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The Plasma Membrane • Functions – Compartmentalization • Plasma membrane, nuclear envelope, mitochondria, chloroplast, lysosome, peroxisome, liposome – Scaffold for biochemical activities • Enclose compartments and are compartments themselves – Ubiquinone dissolved in inner mitochondrial membrane – Diacylglycerol in plasma membrane The Plasma Membrane • Functions – Selective permeability barrier • Prevent unrestricted movement of molecules • Maintain gradients – Transport solutes • contains channels and pores that allow selective movement and establish gradients The Plasma Membrane • Functions – Signal transduction • Location of receptors, some effectors, 2nd messenger precursors (PIP2) – Cell-ECM, cell-cell interactions • Integrins, cadherins – Energy transduction • Proton gradients coupled to ATP synthesis (mitochondria, chloroplast) The Plasma Membrane • Structure – Outer leaflet faces extracellular space – Inner leaflet faces cytoplasm The Plasma Membrane • Structure – Lipid bilayer • Thin sheet held together noncovalent interactions – Fluid mosaic model • Lipids are in a fluid mobile state, not static The Plasma Membrane • Composition • Amphipathic • Phospholipids • Glycerol backbone » Head group » Phosphate » Glycerol » 2 fatty acids (C18) • Composition – Phosphoglycerides – Head group types • Choline (+ charge) • Serine • Ethanolamine (+ charge) • Inositol The Plasma Membrane • Sphingolipids – No glycerol backbone – Sphingosine – Ceramide = sphingosine + fatty acid (amide bond) – Sphingomyelin = choline + phosphate + ceramide – Glycolipids = sugar + ceramide • Cerebrosides and gangliosides • Essential in neuronal function • Cholesterol – Up to 50% of animal membranes • Absent from most plant cells and all bacteria • Planar ring restricts movement of lipid tails The Plasma Membrane • Membrane carbohydrates – ~ 90% attached to protein remainder = glycolipid – Glycosylation: attachment of sugar to a protein – Modification on proteins at amino acids N, S, T – All carbohydrates face away from cytosol • Plasma membrane glycoprotein carbohydrates face outside cell • ER glycoprotein carbohydrates face into lumen The Plasma Membrane • Membrane carbohydrates – Blood type glycolipid • A = allele 1 • B = allele 2 • O = null • AB = allele 1 / allele 2 heterozygote The Plasma Membrane • Membrane proteins – Integral • Receptors, channels, transporters The Plasma Membrane • Membrane proteins – Peripheral • Nuclear lamina • Membrane skeleton The Plasma Membrane • Membrane proteins – Lipid-anchored • Outside = GPI-anchor = Glycosyl-Phosphatidyl-Inositol • Cytosolic = hydrocarbon (palmitoyl) attached (e.g Ras, G-alpha-s/i/q) The Plasma Membrane • Membrane fluidity – Liquid state versus frozen crystalline state • Either extreme is undesirable The Plasma Membrane • Membrane fluidity – Transition temp, melting temp (Tm) • Saturated vs unsaturated C-C bonds – Saturated fatty acids pack well, increase Tm – Unsaturated pack less well, decrease Tm • Length – Long, increase Tm – Short, decrease Tm The Plasma Membrane • Membrane fluidity – Transition temperature, melting temperature (Tm) • Cholesterol disrupts packing but also reduces fatty acid mobility – Broadens melting curve – Increases membrane durability – Decreases permeability (membrane is more hydrophobic) The Plasma Membrane • Movement – Lateral diffusion within a leaflet is possible – Flipping from one leaflet to the other is thermodynamically unfavorable The Plasma Membrane • Membrane skeleton – Required for mechanical strength of membrane – Disease: muscular dystrophy • Mutations in dystrophin – spectrin family member in muscle • Contraction of muscle leads to disruption of the outer membrane The Plasma Membrane • Membrane skeleton – Spectrin alpha/beta heterodimer • Forms a 200nm flexible, elastic fiber • Binds ankyrin, actin, tropomyosin – Ankyrin binds “Band 3” transmembrane protein The Plasma Membrane • Transport – Diffusion: [high] to [low] is spontaneous – H2O reasonably free to diffuse • Aquaporins allow faster movement • Transport – Ions and polar compounds are largely unable to penetrate lipid bilayers The Plasma Membrane • Ion channels – Ligand-gated • allostery – Mechano-gated • Mechanical force on membrane (stretch, compression, etc) – Voltage-gated • Charge difference across membrane – Resting state ~ -70mV – More negative inside cell – Selectivity • K+ ~ 0.27nm diameter • H2O ~ 0.28nm diameter • Na+ ~ 0.19nm diameter • H+ < 0.1 nm diameter – How to let K+ in without letting Na+ in • K+ Ion channels • K+ ~ 0.27nm diameter • H2O ~ 0.28nm diameter • Na+ ~ 0.19nm diameter • H+ < 0.1 nm diameter – Selectivity filter • Backbone carbonyl groups arranged to form “rings” with a 0.30nm diameter • Can substitute for water hydration shell around an ion • Smaller ions have far less favorable bonding interactions with “rings” The Plasma Membrane • Facilitated diffusion – Transporter catalyzes movement from one side to other, driven by movement down concentration gradient – 1st step of glycolysis lowers cytoplasmic concentration – Can be regulated The Plasma Membrane • Active transport – Requires E input to move substances against a concentration gradient [K+]in = 100mM [Na+]in = 10mM [K+]out = 5mM [Na+]out = 150mM – E sources, ATP hydrolysis, light, other substances moving down gradient – Na+/K+ ATPase • ATP hydrolysis achieves: – 3Na+ moved out – 2K+ moved in » Unequal ratio (3:2) directly contributes to net negative charge inside cell (resting potential ~ -70mV) – Ca2+ ATPase (for ER Ca2+) – H+/K+ ATPase secretes acid into stomach (0.16 N HCl, ~ pH 1) The Plasma Membrane • Active transport – H+/K+ ATPase secretes acid into stomach (0.16 N HCl, ~ pH 1) – Prilosec directly inhibits H+/K+ATPase (irreversible) The Plasma Membrane • Co-transport – Use E in existing gradients – Also known as “secondary active transport” – Na+/glucose cotransporter • Symporter • 2Na+ move in • 1 glucose moves in • Can work against a 20,000 fold gradient – Na+/H+ antiporter • Na+ moves in • H+ moves out • Regulates cytoplasmic pH