That's explained in this video coming up. VOICEOVER: Anaerobic respiration in micro-organisms. All living cells respire. It's the process that releases the energy stored in glucose for life ...

Metabolism close metabolismAll the chemical reactions in the cells of an organism, including respiration. is the term used for all the chemical reactions that happen inside an organism's body.

"It may be drawing it from the surrounding fish cells, or it may have a different type of respiration such as oxygen-free breathing, which typically characterizes anaerobic non-animal organisms." ...

Then, via respiration processes ... fuels we use to power our world today are the ancient remains of once-living organisms, and they provide a dramatic example of this cycle at work.