Chapter 7, Problem 7.2PS

Elucidation of Membrane Structure. Each of the following observations played an important role in enhancing our understanding of membrane structure. Explain the significance of each, and indicate in what decade of the timeline shown in Figure 7-3 the observation was most likely made.

1.    (a)            When a membrane is observed in the electron microscope, both of the thin, electron-dense lines are about 2 nm thick, but the two lines are often distinctly different from each other in appearance.
2.    (b)            Ethylurea penetrates much more readily into a membrane than does urea, and diethylurea penetrates still more readily.
3.     (c)            The addition of phospholipase to living cells causes rapid digestion of the lipid bilayers of the membranes, which suggests that the enzyme has access to the membrane phospholipids.
4.    (d)            When artificial lipid bilayers are subjected to freeze-fracture analysis, no particles are seen on either face.
5.     (e)            The electrical resistivity of artificial lipid bilayers is several orders of magnitude greater than that of real membranes.
6.     (f)            Some membrane proteins can be readily extracted with 1 M NaCl, whereas others require the use of an organic solvent or a detergent.
7.    (g)            When halobacteria are grown in the absence of oxygen, they produce a purple pigment that is embedded in their plasma membranes and has the ability to pump protons outward when illuminated. If the purple membranes are isolated and viewed by freeze-fracture electron microscopy, they are found to contain patches of crystalline particles.

Figure 7-3 Timeline for Development of the Fluid Mosaic Model. The fluid mosaic model of membrane structure that Singer and Nicolson proposed in 1972 was the culmination of studies dating back to the 1890s (a–e). This model (f) has been significantly refined by subsequent studies (g and h).