For growth to occur in organisms, the rate of anabolism must exceed that of catabolism. Anabolism is the process of building complex molecules from simpler ones, while catabolism is the breakdown of complex molecules into simpler ones. In order for an organism to grow, it needs to produce more complex molecules (such as proteins and DNA) than it breaks down. This requires a net energy input, which is obtained from food. Therefore, food storage must be high enough to provide the necessary energy and building blocks for anabolism, but it is not the determining factor for growth.