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- Highly selective permeability barriers that contain transport proteins that regulate the solute composition of the interior
- Control the flow of information
- Mediate energy transduction
Receptors on membranes
- attractants/repellants (chemotaxis)
Tyeps of energy transduction mediated by membrane
- electron transport
- oxidative phosphorylation
- light absorption
Give an example of light absorption by a membrane.
light absorption by the purple membrane of halobacterium halobrium (salt-loving) that energizes locomotion
General membrane features
- Molecular assemblies knit together by cooperative but non-covalent forces
- Only a few molecules thick (60-100A)
- Consist mainly of lipids and proteins
- Almost all components undergo lateral but not transverse diffusion
- Assymetric w.r.t. structure
- Electrically polarized
Membrane composition of the interior and exterior are generally ___.
Membranes have a ___ carbohydrate component, ___ and ___, always on the ___ membrane surface.
small, glycolipids, glycoproteins, non-cytoplasmic
Protein to lipid ratios vary ___ to ___.
- 1:4 (myelin sheath - not very active)
- 4:1 (mitochondrial inner membrane - active)
Lipids are ___ and form a ___.
amphiphilic, closed bilayer
List functions of protein in a membrane.
- Energy transducers
Describe the structural functions of proteins in a membrane.
- Integral: part of membrane - covalent
- Peripheral: non-covalent, typically bind to outer layer of membrane (do not penetrate all the way thru)
- Membrane-anchored: properties of both - location of peripheral, covalently bound
In general, membrane components move laterally and ___.
cannot move from one side of a membrane to another.
What affects melting point temperatures of fatty acids?
- A longer length leads to a higher Tm
- A double bond leads to a lower Tm by inhibiting intramolecular contact
- A 2nd double bond isn't nearly as potent as the first.
Carbohydrates are always on ___ side of a cellular membrane.
What are the three types of membrane lipids?
What groups does a glycerophospholopid have?
- Two fatty acid: R-C=O
- Head group: P(O2)OR
How many carbons do fatty acids typically have?
C14-C20, with C16 and C18 the most common
An unsaturated fatty acid is always ___ and no double bonds are ever ___; double bond is always on ___ of glycerol.
cis, conjugated; C2
Fatty acids establish a certain ___ in membrane for membrane ___.
In bacteria, an increase in temperature leads to a ___ in fluidity.
increase (butter/fatty acids)
Glycerophospholipds have a net ___ charge at pH 7.
- Contain a sphingosine plus a fatty acid plus an R group
- H is R group => ceramide
In animals, ceramide is also the base for ___, in which the OH group at the end of a ceramide is in ___ with ___.
glycolipds, transition, a mono or oligosaccharide
What is a cerebroside?
A sphingolipid where glucose or galactose is in the beta-glycosidic linkage
What is a globoside?
A sphingolipid with ceramide base and a neutral oligosaccharide in glycosidic linkage
What is a ganglioside?
A sphingolipid with ceramide base and a negatively-charged oligosaccharide in glycosidic linkage
Waxy steroid of fat; a membrane lipid found in eukaryotes
How does adding cholesterol effect membrane Tm peak?
Adding cholesterol broadens the phase transition state, but does not change the Tm peak.
What are techniques for determining membrane structure/function?
- 1. freeze/fracture (F/F) electron microscropy
- 2. photobleach recovery
- 3. Modification of membrane permeability using ionophores
- 4. Vectorial labeling
- 5. Reconstitution
What is photobleach recovery?
The measurement of the lateral diffusion coefficient (D) of a membrane component
Describe photobleach recovery.
- Component is labelled with a fluorescent probe throughout cell
- Bleaching flash applied killing cells in target area
- Measure time for target area to fluoresce again
The distance travelled (d) is equal to ___
Describe F/F electron microscopy.
Instantly freeze a membrane sample for electron microscopy
What is an ionophore, and what are the two types?
- A lipid-soluble molecule that transports ions across the lipid bilayer of a membrane.
- Carriers and channel formers
What is a carrier? Provide an example.
- Molecule/compound which binds to an ion shielding its charge from environment facilitating transport.
- Valinomycin (K+ specific)
What is a channel former? Provide an example.
- Molecule/compound which forms an aqueous channel across the lipid bilayer that is permeable to select ions.
- Gramicidina (H+, K+, Na+)
Gramicidina is an ___ consisting of alternating ___ and ___ ___ a.a.'s.
oligopeptide, D, L, hydrophobic
The gramicidina molecule forms a curled ___ in which the NH groups point alternately ___ and ___. The R groups all point ___ toward ___.
parallel beta strand, up, down. out, membrane hydrocarbon layer.
What kind of question does vectorial labelling of membrane components answer?
Is this protein intra, trans, or inter? Is it on the inner or outer surface?
Membrane permeable reagents only access the ___ surface of a membrane.
For vectorial labelling, you might take two amino group specific reagents, a ___ one that forms a ___, and an ___ one that doesn't.
permeable, chromophore, impermeable
In vectorial labelling, describe the experiment(s) you would run.
- One experiment with permeable/chromophore and SDS page
- Second experiment with impermeable and SDS page
- Chromophore would label to inner side of protein
What is reconstitution?
Formation of articfical membranes from isolated membrane components.
What are examples of detergents used in reconstitution?
What is "critical micelle concentration" (CMC)?
The concentration of surfactants above which micelles form.
What molecules can pass thru a membrane via unmediated diffusion?
O2, CO2, H2O
For H2O, which can pass thru despite its ___ due to its ___, there are ___ if a high transport rate is required.
polarity, small size, aquaporins
How are transported solutes classfied?
- Number of species and directionality
- Net movement of charge
What are the kinds of classifications by number/directionality of species?
- Uniport: single species in one or both directions
- Symport: two species in same direction
- Antiport: two species in opposite directions
What are the kids of classifications by net movement of charge?
- Electrogenic: movement of charge
- Electroneutral: no movement of charge
Describe delta-G in terms of solute concentrations and charge.
See equation on page 7 of notes on 7Nov2011
Describe mediated passive diffusion.
A transport process moves a solute down its electrochemical gradient
Describe active transport.
A transport process moves a solute up its electrochemical gradient at an energy cost
What are two types of active transport?
- Primary: energized by ATP hydrolysis or light absorption
- Secondary: driven by diffusion of a solute gradient created by a primary process
What are the four types of active transport proteins?
- P: phosphorylated
- V: responsible for the acidification of intracellular organelles
- F: synthesize (not hydrolyze) ATP
- Multipele drug resistance (MDR)
___ is a ___ MW monomer with ___-transmembrane alpha helices that pumps a variety of ___ molecules out of the cell. ___ ATPs are ___ per solute transported.
MDR, 170000, 12, apolar. 2, hydrolyzed