The outer boundary of the cell, also called the
plasma membrane,about 10 nanometers in width. The cell membrane controls the
movement of substances into and out of the cell in a process known in general as
transport.
Highly selective as to the substances that cross
its boundary, the cell membrane is said to be semi-permeable. Using the concept
demonstrated by the fluid mosaic model, biologists explain how some molecules
are able to penetrate the cell membrane while others cannot. According to this
model, the core of the membrane is made up of phospholipids. Large circular
proteins are set into the membrane; smaller proteins lie on the surface. The
proteins and the phospholipids have special functions and form certain
structural pathways that serve to admit or deny passage to specific molecules.
