Explain the three pathways of the breakdown of glucose in living organisms.

(N/A) The breakdown of glucose ($6$-carbon molecule) into pyruvate ($3$-carbon molecule) occurs in the cytoplasm of all living cells. Further breakdown depends on the availability of oxygen:
$1$. Presence of oxygen (Aerobic respiration): This occurs in the mitochondria. Pyruvate is converted into carbon dioxide, water, and a large amount of energy.
$2$. Lack of oxygen (Anaerobic respiration in muscle cells): During vigorous physical activity, when oxygen supply is insufficient in muscle cells, pyruvate is converted into lactic acid ($3$-carbon molecule) and a small amount of energy. The accumulation of lactic acid causes muscle cramps.
$3$. Absence of oxygen (Anaerobic respiration in yeast): In the process of fermentation, yeast converts pyruvate into ethanol ($2$-carbon molecule), carbon dioxide, and a small amount of energy.