In a growing organism, metabolism supplies energy to both maintain existing tissues and create new tissues by cell division. Let Y be the metabolic rate of an organism, Yc the metabolic rate of a single cell, Nc (t) the total number of cells at time t, mc the mass of a cell, and Ec the energy required to create a new cell. The cell properties, Ec, mc, and Yc, are assumed to be constant and invariant with respect to the size of the organism. Thus: