Selectively Permeable Membranes: Definition & Examples

Our bodies are composed of billions of cells. There has to be a way to regulate what enters and leaves these cells. In this lesson, we will examine selectively permeable membranes and their critical role in the function of a cell.
Definition of Selectively Permeable Membranes
All cells are enclosed with a cell membrane. A selectively permeable cell membrane is one that allows certain molecules or ions to pass through it by means of active or passive transport. Active transport processes require the cell to expend energy to move the materials, while passive transport can be done without using cellular energy. It is important to note that the membrane can also block the passage of materials depending on the conditions inside or outside of the cell.

The method used by a cell to pass molecules in or out is determined by the size of the molecule, as well as the concentration of the molecules on the inside and outside of the cell. Most cell membranes are covered with tiny protein channels that allow things to move in and out of the cell. These channels act as gateways.

If a molecule is tiny enough to fit through a special protein channel in the plasma membrane, it will use a form of active or passive transport to move through. If the molecule is too large to fit through a protein channel, it will have to enter or exit the cell by forming a vesicle. A vesicle can be thought of like a tiny pouch that surrounds the molecule and ushers it through an opening in the cell membrane.

Examples of Selectively Permeable Membranes
Selectively permeable membranes can be found around a variety of cells and places. The most common example is the phospholipid bilayer cell membrane that surrounds every cell in our bodies. Another example of a selectively permeable membrane is the inner membranes of an egg.

All cells in our body are surrounded by a phospholipid bilayer. Phospholipids are molecules composed of a hydrophilic, or water-loving, head and a hydrophobic, or water-fearing, tail. In cell membranes the phospholipids are arranged with the heads facing the outer and inner surfaces of the cell with the tails aligned in the middle.