MBC5 Study Guide – Chapter 10 (Membrane Structure)
All living cells have a boundary called the plasma membrane that separates the cytoplasm from
the external environment. Eukaryotic cells also have internal membranes that create organelles
such as the cell nucleus. In this chapter, we will focus on structural aspects of membranes and
membrane proteins. Later chapters will consider functional issues.
THE LIPID BILAYER
When exposed to water, certain types of lipids form a bilayer (Figure 10-1). Membrane proteins
may be embedded in the bilayer structure. In this section, we will consider structural features of
lipids that cause them to form a bilayer, and we will examine the dynamic aspects of lipid
bilayers and biological membranes.
Phosphoglycerides, Sphingolipids, and Sterols are the Major Lipids in Cell Membranes
The most abundant membrane lipids are phospholipids, and the most abundant phospholipids are
phosphoglycerides (Figure 10-2). Phosphoglycerides are amphipathic because they have a
polar head and two nonpolar tail regions. The backbone of phosphoglycerides is the three-
carbon compound glycerol, whereas the backbone of the phospholipid sphingomyelin is
sphingosine. Many membranes contain cholesterol, a sterol that contains a rigid ring structure
attached to a single polar hydroxyl group (Figure 10-4).
Phospholipids Spontaneously Form Bilayers
The nonpolar tails of phospholipids do not favorably interact with water (Figure 10-7). This
unfavorable interaction decreases entropy, because water forms an ordered structure around the
nonpolar tail in an attempt to minimize interactions. To avoid a decrease in entropy, which is
energetically unfavorable, phospholipids in water will spontaneously form a bilayer in which the
nonpolar tails interact with each other and polar heads are exposed to water. This minimizes
entropy because water is more disordered.
The Lipid Bilayer Is a Two-Dimensional Fluid
When exposed to water, phospholipids form a spherical structure called a liposome (Figures 10-
8. 10-9). The bilayer is semi-fluid, allowing certain types of movements and preventing others
(Figure 10-11). Lipids can diffuse laterally (e.g., from left to right) and rotationally (rotate 360o).
However, it is energetically unfavorable for them to flip from one leaflet of the bilayer to the
The Fluidity of a Lipid Bilayer Depends on Its Composition
An optimal fluidity of a biological membrane is needed to ensure the proper integrity of the
membrane and to promote the function of membrane proteins. Fluidity can be altered by altering
the lipid composition. As noted in Table 10-1, certain categories of phospholipids are common.
You should be aware that structural features of lipids make the bilayer more or less fluid. These
include the following:
• Double bonds in the lipid tails make the bilayer more fluid. This is because double bonds
diminish packing between the tails.
• Shorter lipids tails make the bilayer more fluid. Shorter tails are freer to diffuse laterally