solar energy as the driving force for this global process (Fig. 1). All living organisms also require a source of nitrogen, which is necessary for the synthesis of amino acids, nucleotides, and other compounds. Plants can generally use either ammonia or nitrate as their sole source of nitrogen, but vertebrates must obtain nitrogen in the form
yielding metabolism in aerobic organisms. All oxidative steps in the degradation of carbohydrates, fats, and amino acids converge at this final stage of cellular respiration, in which the energy of oxidation drives the synthesis of ATP. Photophosphorylation is the means by which photosynthetic organisms capture the energy of sunlight—the ultimate source of energy in the biosphere—and harness it to make ATP. Together
6.1 An Introduction to Enzymes 191 H20 in the presence of oxygen is a highly exergonic 6.2 How Enzymes Work 193 process, releasing free energy that we can use to think an I 6.3 Enzyme Kinetics as an Approach to Understanding main on the shelf for years without any obvious con-