DIFFUSION IN UNICELLULAR ORGANISMS
All aerobic cells require a source of oxygen, which functions as a terminal electron acceptor in aerobic respiration. When organisms are small, simple diffusion works quickly enough to meet the metabolic demands for oxygen. Thus, many small organisms do not require a complex respiratory system. But because diffusion is a relatively slow process, it cannot meet the oxygen demands of larger organisms.

Even unicellular organisms are limited in size by diffusion. For example, imagine a spherical cell. As the cell increases in size, its volume increases with the cube of its radius (4/3³). However, the cell's surface area, which functions as its respiratory surface across which gas exchange must occur, increases only with the square of its radius (4²). This means that as the cell gets larger, the volume that requires oxygen increases much more rapidly than the surface area, which supplies the oxygen. Eventually, oxygen cannot diffuse across the surface fast enough to supply the cell's oxygen demands. This surface area-to-volume ratio problem explains why there are no giant unicellular organisms, and why large organisms are composed of many small cells.