A biological barrier that selectively allows certain molecules or ions to pass through it by diffusion, osmosis, or active transport. The ability to control the passage of substances is determined by factors such as pore size, charge, and solubility. Cell membranes, which are composed of a phospholipid bilayer embedded with proteins, exemplify this selective permeability. For instance, water molecules can readily pass through these membranes, while larger molecules like glucose require specialized transport proteins to facilitate their movement across the barrier.
The selective nature of these barriers is critical for maintaining cellular homeostasis and carrying out essential biological functions. They enable cells to regulate their internal environment, control the entry of nutrients, and eliminate waste products. Historically, understanding their structure and function has been pivotal in advancing knowledge in fields such as cell biology, physiology, and medicine. The study of transport mechanisms across these membranes has led to the development of targeted drug delivery systems and treatments for various diseases.
