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BIOL 331 (1)
Lecture 3

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BIOL 331
Bruce Reed

Chapter 10 The Lipid Bilayer and Membrane Proteins From the previous lecture... FRET - Protein-protein interactions are key to understanding how cells work - Co-localization of suspected protein partners using immunofluorescence microscopy in fixed cells is possible and popular - Bu the resolution of the fluorescence microscope is much less than the size of a typical protein, co-localization is often a questionable result - FRET allows direct observation of molecular interactions (2-5 nm) in living cells The Plasma Membrane is Crucial to Cellular Life - The cell membrane separates the cell from its environment and creates intracellular compartments (in eukaryotes) - Ion gradients across the membrane are used to make ATP, to allow electrical signals in nerve and muscle cells and to drive transmembrane movement of solutes - All biological membranes have a common structure: a thin layer of lipid molecules with proteins held together by non-covalent interactions - Membranes are fluid, dynamic structures - Lipids found as a bilayer are about 5 nm thick - The lipid bilayer is impermeable to most water soluble molecules - Transmembrane proteins A) Transport specific molecules across the membrane B) Catalyze membrane associated reactions (ATP synthesis) C) Are structural links or cell surface receptors - An estimated 30% of all proteins in the genome are membrane proteins The Lipid Bilayer - The lipid bilayer forms the basic structure of all biological membranes - Lipid bilayer is easily seen by TEM - Structure of the lipid bilayer is inherent to the properties of lipid molecules - Lipid molecules will assemble spontaneously to form lipid bilayers under simple artificial conditions - Animal cell membranes by mass are half lipid/half protein - All lipid molecules in cell membranes are amphipathic (a hydrophobic tail and a hydrophilic head) - Most abundant membrane lipids are the phospholipids (a polar head group and two hydrocarbon tails usually fatty acids 12-24 C atoms) - One of the two tails is usually saturated, the other usually one or more cis C=C bonds - The main phospholipids are the phosphoglycerides (3C glycerol backbone) The Phosphoglycerides E.g. phosphatidylcholine - The four major phospholipids are half of all the membrane lipids by mass Note: Phosphatidylserine carries a net negative charge Cholesterol - Also an important part of the lipid bilayer Lipid Shape and Amphiphilic Nature - Spontaneous assembly of bilayers or micelles - Free edge with water is energetically unfavourable Lipid Bilayer A 2D Fluid - Synthetic lipid bilayers (spheres 25 nm 1 m) liposomes, bilayers - Each side of a bilayer is sometimes called a leaflet - Motion of phospholipids can be measured in artificial membranes (by fluorescent labelling or by electron spin resonance) A) Lateral diffusion is extremely rapid (length of a bacterial cell in 1 s!) B) Flip flop (one leaflet to the other) is extremely rare C) Rapid rotation along long axis, flexible tails, a disordered system
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