5.1 Membrane Components and Structure
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Recognize the structure and describe the function of lipids, proteins, and carbohydrates in cellular membranes.
- Understand the importance of maintaining cellular membranes in a liquid crystalline (fluid) state.
- Explain why and how temperature alters membrane fluidity.
- Evaluate the contribution of phospholipid fatty acid tails, phospholipid head groups, and cholesterol to membrane fluidity.
Membrane Components
The fluid mosaic model describes the overall structure and function of the cell membrane as two fluid layers of phospholipids, with proteins in and on each layer (Figure 5.2). The membrane is described as “fluid” because both proteins and phospholipids can easily move laterally through the membrane. The “mosaic” part of this model refers to the presence of many proteins, lipids, and carbohydrates scattered throughout the lipid bilayer, reminiscent of mosaic tile art pieces (Figure 5.1), which contain many different components. Proteins and lipids are the main components of the membrane, however the protein to lipid ratio varies among different types of cells and organelles. For example, human erythrocyte (red blood cell) plasma membranes have a low protein to lipid ratio, whereas chloroplast membranes have a higher protein/lipid ratio.
The first part of this chapter will consider the structure and function of membrane components. First, let’s give a brief overview of the components in Figure 5.2. Phospholipids arrange in two layers, with their hydrophobic tails pointing towards the inside of the bilayer, and their hydrophilic heads pointing towards the exterior of the bilayer. Sterols (e.g. cholesterol) are additional lipids that can be found in each layer. Integral proteins are embedded in the membrane, peripheral proteins are found on the perimeter of the inner or outer face of the membrane, and lipid-anchored proteins are covalently attached to lipids in the membrane. Glycolipids and glycoproteins are carbohydrates chains attached to lipids and proteins, found in the outer layer of the membrane. Now, let’s dive into some details!
Lipids
Cellular membranes can contain three types of lipids: phospholipids, sphingolipids and sterols. All of these types of lipids are predominantly hydrophobic, but also contain hydrophilic regions, which is important for the overall structure of membranes. Molecules with hydrophobic and hydrophilic portions are known as amphipathic or amphiphilic. This amphipathic property causes the hydrophobic portions of lipids to face each other, and the hydrophilic portions to face outward to interact with an aqueous (water-based) environment, an arrangement that is energetically favourable. This allows amphipathic lipids like phospholipids to form a variety of structures in solution, like spheres, bilayer spheres, or bilayer sheets (Figure 5.3). The cell membrane can be thought of as a lipid-bilaye