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Chap 7 - Membranes.docx

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BIOL 1010
Brent Sellinger

Reece et al., 9 Edition 1 Chapter 7 Page Membrane Structure and Function Membranes • Membranes are of fundamental importance to life • Membranes are the defining boundaries of cells. Membranes permit development of an aqueous solution inside the cell (some say the inside of a cell is an aqueous gel) that is different in composition from the surrounding environment. The membrane accomplishes this while still allowing movement of nutrients into the cell and wastes out of the cell. • Membranes have selective permeability - some substances cross the membrane more easily than others ▯ The hydrophobic region allows passage of nonpolar molecules [O CO (N2te: carbon 2 , dioxide is nonpolar because of the linear arrangement of the two C=O double bonds)] and restricts passage of polar molecules and ions (i.e., hydrophilic compounds). ▯ Water is a polar molecule and consequently diffuses very slowly across the membrane despite its small size; other hydrophilic molecules avoid contact with the hydrophobic lipid bilayer molecules by interacting with transport proteins. These proteins are very specific and selective in what they will transport. I. Membrane Structure ▯ Approximately 8 nm thick ▯ First observed membrane structure with an electron microscope in the 1950s (recall resolving power of microscopes) Fluid mosaic model ▯ currently accepted membrane model ▯ proposed by S.J. Singer and G. Nicolson (1972) as a modification to the original Davson-Danielli sandwich (1935) ▯ Membrane components are arranged in a phospholipid bilayer (recall: the amphipathic nature of phospholipids) and held together by hydrophobic interactions.Amosaic of proteins is embedded in the lipid bilayer or associated with the membrane interacting with integral proteins and/or membrane lipids (Recall: diagram from Chapter 6 notes) ▯ This model is supported by electron microscopy (freeze fracture experiments) The membrane structure ▯ is dynamic and changing ▯ varies in composition - mitochondrial membranes have a higher protein content than the plasma membrane; different cell types have different membrane protein content ▯ inner and outer surfaces are distinct and vary in composition - asymmetrical bifacial quality is determined by the endoplasmic reticulum ▯ Membranes must remain fluid to function - if the membrane solidifies then the permeability changes. This affects membrane function as well as the activities of protein associated with the membrane 2 Factors contributing to fluidity ▯ weak hydrophobic interactions between each of the layers in the lipid bilayer ▯ the presence of unstaturated fatty acids in the phospholipids ▯ cholesterol (reduces fluidity at warm temperatures by restraining phospholipid movement; increases fluidity at lower temperatures by preventing tight packing of phospholipids) ▯ movement of proteins (more restricted – in some cases membrane proteins are immobilized due to attachments to cytoskeleton) II. Membrane Components 1. Lipids ▯ basic fabric of the membrane – phospholipids are amphipathic molecules that contain both hydrophobic and hydrophilic portions ▯ phospholipids and other lipids (e.g., cholesterol) stabilize membrane fluidity. The composition of the membrane will change in response to temperature. For organisms that grow at higher temperatures, membrane lipids will contain more saturated hydrocarbon tails (allowing tighter packing of membrane lipids). Cholesterol reduces fluidity by restraining phospholipid movement. For organisms found at lower temperatures, membrane lipids will contain more unsaturated hydrocarbon tails (preventing tighter packing of membrane lipids). Cholesterol increases fluidity at lower temperatures by preventing tight packing of membrane lipids. 2. Proteins ▯ Proteins determine the specific membrane functions (greater than 50 kinds of proteins are found in red blood cells) ▯ integral proteins - completely (i.e., transmembrane) or partially span lipid bilayer - hydrophobic regions of integral proteins are usually composed of nonpolar amino acids often arranged in one of more alpha-helices that are surrounded by the membrane hydrophobic tails. The hydrophilic regions of the integral proteins are exposed to aqueous cytoplasm or environment ▯ peripheral proteins - associated with membrane surface e.g., fibronectin Functions of membrane proteins ▯ Transport ▯ Enzymatic activity ▯ Receptors – signal transduction ▯ Intercellular junctions ▯ Cell to cell recognition - sorting of cells in tissue formation, foreign cells are recognized by the immune system ▯ Attachment to cytoskeleton and ECM 3. Carbohydrates ▯ branched oligosaccharides – usually < 15 monomers ▯ covalently bonded to lipids (glycolipids) or proteins (glycoproteins) ▯ highly variable in composition - varies from species to species ▯ important in cell to cell recognition – (e.g., blood type -A, B,AB, O is due to variations in oligosaccharides on the red blood cell surface) Reece et al., 9 Edition Chapter 7 3 Page III Mechanisms of Membrane Transport 1. Passive Transport – no energy is expended by the cell during the transport of materials across the membrane a) Diffusion of a substance across a biological membrane 1* Diffusion is a form of passive transport. Diffusion is the movement of a substance from an area of high concentration to low concentration 2* Any substance will diffuse down its own concentration gradient regardless of other substances 3* Aconcentration gradient represents potential energy ▯ this drives diffusion so that there is a net movement from an area of high concentration to that of low concentration. Diffusion results from kinetic energy (thermal motion in this case). The system moves towards increasing entropy (i.e., a measure of disorder or randomness). 4* Diffusion is a spontaneous process that decreases the free energy of the system, which will eventually reach a dynamic equilibrium. 5* Many substances move across the membrane by diffusion (e.g., there is an O gradien2 in respiring cells. The consumption of O ma2ntains the gradient.) b) Osmosis 6* Osmosis is a form of passive transport defined by the movement of free water across a selectively permeable membrane 7* Water moves from an area of higher free water concentrati
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