What are the functions of selectively permeable membranes?

Selectively permeable membrane acts as a gateway that regulates the passage of substances in and out of cells or cellular compartments. It allows only select molecules and ions to pass through it while hindering the passage of other undesirable materials. This provides a protective barrier that prevents harmful substances from freely entering cells, thus safeguarding cellular integrity. 

The selective entry of substances also helps maintain the internal environment of cells and cellular compartments by controlling the balance of ions and molecules. This is crucial for homeostasis and optimal cellular function.

In cellular organelles, like mitochondria and chloroplasts, the selectively permeable membrane helps in creating a concentration gradient, which is necessary for energy production in animals and plants respectively. 

The synaptic membrane which sheaths neurons, plays a role in cell signaling.