Cell, Skin and Specialized Tissue - Transport Mechanisms

The cell membrane regulates the substances that flow in and out of the cell. Because cell membranes allow only certain substances to pass through, determined in part by the cell's role in the body, they are called selectively permeable. Cell membranes may contain several types of receptor protein, each responding to a specific molecule. Some membrane proteins bind to each other, forming connections between cells. 

Simple diffusion - diffusion describes…