What Is a Selectively Permeable Membrane?
A selectively permeable membrane is a biological or synthetic barrier that allows certain molecules or ions to pass through it while blocking the passage of others. It acts as a gatekeeper, controlling which substances can enter or leave a defined space, such as a cell.
Section 2: How It Works
In biology, this property is characteristic of the cell membrane, which is made of a phospholipid bilayer. Permeability is determined by factors like the size, electrical charge, and polarity of the molecules attempting to cross. Small, nonpolar molecules can often pass through directly, whereas larger or charged molecules require specialized protein channels or transporters.
Section 3: A Practical Example
The plasma membrane of a human cell is a perfect example. It allows essential small molecules like oxygen and water to enter freely while keeping out harmful substances. At the same time, it uses protein channels to actively transport necessary nutrients like glucose and ions into the cell and to expel waste products.
Section 4: Importance in Biology
Selective permeability is vital for life as it enables cells to maintain a stable internal environment, a state known as homeostasis. This control over internal conditions is crucial for all cellular processes, including metabolism, energy production, and communication.