The membrane encloses the other cell structures. Much of the chemical work of the cell is done in the cytoplasm, which surrounds the nucleus. The nucleus, enclosed by its own membrane, is the control center of the cell. The Cell Membrane Cells can survive only in a liquid medium that brings in food and carries away waste. This characteristic enables the cell to admit useful substances and to reject harmful substances from the surrounding fluid as well as to force out, or excrete, waste products into the fluid.
Substances probably pass through the cell membrane in several ways.
On the evidence of electron micrographs, biologists believe that it has pores through which certain small molecules pass intact. Small chambers located on the cell membrane called caveolae gather chemical signals and channel them into the cells. The caveolae may also help distribute proteins throughout the cell. Large molecules enter the cell by a process called diffusion, in which they may be dissolved by substances in the cell membrane. They can then pass through the membrane without difficulty.