Plasma Membrane

The plasma membrane serves as the boundary between the machinery in the interior of the cell and the extracellular fluid (ECF) that bathes all cells.

Plasma Membrane Structure

The plasma membrane consists of a combination of phospholipids and proteins. These proteins are not set in any rigid pattern. Instead they float around in the membrane. This is called the fluid mosaic model of the cell membrane. The key to understanding the function of the cell membrane lies in the understanding of these specialized parts.
Phospholipids have a hydrophilic end that is attracted to water and a hydrophobic end that repels water. When mixed with water, phospholipids line up in double layered spheres. These structures are stable and accommodate the needs of both the hydrophilic and the hydrophobic ends of the molecule. The hydrophilic ends are in contact with water, while the hydrophobic ends face each other and do not touch the water.

Membrane Proteins

Floating in the phospholipid bilayer are many types of proteins. PERIPHERAL PROTEINS lie on the surface of the membrane. Other proteins, called INTEGRAL PROTEINS, extend into and sometimes completely through the membrane. Generally, integral proteins fit into three categories: marker proteins, transport proteins, and receptor proteins.

Marker proteins are like nametags that identify the cell to other cells. Each organism has its own unique marker proteins on its membranes. One of the functions of marker proteins is to enable a person's immune system to distinguish its cells from those of invading cells.

TRANSPORT PROTEINS are responsible for shipping and receiving. They move materials in and out of the cell. Some of them function as pores that allow substances to diffuse through the membrane. Others act as pumps that use energy to pull molecules across the membrane, a process called as ACTIVE TRANSPORT. Receptor proteins extend through the cell membrane. As the communication office of the cell, they allow the cell to interact with other cells. The part of the receptor protein on the exterior of the cell surface binds to a molecule. This causes the portion of the protein on the inside of the cell to change shape, triggering a reaction within the cell. The specificity of receptor proteins allow the cell to respond to the outside environment in many different ways.

These three classes of proteins are the real workers of the plasma membrane. They allow the membrane to be a dynamic structure that allows materials to be transported and messages to be communicated to the cell.