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Lec 13-14 Membrane Structure and Function.docx

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University of Lethbridge
BIOL 1010
Igor Kovalchuk

Membrane Structure and Function Lecture 13-14 Structure 1. Membrane Models 2. Fluid mosaic of lipids, proteins, and carbohydrates 3. Selective permeability 4. Passive transport. Osmosis. 5. Active Transport 6. Exocytosis and endocytosis The cell membrane - The plasma membrane is the boundary that separates the living cell from its nonliving surroundings. - Membranes are of crucial importance to life, because a cell must spate itself from the outside environment for two major reasons o It must keep its molecules of life (DNA, RNA. Proteins) from dissipating away. o It must keep out foreign molecules that damage or destroy the cells components and molecules. 1. Membrane Models - Two generations of membrane models o A) The Davidson-Danielli model. Sandwiched phospholipid bilayer between two protein layers. Widely accepted until 1970. o B) The fluid mosaic model disperses the proteins and immerses them in the phospholipid bilayer, which is in a fluid state. Singer and Nicolson proposed - Proteins are individually embedded in the phospholipid bilayer, rather than forming a solid coat spread upon the surface. - Hydrophilic portions of both proteins and phospholipids are maximally exposed to water resulting in a stable membrane structure - Hydrophobic portions of proteins and phospholipids are in the nonaqueous environment inside the bilayer. - Membrane is a mosaic of proteins inserted in a fluid bilayer of phospholipids. The cell membrane - The cell membrane functions as a semi-permeable barrier, allowing very few molecules across it while fencing the majority of organically produced chemicals inside the cell. - The most common molecule in the model is the phospholipid which is polar (hydrophilic)head and two (hydrophobic) tails Artificial membranes - (a) water can be coated with a single layer of phospholipid molecules - The hydrophilic heads of phospholipids are immersed in water, and the hydrophobic tails are excluded from water. - (b) a bilayer of phospholipids forms a stable boundary between two aqueous compartments. - This arrangement exposes the hydrophilic parts of the molecules to water and shields the hydrophobic parts from water. 2. The fluid mosaic of lipids, proteins and carbohydrates - A membrane is held together primarily by the hydrophobic interactions, which are much weaker than covalent bonds. Movement of phospholipids - Most of the lipids and some of the proteins can drift in the plane of the membrane, but not from one layer to another. - Phospholipids move quickly along the membranes plane averaging 2 MicroM per second. Proteins drift more slowly. Evidence for the drifting of membrane proteins - When researchers fuse a human cell with a mouse cell, it takes less than an hour for the membrane proteins of the two species to completely mix in the membrane of the hybrid cell.Membrane fluidity - Tails with kinks are keeping molecules from packing together, enhancing membrane fluidity. Cholesterol within the membrane - Cholesterol reduces membrane fluidity by reducing phospholipid movement at the moderate temperatures and also hinders solidification at low temperature: it make the membrane less fluid at warm temps and more fluid at lower temp. Sidedness of the plasma membrane - The membrane has distinct cytoplasmic and extracellular sides. - The bifacial quality determined when the membrane is first synthesized and modifies by the ER and Golgi - The side facing the inside of the ER, Golgi and vesicles is topologically equivalent to the extracellular surface of the plasma membrane, - The other side always faces the cytosol, from the time the membrane is made by the ER to the time it is added to the plasma membrane by fusion of a vesicle. - The small green trees represent the
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