Final answer:
The primary function of the cell membrane is to regulate cell communication by controlling the entry and exit of substances and facilitating cell signaling and interaction.
Step-by-step explanation:
The function of the cell membrane primarily involves regulating cell communication. The cell membrane offers a protective barrier that controls the entry and exit of substances, thus maintaining the internal environment of the cell. It consists of a phospholipid bilayer with proteins, allowing it to be selectively permeable to different substances. This permeability facilitates various processes such as passive transport for the diffusion of molecules and active transport where energy is utilized to move substances against their concentration gradient. Additionally, the cell membrane plays a crucial role in cell signaling and interaction, vital for tissue and organ formation and the immune response. It's also flexible, which is essential for cells like red and white blood cells that need to change shape in narrow capillaries.