Stages of energy metabolism
Energy metabolism proceeds in three stages. 1) Preparatory stage: large molecules are broken down to simple ones (starch to glucose, protein to amino acids, fat to glycerol and fatty acids); almost all the energy is lost as heat. 2) Glycolysis — oxygen-free breakdown in the cytoplasm: one glucose molecule is split into 2 molecules of a three-carbon compound (pyruvate), forming 2 ATP; when oxygen is scarce, an animal cell turns pyruvate into lactic acid, while yeast and some microorganisms carry out alcoholic fermentation, making ethyl alcohol and CO₂. In the textbook count glycolysis releases 200 kJ, of which 2 × 40 = 80 kJ is stored in ATP. 3) Aerobic oxidation takes place in the mitochondria: the products of glycolysis are broken down with oxygen to CO₂ and H₂O; in the textbook count this gives 36 ATP (altogether 2 + 36 = 38 ATP, 38 × 40 = 1520 kJ), whereas more precise modern estimates are about 30–32 ATP. So aerobic breakdown is far more efficient than anaerobic: without oxygen about 15–20 times more glucose is needed to obtain the same ATP.
“ATP accountant”. Groups calculate ATP, CO₂ and O₂ numbers for conditions on the cards (5 glucose, 3 of them fully broken down) and check on the board.