Biochemistry, 4th Edition P25. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Is the Structure and Chemistry of Polysaccharides 203 b Gram-neratiw bacteria FIGURE The structures of the cell wall and membrane s in Gram-positive and Gram-negative bacteria. The Gram-positive cell wall is thicker than that in Gram-negative bacteria compensating for the absence of a second outer bilayer membrane. Cell Walls of Gram-Negative Bacteria In Gram-negative bacteria the peptidoglycan wall is the rigid framework around which is built an elaborate membrane structure Figure . The peptidoglycan layer encloses the periplasmic space and is attached to the outer membrane via a group of hydrophobic proteins. As shown in Figure the outer membrane of Gram-negative bacteria is coated with a highly complex lipopolysaccharide which consists of a lipid group anchored in the outer membrane joined to a polysaccharide made up of long chains with many different and characteristic repeating structures Figure . These many different unique units determine the antigenicity of the bacteria that is animal immune systems recognize them as foreign substances and raise antibodies against them. As a group these antigenic determinants are called the O antigens FIGURE Lipopolysaccharide LPS coats the outer membrane of Gram-negative lipid portion of the LPS is embedded in the outer membrane and is linked to a complex polysaccharide. 204 Chapter 7 Carbohydrates and the Glycoconjugates of Cell Surfaces and there are thousands of different ones. The Salmonella bacteria alone have well over a thousand known O antigens that have been organized into 17 different groups. The great variation in these O antigen structures apparently plays a role in the recognition of one type of cell by another and in evasion of the host immune system. Cell Walls of Gram-Positive Bacteria In Gram-positive bacteria the cell exterior is less complex than for Gram-negative cells. Having no outer membrane Grampositive cells compensate with a thicker wall. Covalently .
