Flashcards in Membrane Composition And Structure Deck (36):
1
Membrane components
-accomplish transfer of specific substances across the membrane
-cell's communication with the extracellular environment (hormone receptors, cell-cell contact, cell-extracellular matrix interactions)
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Membrane composition
-lipids, proteins and carbohydrate containing molecules.
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Membrane carbohydrate portion
-covalently bound to either proteins (glycoproteins) or to lipids (glycolipids)
-carbohydrate portion of glycolipids extends only into the extracellular space. These lipids are restricted to the outer leaflet of the plasma membrane
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Lipids
-compounds soluble in organic solvent (chloroform/methanol)
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Major lipids of membranes
-phospholipids, glycosphingolipids, and cholesterol
-fatty acids and triacylglycerols may be found as very minor constituents
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Lipid membrane variability
-Mitochrondia inner membrane has high ratio of protein to lipid (transporters and communication b/t cells)
-Myelin has low protein to lipid ratio since it is for insulation.
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Amphipathic nature
Hydrophilic: Phosphate, amino acid derivatives (choline, ethanolamine), and carbohydrate
Hydrophobic: the fatty acid
Form bilayer in an aqueous environment with fatty acid tails facing each other
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Liposomes
Spontaneously formed and similar to a membrane
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Integral protein features
-require detergents to separate them from the membrane
-interact extensively with the hydrophobic core of the membrane
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Integral proteins (usually) span the membrane features
-membrane spanning domains may be alpha-helical (usually about 20 amino acyl residues in an alpha-helix will span the membrane)
-membrane spanning proteins may also be beta-sheets
-may span one of many (7) times
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Peripheral proteins
-do not require detergents to separate them from the membrane
-do not extensively interact with the hydrophobic core of the membrane
-may interact with a lipid component, to with an integral membrane protein
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Membrane lipid variation
Cardiolipin- present in Mitochondria membrane (18%)- restricted to mitochondria
Sphingomyelin- highest membrane percentages in Lysosomes (20%) and Plasma membranes (16%)
Cholesterol- most commonly found in plasma membrane (0.13 mg cholesterol/mg protein)
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Plasma membrane characteristics
-greatest amount of cholesterol
-Major components of Phosphatidylcholine, Phosphatidylserine, Posphatidylethanolamine, Phosphatidylinositol, and Sphingomyelin
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Phospholipids in the membrane
-PC and sphingomyelin are located primarily in the outer leaflet of the plasma membrane
-PS and PE are located primarily in the inner leaflet of the plasma membrane
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Leaflet movement of phospholipids
-Lateral movement of phospholipids is common, but movement from one leaflet to the other occurs only rarely
-Enzymes (translocases and/or flipases) which catalyze the movement of lipids from one leaflet to the other
-These enzymes are responsible for maintaining the lipid asymmetry of the plasma membrane
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Proteins orientation in the membrane
-Orientation is greatly associated with the function of the protein.
-Correct orientation of the protein is usually a function of the synthesis of the protein (membrane insertion and orientation signals are an inherent part of the proteins amino acid sequence)
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Protein in membranes
-may be anchored by the interaction with other cellular elements either outside (extracellular matrix) of inside (cytoskeleton) of the cell.
-Carb portion of the glycoproteins extends only into the extracellular space, so glycoproteins are restricted to the plasma membrane
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Membrane domains
-various regions or "patches" of a membrane may be associated with a specialized function
-"patches" of "domains" may have not only a specific protein composition, but may have a lipid composition which differs measurably from the rest of the membrane
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Membrane fluidity
-bilayer lipids (under most physiological conditions) can exist in either liquid-crystalline phase or in a gel phase
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1. Liquid-crystalline phase
2. Gel phase
1. More fluid like phase
2. More solid-like phase
-for a given composition of lipids, there is a temperature at which this phase transition occurs, called the transition temperature (Tm)
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Physiological consequences of the phase of lipids in a membrane
-lipid bi-layer is less permeable to small molecules when in the gel phase (temperature below the Tm)
-Activities of membrane associated proteins may be altered by changes in the membrane fluidity
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Fatty acid composition and membrane fluidity
-Unsaturated fatty acids have a lower melting temp
Ex:
stearic (18:0) melting temp of 69.6*C
Oleic (18:1) melting temp of 13.4*C
linoleic (18:2) melting temp -5*C
-increasing the unsaturated fatty acids in phospholipid increases the membrane fluidity
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Cholesterol function
-appears to modulate the fluidity of the membrane
-at temps below Tm it increases fluidity
-at temps above the Tm is limits the fluidity
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Simple diffusion, non-mediated
-restricted to hydrophobic molecules or small uncharged polar molecules
Ex. H2O, O2, CO2, urea
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Simple diffusion, mediated by channel proteins
-channel proteins forms a water-filled pore through the membrane
-channel proteins still display exquisite selectively in the molecule they allow through the pore
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Facilitated diffusion
-transport (or carrier) protein is required
-proteins do not form pores, but interact with the transported molecule and due to some change in the molecule-protein complex, release the molecule on the opposite side of the membrane
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Passive diffusion
-no energy input is required for the transport
-the flow of net transport is always from higher concentration to lower concentration
Ex: simple and facilitated diffusion
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Active transport
-requires a specific transport protein
-Requires input of energy
A) ATP hydrolysis to ADP and Pi
B) An ion concentration gradient
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Uniport
Active transport
-only one solute transported
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Symport
Active transport
-two (or mor) different salutes transported in the same direction
-obligatory
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Antiport
Active transport
-two (or more) different salutes transported in the opposite directions
-obligatory
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Electroneutral
-transport results in no difference in charge across the membrane
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Electrogenic
-transport result in a charge difference across the membrane
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Passive mediated characteristics
-saturation kinetics
-specific
-can be specifically inhibited
-solute moves down the concentration gradient
-no energy expenditure
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Active mediated
-saturation kinetics
-specific
-can be specifically inhibited
-solute move against concentration gradient
-energy required
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