Phospholipid Molecules
The main fabric of the membrane is composed of amphiphilic, phospholipid molecules. The hydrophilic or “water-loving” areas of these molecules look like a collection of balls in an artist’s rendition of the model. Look at the image of the fluid mosaic model of the plasma membrane below. These “balls” are in contact with the aqueous fluid both inside and outside the cell.
The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. Image Attribution: OpenStax Biology
Hydrophobic and Hydrophilic Molecules
Hydrophobic, or water-hating molecules, tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster.
The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell.
A phospholipid molecule (see image below) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge.
The head can form hydrogen bonds, but the tail cannot. A molecule with this arrangement of a positively or negatively charged area and an uncharged, or non-polar, area is referred to as amphiphilic or “dual-loving.”
This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. Image Attribution: OpenStax Biology
Lipid Bilayers
This characteristic is vital to the structure of a plasma membrane. This is because, in water, phospholipids tend to become arranged with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they form a lipid bilayer—a barrier composed of a double layer of phospholipids that separates the water and other materials on one side of the barrier from the water and other materials on the other side.
In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward. Conversely, their hydrophobic tails face inward. Image Attribution: modification of work by Mariana Ruiz Villareal
In fact, phospholipids heated in an aqueous solution tend to spontaneously form small spheres or droplets. We refer to these spheres as micelles or liposomes. Their hydrophilic heads form the exterior and their hydrophobic tails stay on the inside (see image above).