As you already know your body is made up of billion of cell-the basic units of structure and function of all living things. Each cell has a cell membrane, that surrounds the cell thus each cell has its own environment. The cell membrane is acting like a guard that identifies what molecules leave or enter the cell. in this way a stable internal environment is maintained (maintaining internal cellular homeostasis).
Check this link for the detailed structure of the cell membrane.http://upload.wikimedia.org/wikipedia/commons/thumb/d/da/Cell_membrane_detailed_diagram_en.svg/75px-Cell_membrane_detailed_diagram_en.svg.png
The cell membrane is picky,decides what molecules pass in and out of the cells. Diffusion and active transport are the two processes that control the mechanism of transporting molecules through the cell. Cells communicate through receptor molecule on the cell membrane. These receptors recognize and respond to messages received from other cells. Examples of these signals received are hormones, neurotransmitters.
