In Chapter 2, we learned that phospholipids are the principal building blocks of biomembranes. The most common phospholipids in membranes are the phosphoglycerides (see Figure 2-20), but as we will see in this chapter, there are multiple types of phospholipids. All phospholipids are amphipathic molecules that consist of two segments with very different chemical properties: a fatty acid–based (fatty acyl) hydrocarbon “tail” that is hydrophobic (“water fearing”) and partitions away from water, and a polar “head group” that is strongly hydrophilic (“water loving”) and tends to interact with water molecules. The interactions of phospholipids with one another and with water largely determine the structure of biomembranes.

Besides phospholipids, biomembranes contain smaller amounts of other amphipathic lipids, such as glycolipids and cholesterol, which contribute to membrane function in important ways. We first consider the structure and properties of pure phospholipid bilayers and then discuss the composition and behavior of natural cellular membranes. Then we consider how the precise lipid composition of a given membrane influences its physical properties.