Summaries - CHAPTER 11: MEMBRANE STRUCTURE AND FUNCTION...

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11.1 Phospholipids and Glycolipids Form Bimolecular Sheets: Membrane lipids spontaneously form extensive bimolecular sheets in aqueous solutions. The driving force for membrane formation is the hydrophobic effect. The hydrophobic tails then interact with one another by van der Waals interactions. The hydrophilic head groups interact with the aqueous medium. Lipid bilayers are cooperative structures held together by many weak bonds. These lipid bilayers are highly impermeable to ions and other polar molecules, yet they are quite fluid, which allows them to act as a solvent for membrane proteins 11.2 Proteins Carry Out Most Membrane Processes Specific proteins mediate distinctive membrane functions, such as transport, communication, and energy transduction. Many integral membrane proteins span the lipid bilayers, whereas others are only partly embedded in the membrane. Peripheral proteins are bound to membrane surfaces by electrostatic and hydrogen-bond interactions. Membrane spanning proteins have regular structures, including β strands, although the α helix is more common among the membrane-spanning proteins. 11.3 Lipids and Many Membrane Proteins Diffuse Laterally in the Membrane Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plain of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face to the other (transverse diffusion, or flip- flopping) is usually very slow. Proteins don’t rotate across the bilayers; hence membrane asymmetry can be preserved. 11.4 Membrane Fluidity is Controlled by Fatty Acid Composition and Cholesterol Content The degree of fluidity of a membrane partly depends on chain length of its lipids and the extent to which their constituent fatty acids are unsaturated. In animals, cholesterol content also regulates membrane fluidity. 11.5 A Major Role of Membrane Proteins is to Function as Transporters For a net movement of molecules across a membrane, two features are required: (1) the molecules must be able to cross a hydrophobic barrier and, (2) an energy source must power the movement. Lipophilic molecules can pass through a membrane’s hydrophobic interior by simple diffusion. These molecules will move down their concentration gradients. Polar or charged molecules require proteins to form passages through the hydrophobic barrier. Passive transport or facilitated diffusion take place when an ion or polar molecule moves down its concentration gradient. If a molecule moves against its concentration gradient, an external energy source is required; this movement is referred to as active transport and results in the generation of concentration gradients. Active transport is often carried out at the expense of ATP hydrolysis. P-type
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Summaries - CHAPTER 11: MEMBRANE STRUCTURE AND FUNCTION...

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