TAILIEUCHUNG - Biochemistry, 4th Edition P31

Biochemistry, 4th Edition P31. 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 Are the Dynamic Processes That Modulate Membrane Function 263 Before transition Main transition Anti conformation CENGAGENOW ANIMATED FIGURE The gel-to-liquid crystalline phase transition which occurs when a membrane is warmed through the transition temperature Tm. In the transition the surface area increases the membrane thickness decreases and the mobility of the lipid chains increases dramatically. Membrane phase transitions can be characterized by measuring the rate of heat absorption by a membrane sample in a calorimeter. Pure homogeneous bilayers containing only a single lipid component give sharp calorimetric peaks. As membrane heterogeneity increases the calorimetric peaks broaden. Below phase transitions lipid chains primarily adopt the anti conformation. Above the phase transition lipid chains have absorbed a substantial amount of is reflected in the adoption of higher-energy conformations including the gauche conformations shown. See this figure animated at login. bilayer thickness is maximal. At higher temperatures the acyl chains undergo much more motion with rotations around the acyl chain C-C bonds and significant largescale bending of the acyl chains. The membrane is then said to be in a liquid crystalline phase or liquid-disordered state Ld state Figure . In this less ordered state the surface area per lipid increases and the bilayer thickness decreases by 10 to 15 . Under most conditions the transition from the gel phase to the liquid crystalline phase is a true phase transition and the temperature at which this change occurs is referred to as a transition temperature or melting temperature Tm . The sharpness of the transition in pure lipid preparations shows that the phase change is a cooperative behavior. This is to say that the behavior of one or a few molecules affects the behavior of many other molecules in the vicinity. The sharpness of the transition then reflects the number of molecules that are

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