Respiration can be divided into two stages based on the presence or absence of oxygen.
1. Aerobic Respiration: This respiration requires free oxygen and produces more energy. Respiration occurs in numerous bacteria, most fungi, algae, bryophyta, pteridophyta, plants and animals.
2. Anaerobic respiration: This respiration does not require free oxygen and produces less energy. Abiotic respiration occurs in some bacteria and single-celled fungi.