Advantages to being bigger include being able to prey on other organisms and also being able to resist or work against forces in the environment, such as wave forces in reefs. There is a problem, however, that limits how big single-celled organisms can get. All exchanges with the environment, such as uptake of nutrients and elimination of wastes, occur across the cell membrane, so the extent of these exchanges is limited by the surface area of the cell membrane. *Cell surface area increases with the square of the linear dimension of the cell, but the needs of the cell increase as a function of the volume of the cell, which is a function of the cube of the linear dimension. Thus there is a size limit beyond which single-celled organisms cannot satisfy their needs.

Cell specialization is essential for multicellular animals to grow bigger. If a multicellular animal consists of only a few layers of cells, those cells can carry out their own exchanges directly with the environment. A good example is a sponge (Figure 39.1). Bigger organisms have cells that are not in contact with the external environment. The needs of those cells must be supplied by an internal environment—extracellular fluid that bathes all the cells of the animal’s body. Different cells and groups of cells must be specialized to contribute to the maintenance of that internal environment. Thus each cell contributes to the internal environment, and each cell is supported by that internal environment.

Cell specializations have tremendous adaptive value for even the simplest multicellular animals. Some cells can be specialized for movement, some for intake of food, and some for reproduction. But evolving a specialized function usually comes at the expense of other cellular functions, which may be lost or at least compromised. The specialized cells of an organism thus engage in a kind of collaborative enterprise by providing services for all of the cells of the organism in return for different services provided by other cells. Thus the cells specialized for movement do not have to also capture and process food, and the reproductive cells do not have to move the whole animal to find a mate. For example, male sex cells are motile cells that are good at seeking out nonmotile female sex cells, but the male sex cells have lost the ability to do many other cell functions such as acquire and process food.