The term "aerobic" really means "requiring air," or "requiring oxygen." Therefore, the purpose of oxygen in aerobic respiration is quite critical in this kind of system. Without oxygen, the respiration process fails altogether. Not all organisms on this planet rely on it, though. Others survive in living conditions that have little or no oxygen molecules at all. Their mode of breathing is then known as anaerobic respiration, or that which is "not requiring air" or "not requiring oxygen." Human beings, obviously rely greatly on aerobic respiration. But what exactly is the purpose of oxygen in it? For this we have to look into this process in a micro-cellular level.
For aerobic organisms like human beings, our cells need to convert biochemical energy from the nutrients that we consume. This process is actually quite complicated. But for the sake of simplification, let us just assume that: the nutrients are already digested, and floating in the blood stream. We will also assume that the cells have already absorbed what they need from the bloodstream through the simplest process of cell wall absorption (osmosis, for example.) However, our cells need to convert those nutrients into ATP or adenosine-5- triphosphate. ATP is a nucleotide that has many functions, but it is known primarily as the most basic molecular currency that is responsible for cell-to-cell energy transfer.
ATP is known as molecular currency for the simple reason that this is the nucleotide that makes everything move. Without this molecular currency, there will be no intracellular transfer of chemical energy. And without this cellular level chemical energy transfer, then cells cannot break down the nutrients in food. Cells cannot transfer information through nerve endings and synapses. Cells cannot process or eliminate waste products. In short, ATP is the most basic "battery" of all aerobic organisms. Without the mandatory chemical energy transfer, cells begin to die quickly, and the organism begins to fade as more and more cells are lost.
Having established that, it is now easy to define the purpose of oxygen in aerobic respiration. Oxygen is the essential material - that required electron that triggers the ATP to release its stored chemical energy, transferring molecular currency to other cells while releasing waste products into the blood stream. Without oxygen, the ATP remains stagnant within the cells, which in turn literally "freezes" all other cellular activity. Additionally, if oxygen is not introduced in blood stream, the waste materials within begins to "poison" the ATP cells, further hampering its functions, and ultimately leading to cell demise.
For human beings, part of the respiration process includes the micro-cellular release of essential amino acids, fatty acids and glucose in the blood stream. Without these 3 primary components, internal organs (particularly the brain, which is heavily dependent on glucose supply) begin to shut down and die. As such, it only takes 5 minutes of oxygen deprivation before the brain starts to deteriorate. From then on, permanent and irreversible damage happens, even with the re-introduction of oxygen into the system.
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