Biology  5/e   Raven/Johnson
	Student   Online Learning Center

As you learned in the last chapter, organisms need a continual input of energy if they are to stay alive. Work must be done if an organism or cell is to live, and energy must be expended for work to be done. All living organisms – from bacteria and protists to fungi, plants, and animals – obtain energy by carrying out cellular respiration. They break down energy-rich food molecules and then use the released energy to synthesize ATP, the universal energy currency. The food molecules are oxidized, and ADP is reduced to ATP. Glycolysis, the conversion of glucose to pyruvate, is the first stage of cellular respiration for almost all organisms and results in the net production of 2 ATP for each molecule of glucose. What happens next depends on what molecules are available to act as electron acceptors. If oxygen is present, aerobic respiration occurs and 32 ATP are generated by chemiosmosis for each initial molecule of glucose. These 32 ATP, plus the 2 from glycolysis and 2 more from substrate-level phosphorylation in aerobic respiration, result in a net theoretical yield of 36 ATP. If oxygen is not present, fermentation (anaerobic respiration) occurs and an organic molecule acts as the final electron acceptor instead of oxygen. Anaerobic metabolism yields only the 2 ATP from glycolysis for each initial glucose molecule.

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