(i) In man, it occurs during the heavy exercise in the muscles. In yeast, the anaerobic respiration occurs in fermentation.
(ii) It is because of the following:
(a) the end products of anaerobic respiration can be further oxidised to release energy,
(b) the regeneration of NAD does not yield ATP, as the electrons are not transported to oxygen.
(iii) In the membranes of mitochondria.
(iv) Oxygen.