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Chapter 6 - Bio 102

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BIOL 102
Wayne Snedden

Midterm- TUESDAY OCTOBER 23RD Membrane structure and transport Biological Membranes • Basic framework is phospholipid bilayer • Phospholipids are amphipathic – Hydrophobic region faces in – Hydrophilic region faces out • Also contain proteins & carbohydrates – Relative amount of each vary •Amphipathic Fluid Mosaic Model Modifications on surfaces ie polar head group, or orientation will effect the function. Mosaic – complex/mixture of things. Mainly proteins. Integral protein- cut from one side to another -> see the purple pieces that extend from the top to the bottom The way it is folded has association with the bilayer. Some proteins may still fold properly, however most if they are pulled out will not work. Trans membrane domain- can calculate based on computer systems Modifications- many ways, interior, lipids etc Often have accessory compounds that help in the structural form of the cell. Proteins bound to membranes • Integral membrane proteins
 -Trans membrane proteins -Lipid anchors – hard to get off. Can “cleave” or use harsh chemicals to separate. • Peripheral membrane proteins -Non-covalently bound to integral membrane proteins or to the polar head group of the PLs Proteins are a critical aspect of membrane structure and function can change orientation so they switch sides, however the trans membrane protein will not flip as it will not function. Protein-protein interaction is on the inside of the cell. The proteins will recognize each other and come together, they have an affinity. Bind which induces a conformational change. In almost all cases when it binds, it induces a conformational change, which can change its shape and ability to interact. Hormone binds- change in shape so it cannot bind, and leaves, and can now pass it along to the rest of the cell and go into nucleus, or do something else once the hormone has been detected. Approximately 25% of All Genes Encode Membrane Proteins • Membranes are important biologically and medically (70% drugs target membrane proteins) • Can predict TMs via algorithms • ~ 20–30% of all genes predicted • Function of many proteins unknown The synthesis of proteins take place: a) On a ribosome b) in the cytoplasm c) on the surface of the rough er d)in prokaryotic cells e) all of the above semi fluid and semi permeable. A is free to wander and hang out doin its own thing… generally lateral movement and some rotational movement. What they cannot do is flip, without the addition of energy. Can move from the outside to the inside and then be modified.  Factors affecting fluidity To much fluidity or too rigid is a problem so it must be controlled • Length of fatty acyl tails
 – Shorter acyl tails are less likely to interact, which makes the membrane more fluid
 – Longer they are, the tighter they can bond together and increase rigidity • Presence of double bonds in the acyl tails – DB creates kink tail, making it more difficult for neighboring tails to interact = more fluid – More fluid because it is less tightly packed <>-~< compared to --- • Presence of cholesterol (animals) – Cholesterol tends to stabilize membranes – Effects depend on temperature – can be added to help stabilize the membrane structure cant become leaky so proteins function properly Experiments on lateral transport • Larry Frye and Michael Edidin conducted an experiment that verified the lateral movement of membrane proteins • Mouse and human cells were fused • Temperature treatment- 0°C or 37°C • Mouse membrane protein H-2 fluorescently labeled • 0°C cells- label stays on mouse side • 37°C cells- label moves over entire cell hypothesis: are inter-membrane proteins free to diffuse laterally in the membrane? antibody- tool to help us recognize the binding and interactions specifically with a region on a target. Purify the antibody from rabbit, and these will now bind only to the mouse protein. An antibody stuck on a protein cant be seen, however if the antibody has been covalently bond to a “flag” we can see where the antibodies are. Integral membrane proteins- can be anchored. Attached to cytoskeletal filament. Maybe you want it held in place because it is attached to something else, like a neighboring cell and it is working together. The results couldn’t ultimately determine that proteins don’t diffuse, as there can be exceptions. Glycosylation • Covalently attach carbohydrate to a protein or lipid 
 – Glycolipid – carbohydrate to lipid – Glycoprotein – carbohydrate to protein • Can serve as recognition signals for other cellular proteins • Often play a role in cell surface recognition • Protective effects 
– Cell coat or glycocalyx - carbohydrate-rich zone on the cell surface shielding cell Electron microscopy • Transmission electron microscopy (TEM), uses a biological sample that is thin sectioned and stained with heavy-metal dyes • Dye binds tightly to the polar head groups of phospholipids, but it does not bind well to the fatty acyl chains FFEM • Freeze fracture electron microscopy, specialized form of TEM, can be used to analyze the interiors of phospholipid bilayers – Sample is frozen in liquid nitrogen and fractured with a knife – Due to the weakness of the central membrane region, the leaflets separate into a P face (the protoplasmic face that was next to the cytosol) and the E face (the extracellular face) – Can provide significant three-dimensional detail about membrane protein form and shape Phospholipid bilayer is a barrier • Hydrophobic interior makes formidable barrier • Selectively-permeable 
• Diffusion – Movement of solute from an area of higher concentration to an area of lower concentration Passive diffusion- without transport protein • Solutes vary in their rates of penetration – P
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