100%(1)1 out of 1 people found this document helpful
This preview shows page 1 - 2 out of 4 pages.
CHAPTER 5 Outline: Cell Membrane Structure and Function5.1 How Is the Structure of a Membrane Related to Its Function?A. Cell Membranes Isolate the Cell Contents While Allowing Communication with the Environment1. Cell membranes perform several crucial functionsa. Selectively isolate the cell’s contents from the external environmentb. Regulate the exchange of essential substances between the cell and the extracellularenvironmentc. Communicate with other cellsd. Create attachments within and between cellse. Regulate many biochemical reactions2. Membrane function is dependent on membrane structureB. Membranes Are “Fluid Mosaics” in Which Proteins Move Within Layers of Lipids1. The fluid mosaic model was developed in 1972 by S. J. Singer and G. L. Nicolson2. The membrane consists of a mosaic of proteins within the double layer of phospholipids(Figure 5-1)C. The Phospholipid Bilayer Is the Fluid Portion of the Membrane1. Phospholipids have a polar “head” and two nonpolar “tails” (Figure 5-2)2. The plasma membrane is a phospholipid bilayer (Figure 5-3)a. In a bilayer, the tails point inward and the heads point toward the exterior wateryenvironment (the extracellular fluid), or the internal watery environment (cytoplasm)b. The polar heads form hydrogen bonds with water; hydrophobic interactions keep tailsfacing each otherc. Membrane is fluid because phospholipid molecules are not bonded to each otherd. Some phospholipids have “kinks” in their “tails” which increase membrane fluidity(Figure 5-4)e. The phospholipid bilayer selectively isolates the internal environment from the externalenvironmenti. Most biological molecules are hydrophilic and cannot easily pass through themembraneii. Some very small molecules or uncharged, lipid-soluble molecules can freelypass through membranef. Cholesterol in animal cell membranes makes the bilayer stable, less fluid, and lesspermeable to water-soluble substancesg. The flexible and fluid nature of the phospholipid bilayer is very important to itsfunctionD. A Variety of Proteins Form a Mosaic Within the Membrane1. Thousands of proteins are embedded within or attached to the surface of the membrane’sphospholipid bilayer2. Many membrane proteins are glycoproteins that have an attached carbohydrate group