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Download 16-17 membrane notes
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structure and function Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein Transmembrane proteins Cytoplasm Filaments of cytoskeleton 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model Membrane structure and function PLASMA MEMBRANE- surrounds ALL cells • Made of PROTEIN and PHOSPHOLIPIDS • PHOSPHOLIPIDS = AMPHIPATHIC = Have both philic and phobic regions • Form a BILAYER with polar heads out/phobic tails in FLUID MOSAIC MODEL • Current model for animal plasma membrane • Held together by weak phobic interactions that make membrane fluid • Components can move laterally FLUIDITY of MEMBRANES • Different species have different FATTY ACID tails in their phospholipids • UNSATURATED FATTY ACIDS make “kinks” so phospholipids can’t pack as close together (remain fluid @ colder temps) CHOLESTEROL (in animal cells only) makes membranes less fluid at higher temps (keep phospholipids from moving around) makes membranes more fluid at lower temps (keep phospholipids from packing closely together) Membrane Proteins • Proteins determine membrane’s specific functions • cell membrane & organelle membranes each have unique collections of proteins • Classes of membrane proteins: • peripheral proteins • loosely bound to surface of membrane • ex: cell surface identity marker (antigens) • integral proteins • Embedded in lipid bilayer, usually across whole membrane • transmembrane protein – span entire membrane • ex: transport proteins • channels, permeases (pumps) Proteins domains anchor molecule • Within membrane • nonpolar amino acids Polar areas of protein • hydrophobic • anchors protein into membrane • On outer surfaces of membrane in fluid • polar amino acids • hydrophilic • extend into extracellular fluid & into cytosol Nonpolar areas of protein Many Functions of Membrane Proteins “Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton “Antigen” OTHER MEMBRANE COMPONENTS • CARBOHYDRATES (Ex: attached to GLYCOPROTEINS) ~ important in cell-cell recognition/immune system function and tissue development/ differentiation EX; important in blood transfusions/organ transplants/recognition of invaders by immune system FUNCTION OF PHOSPHOLIPIDS • phospholipid tails provide phobic barrier • separate cell from liquid environment • SELECTIVELY PERMEABLE- due to hydrophobic/hydrophilic arrangement of phospholipids ~ allows certain molecules to pass through- non-polar, hydrophobic, gases (hydrocarbons, CO2 & O2) ~ prevents other substances from passing through- ions, polar/hydrophilic, large TRANSPORT PROTEINS/VESICLES assist movement of substances across bilayer that can’t go on their own