What is a key characteristic of the plasma membrane?

A key characteristic of the plasma membrane is that it is a phospholipid bilayer. This structure is essential in providing a selective barrier that allows certain substances to pass while preventing others from crossing. The bilayer formation occurs because phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. As a result, in an aqueous environment, they arrange themselves into two layers with the tails facing inward, away from water, while the heads face outward towards the water, creating a stable boundary.

This bilayer structure is important for cell function as it maintains the integrity of the cell, facilitates communication, and plays a critical role in the transport of ions and molecules into and out of the cell. Additionally, embedded within this bilayer are proteins that perform various functions, such as signaling and transport, contributing to the membrane's dynamic nature and functionality.

The other choices do not accurately describe the plasma membrane's structure. For instance, the membrane is not fully permeable to all substances, and it is not made up solely of cholesterol; rather, cholesterol is interspersed within the phospholipid bilayer, contributing to its fluidity. Moreover, proteins and carbohydrates are integral components of the membrane