Lesson: Chapter - 4

The Cell Membrane

The cells of all organisms, prokaryotic and eukaryotic alike, are surrounded by a thin sheet called the cell membrane. This barrier keeps cellular materials in and foreign objects out. The membrane is key to the life of the cell. By regulating what gets into and out of the cell, the membrane maintains the proper chemical composition, which is crucial to the life processes the cell carries out.

Structure of the Cell Membrane

The cell membrane is made up of two sheets of special fat molecules called phospholipids, placed on top of each other.

This arrangement is known as a phospholipid bilayer. Phospholipid molecules naturally arrange in bilayers because they have a unique structure. The long chains of carbon and hydrogen that form the tail of this molecule do not dissolve in water; they are said to be hydrophobic or “water fearing.” The hydrophilic phosphorous heads are attracted to water. Forming a bilayer satisfies the water preferences of both the “heads” and “tails” of phospholipids: the hydrophilic heads face the watery regions inside and outside the cell, and the hydrophobic tails face each other in a water-free junction. The bilayer forms spontaneously because this situation is so favorable.

The sea of phospholipid molecules and gatekeeper membrane proteins is in constant motion. The membrane’s fluidity keeps the cell from fracturing when placed under strain.

Transport Through the Cell Membrane

The most important property of the cell membrane is its selective permeability: some substances can pass through it freely, but others cannot. Small and nonpolar (hydrophobic) molecules can freely pass through the membrane, but charged ions and large molecules such as proteins and sugars are barred passage. The selective permeability of the cell membrane allows a cell to maintain its internal composition at necessary levels.

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