organisms which do not require oxygen, infact most of them would find oxygen poisoness Like or for example yeast cells....etc
Mitochondrion. It happens in the matrix, i believe.
Since muscles does not receive any oxygen, it relies on the food (food = energy) that you have eaten.
There are many organisms, including us, that are able to utilize both aerobic and anaerobic respiration.
Because they're is no oxygen available.