank. Without it, not all, but most of the energy within the glucose molecule would be inaccessible. Although the chemical pathways leading to it are complex, in the final analysis, oxygen supplies this critical function simply by taking on electrons and combining with hydrogen to produce water.
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