Membrane Composition And Structure Flashcards
(36 cards)
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
Lipids
-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
Liposomes
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
- Liquid-crystalline phase
2. Gel phase
- More fluid like phase
- 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 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
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
