12.1 : What are Membranes?

A cell's plasma membrane demarcates the cell's borders and determines the nature of its interaction with the environment. Cells exclude certain substances, take in others, and excrete some others in controlled quantities. The plasma membrane must be flexible to allow certain cells, such as red and white blood cells, to change their shape while passing through narrow capillaries. These are the more obvious plasma membrane functions. In addition, the plasma membrane's surface carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and later plays a role in the immune response's "self" versus "non-self" distinction.

The plasma membrane also carries integral proteins, the receptors, the attachment sites for specific substances that interact with the cell through transmitting signals. These proteins act both as extracellular input receivers and as intracellular processing activators. For example, surface receptors of the membrane create modifications in the interior, such as changes in enzymes of metabolic pathways. These metabolic pathways might be vital for providing energy to the cell, producing specific substances for the cell, or breaking down cellular waste or toxins for disposal. These membrane receptors also provide extracellular attachment sites for effectors like hormones and growth factors, activating intracellular response cascades when their effectors are bound. Occasionally, some recognition sites are used by viruses as attachment points. Although they are highly specific, pathogens such as viruses may evolve through mutations to exploit receptors to gain entry into a cell by mimicking the specific substance that the receptor is meant to bind. This specificity helps to explain why human immunodeficiency virus (HIV) or any hepatitis viruses invade only specific cells.

This text is adapted from Openstax 2e, Section 5.1 and Openstax, Cell structure and function, section 3.4: The cell membrane structure.