Why is oxygen essential for the continued activity of cells?

Answer:
There are 2 ways of producing energy. Oxidative - in the presence of oxygen.
Anoxidative - in the absence of oxygen.Oxidative production is more common and liberates more energy per molecule of glucose. energy can also be produced from protein and fat. Certain organs like brain rely solely on oxidative energy production.Anoxidative production of energy is less common and liberates less energy. It also liberates lactic acid. This commonly happens when u exercise. When you exercise, the demand of energy is high but oxygen is short supply. So the body temporarily switches to anoxidative cycle. Due to the accumulation of lactic acid you experience pain. Also the body needs lot of oxygen to convert this lactic acid into harmless chemicals. This is results in air hunger, to clear the oxygen debt.One more interesting information is that cyanide poisoning dissociates the combustion of glucose by oxygen and production of ATP (energy pockets). Hence energy is not produced and the person dies.

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short version- it is used to produce atp (energy molecules)long version- way too long and in depth, very detailed chemical reaction. google "kreb cycle" if you want the long version (someone else might be able to explain it better in laymans terms than i).

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