The cell membrane demarcates the cell from its environment. It is made up of liquid fat molecules and incorporated proteins; its shape is a double layer.

For a long time, it was not clear how cells exchange substances with their environment and how they pass on information. The cell membrane is not permeable even for the smallest particles. Today we know that there are pores in the cell membrane that act as channels. The pores are made up of proteins. Each channel only lets through certain select substances.