Solution

Breakdown of glucose is done through various pathways to release energy. The very first stage includes the breakdown of glucose which is a 6-carbon molecule to convert it into 2 molecules of pyruvate which is a 3-carbon compound in the cytoplasm. This process of breakdown is called glycolysis. Once the pyruvate has formed, further breakdown can take place by various methods. (i) In the absence of oxygen: The incomplete oxidation of pyruvate will produce end products like ethanol which is 2 carbon molecules, carbon dioxide and release a small amount of energy. It occurs mainly in yeast cells. It is the anaerobic way of cellular respiration. Pyruvic acid decarboxylase and alcohol dehydrogenase enzymes will be used to catalyze this reaction (ii) In the lack of oxygen: The pyruvic acid is reduced to form lactic acid by an enzyme lactate dehydrogenase, also a small amount of energy will be released. It occurs mainly in muscle while excercising. (iii) In the presence of oxygen: This resakes place in the mitochondria of a cell. The end products of this reaction are carbon dioxide, water and more amounts of energy will be released. This is called aerobic cellular respiration.