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Overall Energy Harvest

Given that glucose, if burned in a calorimeter, releases 686 kilocalories/mole, and ATP hydrolysis usually yields approximately 7 kilocalories/mole, we can calculate the efficiency of aerobic respiration. If we harvest 36 ATP molecules and multiply by 7 kilocalories/mole, we have obtained 252 kilocalories/mole of glucose burned. This represents an efficiency of 252/686, or approximately 37%. While far from perfect, it is certainly preferable to the efficiency of fermentation, which by the same logic is approximately 2%. Where does the rest of the energy go? The first law of thermodynamics tells us that energy can never be created or destroyed, so it must have been transformed into another type of energy. In fact, it was converted to heat, and is in effect “wasted” energy. Birds and mammals, however, have figured out another use for this energy so that it is not completely wasted: it is the major source of internal heat used to maintain a relatively high body temperatures (see Chapter 17).

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