What is the primary function of a cell membrane?

The primary function of a cell membrane is to regulate the entry and exit of substances. This vital role is essential for maintaining homeostasis within the cell, as the membrane selectively allows certain molecules to pass through while blocking others. This selective permeability is achieved through the structure of the cell membrane, which consists of a phospholipid bilayer with embedded proteins that facilitate transport.

By controlling which substances can enter or leave the cell, the membrane helps manage the internal environment, ensuring that essential nutrients, ions, and water can enter, while waste products and harmful substances can be prevented from accumulating. This regulatory function is critical for cellular functions, signaling, and communication with other cells.

The other options, while related to cellular functions, do not capture the primary purpose of the cell membrane. For instance, protein synthesis occurs in the ribosomes, nutrient storage is managed by different cellular organelles, and structural support is provided primarily by the cytoskeleton and cell wall in certain organisms, rather than the cell membrane itself.