recap

6.1 recap

The fluid mosaic model applies to the cell membrane and the membranes of organelles. In the aqueous environment of the cell, phospholipids spontaneously assemble into a bilayer. The cell membrane is considered fluid because the phospholipids and some of the proteins that compose it can move within their monolayer. An integral membrane protein has both hydrophilic and hydrophobic domains, which affect its position and function in the membrane. Peripheral membrane proteins are bound to one side or the other of the membrane. Carbohydrates that attach to lipids and proteins on the outside of the membrane serve as recognition and adhesion sites for adjacent cells.

learning outcomes

You should be able to:

  • Explain how the hydrophobic and hydrophilic regions of phospholipids are involved in membrane formation.

  • Differentiate between integral proteins and peripheral proteins.

  • Design an experiment to compare the properties of membranes in different types of cells.

  • Compare and contrast information provided by freeze-fracturing techniques and cell fusion experiments.

Question 1

How do the hydrophobic and hydrophilic regions of phospholipids form a membrane bilayer?

The hydrophilic “heads” of fatty acids are the polar ends, and the hydrophobic “tails” are the nonpolar ends. So the heads tend to associate with water molecules, and the tails away from water molecules. Placed in an aqueous environment, the fatty acids will arrange themselves so that their tails interact with one another while their heads face the water of the environment and cytoplasm, forming a bilayer.

Question 2

What differentiates an integral protein from a peripheral protein?

An integral membrane protein is embedded in the phospholipid bilayer by hydrophobic interactions with the lipid interior. It must have amino acids with hydrophobic R groups to insert into the nonpolar fatty acid tail region of the membrane bilayer. A peripheral membrane protein lacks hydrophobic regions and does not interact with the hydrophobic core of the phospholipid bilayer. Instead it is usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid polar head groups.

Question 3

What information about membranes is derived from freeze-fracturing and cell fusion experiments?

Both freeze-fracturing and cell fusion experiments indicate that nonpolar membrane proteins are inserted into the hydrophobic interior of the lipid bilayer. Cell fusion experiments also show that the proteins can move in the plane of the membrane.

Question 4

When a normal lung cell becomes a lung cancer cell, there are several important changes in cell membrane properties. How would you investigate the observation that the cancer cell membrane is more fluid, with more rapid diffusion in the plane of the membrane of both lipids and proteins?

To measure membrane fluidity, label a small amount of a lipid or protein with a dye and allow it to incorporate into the membrane of a cancer cell and a noncancer cell. This may make a localized labeled spot on the cells. The localized region will be seen to diffuse over the cells over time. In the cancer cell, this rate of diffusion may be faster.

Now that you understand the structure of biological membranes, let’s see how their components function. In the next section we’ll focus on the membrane that surrounds individual cells: the cell membrane. We’ll then look at how the cell membrane allows individual cells to be grouped together into multicellular systems of tissues.