4.2: The Cell or Plasma Membrane
The cell membrane, also called the plasma membrane, protects the contents within the cell and monitors all of the components that enter and exit it. The cell or plasma membrane also plays a role in communication and attachment in tissues. Its structure is made of phospholipids and proteins. These biological molecules play an important role in the different actions of the cell’s membrane. In this section, we discuss the structure and function of the membrane and its impact on the life of the cell.
Watch this short video for a visual overview of the molecular structure and function of the cell or plasma membrane. Note that the membrane is composed mainly of phospholipids and proteins. It is selectively permeable, allowing certain organic molecules to pass into and out of the interior.
The chief components of the cell or plasma membrane are phospholipids. Each phospholipid molecule is an amphipathic molecule (polar at one end and non-polar at the other end). This explains why plasma membranes form.
In the presence of water, phospholipids self-assemble into a bilayer, with the non-polar tails in each monolayer pointing toward the non-polar tails of the other monolayer, and the polar heads of each monolayer pointing toward the watery solution on its side of the membrane (the water interior of the cell for one monolayer, and the water exterior of the cell for the other monolayer). In addition to the phospholipid bilayer, a plasma membrane features various other macromolecules, including proteins, sterols, and polysaccharides.
As you read this chapter on the cell membrane, pay attention to how certain molecules "transport" across the cell membrane. We will examine these mechanisms in more detail later in the unit.
After you read, you should be able to describe how phospholipids and most other membrane constituents are amphipathic molecules, list the major functions of membrane proteins, and define the three types of transport: selective permeability, active transport, and passive transport.