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How is the concentration of molecules that can freely pass through cellular membranes controlled inside cells? Many hormones are small, nonpolar molecules that can freely pass through a plasm membrane. Hormones are used in cell signaling and are used to turn specific genes on or off in specific cells. However, if hormones can freely diffuse in and out of cells it would seem to be difficult to build up concentrations in specific parts of the body. If the concentration of a hormone is uniform throughout the body then it has little value as a cellular signal. The answer to this problem is that, once hormones enter target cells, they bind to receptor proteins to from a complex that activates gene expression. Once bound the hormone cannot diffuse back out of the cell and the concentration can increase.