Which feature characterizes the lipid bilayer of the plasma membrane?

The lipid bilayer of the plasma membrane is characterized by its fluidity and flexibility, which is essential for the proper functioning of cells. This structure is formed primarily by phospholipids arranged in two layers, with their hydrophobic tails oriented inward, away from water, and their hydrophilic heads facing outward toward the aqueous environment.

The fluid nature of the bilayer allows proteins and lipids to move laterally within the membrane, creating a dynamic environment that supports various cellular processes, such as signaling, transport, and cell interactions. The flexibility provided by this arrangement enables the membrane to change shape and adapt to different conditions, allowing for processes like endocytosis and exocytosis.

In contrast to the correct answer, the other choices do not accurately describe the nature of the lipid bilayer. The membrane is not rigid and inflexible, as that would impede essential functions. Moreover, it does not allow the free movement of all substances; rather, it selectively permits some molecules to pass while blocking others. Lastly, the membrane is not solely composed of proteins, as it primarily consists of lipids, with proteins embedded within or associated with the lipid bilayer, contributing to its functionality.