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Flashcards in Membrane Composition And Structure Deck (36):

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)


Membrane composition

-lipids, proteins and carbohydrate containing molecules.


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



-compounds soluble in organic solvent (chloroform/methanol)


Major lipids of membranes

-phospholipids, glycosphingolipids, and cholesterol
-fatty acids and triacylglycerols may be found as very minor constituents


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.


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



Spontaneously formed and similar to a membrane


Integral protein features

-require detergents to separate them from the membrane
-interact extensively with the hydrophobic core of the membrane


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


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


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)


Plasma membrane characteristics

-greatest amount of cholesterol
-Major components of Phosphatidylcholine, Phosphatidylserine, Posphatidylethanolamine, Phosphatidylinositol, and Sphingomyelin


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


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


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)


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


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


Membrane fluidity

-bilayer lipids (under most physiological conditions) can exist in either liquid-crystalline phase or in a gel phase


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)


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


Fatty acid composition and membrane fluidity

-Unsaturated fatty acids have a lower melting temp
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


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


Simple diffusion, non-mediated

-restricted to hydrophobic molecules or small uncharged polar molecules
Ex. H2O, O2, CO2, urea


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


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


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


Active transport

-requires a specific transport protein
-Requires input of energy
A) ATP hydrolysis to ADP and Pi
B) An ion concentration gradient



Active transport
-only one solute transported



Active transport
-two (or mor) different salutes transported in the same direction



Active transport
-two (or more) different salutes transported in the opposite directions



-transport results in no difference in charge across the membrane



-transport result in a charge difference across the membrane


Passive mediated characteristics

-saturation kinetics
-can be specifically inhibited
-solute moves down the concentration gradient
-no energy expenditure


Active mediated

-saturation kinetics
-can be specifically inhibited
-solute move against concentration gradient
-energy required


Na+ K+ ATPase

-electrogenic antiport active transport protein
-responsible for transporting 3 Na+ out of the cell, as it (obligatorily) transports 2 K+ into the cell at the expense of 1 ATP