On the basis of respiration, organisms can be classified as following: 1. Obligate aerobes: These organisms respire only under aerobic conditions, i.e., O2 is essential for their survival, e.g., most of the animal and plants. 2. Facultative aerobes: These organisms are basically anaerobes but under certain conditions, they can respire under aerobic conditions also. e.g., yeast. 3.Obligate anaerobes: These organisms respire only under anaerobic conditions. These organisms cannot survive in the presence of O2 only. e.g., Clostridium bacteria. 4.Fucultatlve anaerobes: These organisms are basically aerobes but under certain conditions, they can respire under anaerobic conditions also, e.g., lactic acid bacteria.