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In biological systems, the primary function of a membrane, specifically the cell membrane, is to form the outer boundary of the cell. This semipermeable membrane consists mainly of a lipid bilayer that not only encapsulates the cell's contents but also plays a crucial role in regulating the movement of substances in and out of the cell. It helps maintain homeostasis by controlling what enters and leaves, ensuring that essential nutrients can enter the cell while waste products are expelled.

This boundary also serves as a site for communication with other cells through receptor proteins, which are embedded in the membrane, allowing the cell to respond to external signals. In this way, the membrane is integral to maintaining cellular integrity and facilitating interactions with the environment.

The other options do not accurately describe the primary function of a membrane. While membranes do play a role in nutrient transport, storing nutrients is not their main purpose. Membranes do not provide energy directly as organelles like mitochondria do, and while they can house proteins or assist in protein synthesis, they do not produce proteins themselves.