Which of the following best describes the nature of the cell membrane?

The cell membrane, often referred to as the plasma membrane, is accurately described as fluid and selectively permeable. This means that the membrane is not rigid; rather, it has a flexible structure that can allow some substances to pass through while blocking others.

The fluid nature of the cell membrane is primarily due to the phospholipid bilayer that makes up its structure. The phospholipids are arranged in a way that allows them to move laterally, creating a dynamic and adaptable environment. This fluidity is essential for various cellular functions, such as allowing the movement of proteins and lipids within the membrane and facilitating communication and transport processes.

Selective permeability is a critical characteristic of the cell membrane. It allows the cell to maintain homeostasis by controlling what enters and exits the cell. Small nonpolar molecules, for example, can pass through the membrane easily, while larger or charged molecules require specific transport proteins to facilitate their movement.

Overall, describing the cell membrane as fluid and selectively permeable highlights its essential role in maintaining the internal environment of the cell while responding to changes in the external environment.