A semipermeable membrane is a barrier that allows certain molecules to pass through while blocking others. This selective permeability is essential in biological systems, especially within cells, where semipermeable membranes maintain homeostasis by regulating the movement of ions, nutrients, and waste products. Examples include the plasma membrane surrounding cells and the membranes of organelles. This property is vital for processes such as osmosis and diffusion, contributing to cellular functions and overall organismal health.