Components of Plasma Membranes

By the end of this lesson and the next few, you should be able to:

• Understand the fluid mosaic model of cell membranes
• Describe the functions of phospholipids, proteins, and carbohydrates in membranes
• Discuss membrane fluidity

Structure and Function of Plasma Membranes

Despite its seeming hustle and bustle, Grand Central Station functions with a high level of organization. People and objects move from one location to another. They cross or remain within certain boundaries. In addition, they provide a constant flow as part of larger activity. Analogously, a plasma membrane’s functions involve movement within the cell and across boundaries in the process of intracellular and intercellular activities. Image Attribution: Modification of work by Randy Le’Moine / OpenStax Biology (CC BY 4.0)

The plasma membrane, which is also called the cell membrane, has many functions, but the most basic one is to define the borders of the cell and keep the cell functional. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, and occasionally, even energy investment for crossing.

Components and Structure of Plasma Membranes

A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment. Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane must be very flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries.

These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and which later plays a role in the “self” versus “non-self” distinction of the immune response.

Among the most sophisticated functions of the plasma membrane is the ability to transmit signals. This is done by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors. Furthermore, they activate intracellular response cascades when their effectors are bound.

Occasionally, receptors are hijacked by viruses (HIV, human immunodeficiency virus, is one example). In fact, these viruses use them to gain entry into cells. At times, the genes encoding receptors become mutated. As a result, the process of signal transduction to malfunction arises with disastrous consequences. In the next lesson, we will start looking at the fluid mosaic model for describing the plasma membrane.