Yeast are simple unicellular organisms belonging to the Kingdom Fungi. They
are relatives of molds, mildews and mushrooms. Like all organisms, yeast need energy to
do the work of living (synthesis, transport, reproduction, etc.) They obtain this energy
through a process called cellular respiration, which involves the oxidation of organic
molecules. Some of the energy produced by this oxidation is stored in the chemical
bonds of adenosine triphosphate (ATP) - the energy currency of the cell. There are two
types of cellular respiration - aerobic and anaerobic.