Selectively permeable membranes are made up of a phospholipid bilayer, which is a double layer of phospholipids. Phospholipids are molecules with a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail. The hydrophilic heads face outward, where they interact with water, and the hydrophobic tails face inward, where they avoid water.
This arrangement of phospholipids creates a barrier that is impermeable to most molecules, including water. However, some molecules, such as oxygen, carbon dioxide, and glucose, can pass through the membrane by diffusion. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.
Other molecules, such as sodium and potassium ions, cannot pass through the membrane by diffusion. These molecules can only pass through the membrane through channels or pumps. Channels are proteins that form pores in the membrane, allowing specific molecules to pass through. Pumps are proteins that use energy to move molecules against their concentration gradient, from an area of low concentration to an area of high concentration.
The selective permeability of cell membranes is essential for maintaining the proper functioning of cells and organisms. It allows cells to take in nutrients and get rid of waste products. It also allows cells to maintain a proper water balance and to respond to changes in their environment.
