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Summary 2-Membranes.doc

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Biology 1202B
Richard Gardiner

Membranes Summary Proteins in the Plasma Membrane • Membrane must have phospholipids that contain proteins to perform a variety of functions 1. Transport – may or may not involve energy 2. Enzymatic Activity – number of enzymes are actually membrane proteins 3. Signal Transduction – receptors 4. Intercellular Joining – tight junctions, etc. 5. Cell-Cell Recognition – identifying foreigners by orientation/sensing each other for development 6. Attachment to the cytoskeleton and extracellular matrix Phospholipids • Made up of phosphate + glycerol (hydrophilic) and 2 fatty acid chains (hydrophobic) • Types: Micelle (all tails point in), Liposome (inner + outer portions), bilayer (many phospholipids) • Bilayers are studied by separating the 2 layers (bumps and matching concavities show proteins) Fluid Mosaic Model • Hydrophilic regions of proteins/phospholipids are in maximum contact with water, hydrophobic regions are in a nonaqueous environment Fluidity of Membranes • Membrane molecules are held in place by relatively weak hydrophobic interactions o Most lipids and some proteins can drift laterally  Phospholipids are quick/ larger proteins are slow (directed manner may be due to motor proteins of the cytoskeleton) or don’t move at all (anchored by cytoskeleton)  Red and green membrane proteins mix freely in experiment • Influenced by temperature (cooling makes the membrane less fluid) o Membranes rich in unsaturated fatty acids (contain double bonds) are more fluid  Unsaturated hydrocarbon tails have kinks • Thickness- approx. 1-9 nm Cholesterol • Amount varies by type of membrane (plasma has 1 per phospholipid molecule/others have none) • Cells can alter membrane lipid composition to compensate for temperature changes o Cold-adapted organisms increase percentage of unsaturated phospholipids in autumn o Also can hold phospholipids together when it gets too hot Membrane Proteins • Determine most of the membrane’s specific functions o Peripheral Proteins – not embedded in lipid bilayer (loosely bound to the either surface) o Integral Proteins- penetrate hydrophobic core of lipid bilayer, often spanning membrane  Hydrophobic regions with nonpolar amino acid (alpha-helices) that Membranes Summary contact core  Hydrophilic amino acids where they contact aqueous environment • Reinforce shape of cell and provide framework (attach to cytoskeleton on the inside and fibers of the extracellular matrix on the outside)
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