When considering a cell’s use of energy, it is helpful to also consider the cell’s sources of carbon because carbon is the backbone of the organic molecules that make up cells and because cells often use carbon-based compounds as a stable form of energy storage. How organisms obtain the energy and carbon needed for growth and other vital functions is so fundamental that it is sometimes used to provide a metabolic classification of life (Fig. 6.1). Simply put, organisms have two ways of harvesting energy from their environment and two sources of carbon. Together, this means that there are four principal ways in which organisms acquire the energy and materials needed to grow, function, and reproduce.