biological membranes Flashcards
hydrophobic effect
stabilizes structures (lipid bilayers & vesicles) in which lipids with some polar and some nonpolar regions can protect their nonpolar regions from interaction with the very polar solvent, water
biological membranes functions
define external boundaries, control molecular traffic, divide cells-organelles, organize reaction sequences, cell-to-cell communication, transporters, receptors, adhesion molecules, within the cell-energy transduction
glycerophospholipids, sphingolipids, and sterols are virtually insoluble in water ->
spontaneously form microscopic lipid aggregates when mixed with water
micelles
spherical structures containing amphipathic molecules
bilayer
lipid aggregate in which two lipid monolayers (leaflets) form a two-dimensional sheet
vesicle (liposome)
forms spontaneously when a bilayer sheet folds back on itself to form a hollow sphere
the _________________ of each membrane type is reflected in its unique lipid composition
functional specialization
single membrane surrounds…
endoplasmic reticulum, golgi apparatus, lysosomes, various small vesicles
double membrane surrounds…
nucleus, mitochondrion, chloroplasts
fluid mosaic
pattern formed by individual lipid and protein units in a membrane -> pattern can change while maintaining the permeability of the membrane
transporters
move specific organic solutes and inorganic ions across the membrane
receptors
sense extracellular signals and trigger molecular changes in the cell
ion channels
mediate electrical signaling between cells
adhesion molecules
hold neighboring cells together
peripheral membrane proteins
associate with the membrane through electrostatic interactions and hydrogen bond -> released by mild detergents or by interfering with charge/hydrogen bonding
integral membrane proteins
firmly embedded within the lipid bilayer; monotopic, polytopic, removable only by hydrophobic agent
amphitropic proteins
associate reversibly with membranes- found in both membranes and the cytosol
monotopic proteins
have small hydrophobic domains that interact with only a single leaflet of the membrane
bitopic proteins
span the bilayer once, extending on either surface- have a single hydrophobic sequence somewhere in the molecule
polytopic proteins
cross the membrane several times- have multiple hydrophobic sequences of ~20 residues that each cross the membrane when in the a-helical conformation
type 1 integral membrane protein
glycophorin
type III integral membrane protein
bacteriorhodopsin
hydropathy index
expresses the free-energy change associated with the movement of an amino acid side chain from a hydrophobic environment to water- ranges from highly exergonic to highly endergonic
deltaG<0
hydrophilic
deltaG>0
hydrophobic
hydropathy plot
average hydropathy index plotted against residue number
window: segment of given length
hydropathy index (y-axis): average hydropathy for a window
residue number (x-axis): residue in the middle of the window
B-barrel
structural motif in which 20+ transmembrane segments form B sheets that line a cylinder- stabilized by intrachain hydrogen bonds
porins
proteins that allow certain polar solutes to cross the outer membrane of gram-negative bacteria
what side chains serve as membrane interface anchors?
tyr & trp
positive-inside rule
positively charged Lys & Arg residues in the extramembrane loop of membrane proteins occur more commonly on the cytoplasmic face
membrane trafficking
process by which membrane lipids and proteins that are synthesized in the ER move to their destination organelles or to the plasma membrane
lipids and proteins undergo____________ in the _____. this dictates the eventual location of the mature protein.
covalent modifications; golgi
glycosylation
attachment of oligosaccharides to proteins- typically on the outer face of the plasma membrane; attachment of 1+ lipids: serve as hydrophobic anchors or targeting tags
GPI-anchored protein
exclusively on the outer face and are clustered in certain regions
