Cell membranes are like gates.

Perhaps you'd be assigned to the warehouse. Here, you would be responsible for shipping and receiving. A factory requires a constant supply of raw materials, as well as a way to send out the finished product. This department is usually located along an outside wall of the factory. Working here, you would be one of the factory's contacts with the outside world.

You might take a job as a receptionist and sit at a desk near the front door of the factory. A phone would allow you to contact anyone else in the building. Also, all incoming and outgoing calls would go through you. As a receptionist, you may speak for the factory and allow it to communicate with the outside messengers.

Maybe you wouldn't want to lift heavy crates in the warehouse or answer phones . Another possibility might be to take a job with the security department. Security personnel are posted at every entrance to the building. You would be responsible for checking ID cards and admitting only those individuals who belong in the factory.

Mail, reception, and security are separate departments in a real factory. But in a cell, these jobs are all performed by the plasma membrane.

Plasma Membrane Structure

Plasma membrane phospholipid bilayer

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

Integral and peripheral membrane 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.

Active transport requires the use of energy.

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.

So it is evident that proteins do much of the work in a cell. But who makes the decisions?