What is a key characteristic of the plasma membrane?

The plasma membrane is characterized by its selective permeability, which means it allows certain substances to pass while restricting others. This property is crucial for maintaining homeostasis within the cell, as it regulates the entry and exit of ions, nutrients, and waste products. This selective control is primarily due to the lipid bilayer structure of the membrane, composed of phospholipids with hydrophilic heads and hydrophobic tails, along with embedded proteins that facilitate the transport of specific molecules.

Other options, such as being completely impermeable, do not accurately depict the nature of the plasma membrane, as it needs to be permeable to essential substances. Additionally, the statement about being made of carbohydrates and nucleic acids is incorrect; the plasma membrane's primary components are lipids and proteins. Lastly, while proteins are significant in the structure and function of the membrane, relying solely on proteins neglects the crucial role of the lipid bilayer and does not encompass the full complexity of the membrane's composition. Therefore, selective permeability is the key characteristic that accurately describes the plasma membrane's function and structure.