All living organisms carry out some form of cell respiration (aerobic or anaerobic) in order to produce ATP

• Organisms can be classed as facultative or obligate according to their tolerance for other types of respiration
  

Obligate anaerobes

• Obligate anaerobes are single celled organisms that only carry out anaerobic respiration – they cannot survive in the presence of oxygen

• The first bacteria were likely obligate anaerobes as they were able to survive in Earth's reducing atmosphere prior to photosynthesis

• Modern obligate anaerobes can only be found in oxygen-free environments – such as the deep sea, lower layers of soil or within the bodies of certain organisms

Obligate aerobes

• Obligate aerobes are organisms that cannot survive in the absence of oxygen – they rely on aerobic respiration to produce sufficient quantities of ATP

• These organisms may be able to carry out anaerobic respiration in conjunction with aerobic respiration, but cannot survive on anaerobic respiration alone

• Most animals, plants and fungi are obligate aerobes, along with certain types of bacteria

Facultative anaerobes

• Facultative anaerobes will normally respire aerobically, but can fully switch to anaerobic respiration in the absence of oxygen

• These organisms can tolerate oxygen but also experience no long-term negative effects from its absence

• Examples of facultative anaerobes include yeast (S. cerevisiae) and certain types of bacteria (E. coli)

Respiration Patterns