Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
ATP Cycle and Cellular Bioenergetics
Cells require free energy

Heat is not a significant source of energy for Cells, because heat can only perform work when it passes from a warmer body to a colder one, or from a higher-Temperature zone to a lower-temperature zone. Moreover, as is well known, the efficiency of a heat engine depends on the temperature difference between the hot and cold bodies; the greater this difference, the larger the fraction of thermal energy that can be converted into work. Since the temperature at any point within living cells is practically uniform, they are unable to utilize thermal energy. Heat serves them solely to maintain an optimal operating temperature.

The form of energy usable by cells—that is, the form they can and must utilize—is Free energy, which is capable of performing work at constant temperature and constant pressure. Heterotrophic cells extract the energy they need from energy-rich nutrients, whereas for photosynthetic cells, the source is captured sunlight. Both types of cells convert the acquired free energy into a single common form: chemical energy, which is then used to perform work in processes that do not involve any appreciable temperature gradient. Simply put, cells are chemical engines operating under conditions of constant temperature and pressure.

We will now examine how the Free energy of Chemical Reactions is measured and in what form it is expressed.