ATP is continuously being hydrolyzed to provide energy for many cellular activities. Some estimates suggest that humans daily hydrolyze a mass of ATP equal to their entire body weight. Clearly, to continue functioning, cells must constantly replenish their ATP supply. Constantly replenishing ATP requires that cells obtain energy from their environment. For nearly all cells, the ultimate source of energy used to make ATP is sunlight. Some organisms can use sunlight directly. Through the process of photosynthesis, plants, algae, and certain photosynthetic bacteria trap the energy of sunlight and use it to synthesize ATP from ADP and P_i. Much of the ATP produced in photosynthesis is hydrolyzed to provide energy for the conversion of carbon dioxide to six-carbon sugars, a process called carbon fixation:

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6 CO₂ + 6 H₂0 → C₆H₁₂O₆ + 6 O₂ + energy

The sugars made during photosynthesis are a source of food, and thus energy, for the photosynthetic organisms making them and for the non-photosynthetic organisms, such as animals, that consume the plants either directly or indirectly by eating other animals that have eaten the plants. In this way, sunlight is the direct or indirect source of energy for most organisms (see Chapter 12).

In plants, animals, and nearly all other organisms, the free energy in sugars and other molecules derived from food is released in the processes of glycolysis and cellular respiration. During cellular respiration, energy-rich molecules in food (e.g., glucose) are oxidized to carbon dioxide and water. The complete oxidation of glucose,

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂0

has a ΔG°′ of –686 kcal/mol and is the reverse of photosynthetic carbon fixation. Cells employ an elaborate set of protein-mediated reactions to couple the oxidation of 1 molecule of glucose to the synthesis of as many as 30 molecules of ATP from 30 molecules of ADP. This oxygen-dependent (aerobic) degradation (catabolism) of glucose is the major pathway for generating ATP in all animal cells, all non-photosynthetic plant cells, and many bacterial cells. Catabolism of fatty acids can also be an important source of ATP. We discuss the mechanisms of photosynthesis and cellular respiration in Chapter 12.

Although light energy captured in photosynthesis is the primary source of chemical energy for cells, it is not the only source. Certain microorganisms that live in or around deep-sea hydrothermal vents, where adequate sunlight is unavailable, derive the energy for converting ADP and P_i into ATP from the oxidation of reduced inorganic compounds. These reduced compounds originate deep in the earth and are released at the vents.