Membrane Permeability Facts

When you're learning about cell membranes in animals, you might hear the words "membrane permeability," "phospholipid bilayerosmosis" and the tongue twisting "differentially permeable membrane." What do they mean? And which facts do you need to remember?

What is a cell membrane?

A membrane is a thin layer of tissue that covers a surface and separates or joins connecting structures or organs. The Merriam-Webster dictionary describes a membrane as "a thin soft pliable sheet or layer especially of animal or plant origin." An animal cell membrane is around 8 nanometers thick and provides protection and structure for the cell's inner material. You may also hear it referred to as a plasma membrane or plasmalemma.

Phospholipid bilayer

The cell membrane consists of two layers of phospholipids, a kind of lipid derivative with a glycerol-phosphate head and tails made up of two fatty acids. The hydrophilic-attracted to water-heads point out towards the cell's outer environment and into the cell's interior, while the hydrophobic-repelled from water-tails point towards each other. This arrangement, known as a 'fluid-mosaic model,' allows small, nonpolar molecules to pass easily through the cell membrane.


Permeability relates to the extent to which liquids or gasses can penetrate or pass through the barrier. A nonpermeable membrane would allow nothing to pass. A semipermeable membrane allows some things to pass, as in the example of the phospholipid bilayer, which allows nonpolar molecules to pass, but prevents most polar molecules from passing. A differentially permeable membrane allows smaller molecules to pass, while preventing larger ones.


Osmosis is the process by which water molecules pass through a semipermeable membrane from a higher concentrated solution to a lower one. We call the natural movement of molecules from a higher concentrated solution to a lower one to achieve balance diffusion. While nonpolar molecules can move freely through a semipermeable membrane, most polar molecules cannot. Water molecules, however, are small enough to pass through the membrane. It is this movement of water molecules through the membrane that we call osmosis.

If large polar molecules are present on one side of the membrane but not the other, they will interact with polar water molecules. This interaction reduces the number of smaller water molecules that are free to pass through the membrane without reducing the number of overall water molecules in the system. As a result, there will be more water molecules passing through the membrane to the side with the other molecules than are passing back to the side without, resulting in a difference in volume on the two sides. When the two sides are balanced, they are isotonic, and when they are out of balance, the side with more volume is called the hypertonic side while the side with less is the hypotonic side.

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