Chapter 7. Cell Biology: Membrane Transport
Scott Crousillac
Unit Outline
Learning Objectives
At the end of this unit, you should be able to:
I. Describe the “fluid mosaic” model of membrane structure.
II. Describe how the structure of the cell membrane affects membrane permeability.
III. Describe the following passive transport processes: diffusion, facilitated diffusion, and osmosis.
IV. Describe the effects of placing red blood cells in hypertonic, hypotonic, and isotonic solutions, respectively.
V. Describe the following active transport processes: primary and secondary active transport, endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis), and exocytosis
Part 1: The Cell Membrane
Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.
Structure and Composition of the Cell Membrane
The cell (plasma) membrane is described by the fluid mosaic model; it is an extremely pliable structure composed primarily of stacked phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions.
A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 7.1). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water and is water soluble. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic, or “water fearing,” and are NOT water soluble. A hydrophobic molecule (or region of a molecule) repels water. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water, while the hydrophobic portion can trap grease in micelles that then can be washed away.
The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 7.2). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is th