Obligate anaerobes: Primitive bacteria (eg Clostridium) for them O₂ is poisonous.

Facultative anaerobes: Yeasts (also muscle tissue) can withstand a lack of oxygen for short periods. Can also respire aerobically.

Anaerobic respiration is less efficient. The electron transport chain needs free oxygen to function. Only a few molecules of ATP are produced by reactions directly coupled to the oxidation of glucose in glycolysis.

Net ATP gain per molecule of glucose = 2ATP

But what is to be done with all the spare electrons and protons?

These are used to reduce:

Facultative anaerobes must control themselves:

Much more ATP is produced in the presence of O₂ (18 times more)

ATP and Citrate (formed in the Kreb's Cycle) inhibit the enzyme that phosphorylates glucose (phosphofructokinase) at the beginning of glycolysis. This is known as the Pasteur effect, a good example of allosteric inhibition.

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