Question: why do we need oxogen to breathe

David Walker-Sünderhauf answered on 6 Mar 2019:

We need oxygen, because our cells need it to produce energy. They use glucose and oxygen to produce ATP (the molecular energy “currency” cells use to do their different processes). This is one of the main reasons for our blood stream – when you breath fresh air into your lungs, the oxygen gets bound to haemoglobin in your blood stream near the lungs, and then rapidly transported through your whole body so that every cell can have some to produce ATP.

Some other organisms don’t need oxygen to produce energy, but these have alternative ways of producing ATP – fermenting bacteria break glucose down by producing alcohol instead (and are therefore used in wine & beer making), and plant cells can produce ATP using sunlight through photosynthesis.