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Chapter 5 Membranes and Transport Summary Notes Complete summary of chapter 5, including all topics covered in lecture in depth, with definitions, and explanations. Cuts all the excess fluff in the textbook, straight to the point biology

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Wilfrid Laurier University
Diane Williams

BI110 CHAPTER 5 MEMBRANES AND TRANSPORT 5.1 An Overview of the Structure of Membranes • plasma membrane a selectively permeable membrane that allows for the uptake or the key nutrients and elimination of waste products while maintaining a protected environment in which metabolic processes can occur 5.1 The Fluid Mosaic Model of Membranes • the Fluid Mosaic Model proposes that membranes are not rigid with molecules locked into place but rather consist of fluid lipid molecules in which proteins are embedded and float freely • the lipid molecules of all biological membranes exist in a double layer, called a bilayer, that is less than 10nm thick. Lipids in either side of the bilayer can move, but not switch between layers. Very dynamic. • membranes contain a wide variety of proteins, each with a specific function • the relative proportions of lipid and protein within a membrane vary considerably depending on the type of membrane (ie. myelin sheaths are mostly lipid, inner mitochondial membrane is mostly protein for the electron transport chain) • the proteins and other components of one half of the lipid bilayer are different from those that make up the other half of the bilayer -> membrane asymmetry. reflects the differences in the functions performed by each side of the membrane 5.1b Experimental Evidence in Support of the Fluid Mosaic Model • Membranes are Fluid - mouse proteins + human proteins on a cell, they moved around the cell and mixed • Membrane Asymmetry - cryo freeze a cell and slice the bilayer and look at with electron microscope to see the bilayer 5.2 The Lipid Fabric of a Membrane 5.2a Phospholipids are the Dominant Lipids in Membranes • phospholipids consist of two fatty acid “tails” linked to one of several types of alcohols or amino acids by a phosphate group • they are amphipathic, they each contain a region that is hydrophobic (tail) and hydrophilic (head) • when added into an aqueous solution, phospholipids associate with each other and form into a bilayer spontaneously because it represents the lowest energy state (most stable) 5.2b Membrane Fluidity • the fluidity of the lipid bilayer is dependent on how densely the individual lipid molecules can pack together. Influenced by two factors: 1.1. Composition of the Lipid Molecules • more unsaturated fats on the tails results in a more fluid membrane BI110 1.2. Temperature • the lower the temperature the more densely the molecules are packed (less movement) 5.2c Organisms Can Adjust Fatty Acid Composition • exposure to low temperatures may result in membrane viscosity to decrease to the point where normal membrane permeability is inhibited. • if the membrane solidifies, electron transport ceases to operate • membranes may become too fluid and liquid due to the increase in molecular motion, which can result in membrane leakage, irreversible disruption of cellular ion balance can lead to cell death • most organisms can adjust the fatty aid composition of their membranes such that proper fluidity is maintained over a relatively broad range of temperatures • desaturases are enzymes that produce unsaturated fatty acids through fatty acid synthesis. Act on saturated fatty acides by catalyzing a reaction that removes two hydrogen atoms from neighbouring carbon atoms and introduces a double bond. • the abundance of desaturases increases as the temperature is lowered to maintain membrane fluidity • Sterols act as membrane buffers: at high temperatures, they help restrain the movement of lipid molecules, thus reducing the fluidity of the membrane 5.3 Membrane Proteins 5.3a The Key Function of Membrane Proteins 1. TRANSPORT - of substances into and out of the cell through channels, or transport proteins 2. ENZYMATIC ACTIVITY - enzyme membrane proteins such as those associated with the electron transport chain 3. SIGNAL TRANSDUCTION - receptors that bind to signal molecules such as hormones that 4. ATTACHMENT/RECOGNITION - attachment points for a range of cytoskeleton elements, as well as components involved in cell to cell recognition 5.3b Integral Membrane Proteins • integral membrane proteins are proteins that are embedded in the phospholipid bilayer, most are transmembrane proteins, spanning the entire membrane bilayer • composed of both hydrophobic (nonpolar) and hydrophilic (polar) domains to interact with the bilayer and the external and internal aqueous environments generally 17-20 amino acids long to span the entire bilayer, predominantly nonpolar, and often • coiled into alpha helices 5.3c Peripheral Membrane Proteins peripheral membrane proteins are on the surface of the membrane and thus do not interact • with the hydrophobic core. • bound to the surface by noncovalent bonds, hydrogen or ionic bonds, usually by interacting with the exposed portions of integral proteins a well as directly with membrane lipid molecules mostly on the cytoplasmic side of the membrane • BI110 5.4 Passive Membrane Transport 5.4a Passive Transport is Based on Diffusion • passive transport is defined as the movement of a substance across a membrane without the need to expend chemical energy such as ATP • driven by diffusion the net movement of a substance from a region of higher concentration to a region of lower concentration • the driving force behind diffusion is an increase in entropy • the rate of diffusion depends on the concentration difference or concentration gradient that exists between two ares or across a membrane, bigger gradient, faster diffusion • when conc
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