Second, it is permeable, allowing nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

The membrane is made of a two molecule thick layer (bilayer) of phospholipids, an oily substance found in all cells. This layer is embedded with many diverse proteins and has carbohydrates attached to its outer surface. The lipids in the membrane can exist either in a gel-like, nearly solid, state or in a liquid-like state, which gives the lipid molecules more mobility. In living cells, the membrane seems to be in a transition between the two states, depending on physical conditions and what lipids and proteins are present in the membrane layer.

In prokaryotes and plants, the plasma membrane is the inner layer of protection. A rigid cell wall forms the outside boundary for the cell. While it has pores that allow materials to enter and leave the cell, they are not as selective about what passes through. The membrane, which lines the cell wall, provides the final filter between the cell interior and the environment.

Eukaryotic animal cells probably descended from prokaryotes that lost their cell walls. With only the flexible membrane left to enclose them, they were able to expand in size and complexity. Eukaryotic cells are generally ten times larger than prokaryotic cells and have membranes enclosing interior components, the organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials, allowing the cell to segregate its chemical functions into discrete internal compartments.

Although plants have evolved another version of the cell wall, in animal cells the plasma membrane is the only barrier between the cell and its environment.

Information provided by: http://micro.magnet.fsu.edu