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 Fluid Mosaic Model

 

    Phospholipids form the fundamental structure of the cell membrane, but they are not the only substance found there. According to the fluid-mosaic model of the cell membrane, special proteins called membrane proteins float in the phospholipid bilayer like icebergs in a sea.

 

    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.

 

    Molecules that can pass freely through the membrane follow concentration gradients, moving from the higher concentration area to the region of lower concentration. These processes take no energy and are called passive transport. The molecules that cannot pass freely across the phospholipid bilayer can be carried across the membrane in various processes that require energy and are therefore called active transport.

 

    Passive Transport

 

    There are three main types of passive transport: diffusion, facilitated diffusion, and osmosis. In fact, osmosis is simply the term given to the diffusion of water.

 

    Diffusion

 

    In the absence of other forces, substances dissolved in water move naturally from areas where they are abundant to areas where they are scarce—a process known as diffusion. If there is a higher concentration of carbon dioxide gas dissolved in the water inside the cell than in the water outside the cell, carbon dioxide will naturally flow out from the cell until its distribution is balanced, without any energy required from the cell.