26.3. Lipids http://www.ck12.org
FIGURE 26.18
Some common foods and oils, along with their percentages of saturated, monounsaturated, and polyunsaturated
fats.Phospholipids
Aphospholipidis a lipid that contains a phosphate group and one or more long hydrocarbon chains. Phospholipids
are the primary component of cell membranes. Each phospholipid molecule consists of a hydrophilic (water-loving)
head and hydrophobic (water-fearing) tails (Figure26.19).
Following the rule of “like dissolves like,” the hydrophilic head of the phospholipid molecule dissolves readily in
water. The long carbon chains of a phospholipid are nonpolar, so they avoid water because of their insolubility.
In water, phospholipids spontaneously form a double layer called a lipid bilayer, in which the hydrophobic tails of
phospholipid molecules are sandwiched between two layers of hydrophilic heads (Figure26.20). In this way, only
the heads of the molecules are exposed to the water, while the hydrophobic tails interact only with each other.
Phospholipid bilayers are critical components of cell membranes. The lipid bilayer acts as a barrier to the passage of
molecules and ions in and out of the cell. However, an important function of the cell membrane is to allow selective
passage of certain substances. This is accomplished by embedding various protein molecules that span the lipid
bilayer (Figure26.21). These proteins form channels through which certain specific ions and molecules are able
to move. Many membrane proteins are also attached to carbohydrates on the outside of the lipid bilayer. These
carbohydrates serve a number of purposes that we are only beginning to understand, including the ability to provide
identifying information about the type of cell to which it is attached.