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The Fluid-Mosaic Model of the Cell Plasma Membrane

The fluid-mosaic model describes the plasma membrane of animal cells. The plasma membrane that surrounds these cells has two layers (a bilayer) of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid). And the structure of the plasma membrane supports the old saying, “Oil and water don’t mix.”

Each phospholipid molecule has a head that is attracted to water (hydrophilic: hydro = water; philic = loving) and a tail that repels water (hydrophobic: hydro = water; phobic = fearing). Both layers of the plasma membrane have the hydrophilic heads pointing toward the outside; the hydrophobic tails form the inside of the bilayer.

Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), the plasma membrane forms a circle around each cell so that the water-loving heads are in contact with the fluid, and the water-fearing tails are protected on the inside.

The fluid-mosaic model of plasma membranes.
The fluid-mosaic model of plasma membranes.

Proteins and substances such as cholesterol become embedded in the bilayer, giving the membrane the look of a mosaic. Because the plasma membrane has the consistency of vegetable oil at body temperature, the proteins and other substances are able to move across it. That’s why the plasma membrane is described using the fluid-mosaic model.

The molecules that are embedded in the plasma membrane also serve a purpose. For example, the cholesterol that is stuck in there makes the membrane more stable and prevents it from solidifying when your body temperature is low. (It keeps you from literally freezing when you’re “freezing.”) Carbohydrate chains attach to the outer surface of the plasma membrane on each cell. These carbohydrates are specific to every person, and they supply characteristics such as your blood type.

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