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Biological membranes
- Highly selective permeability barriers that contain transport proteins that regulate the solute composition of the interior
- Control the flow of information
- Mediate energy transduction
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Receptors on membranes
- Hormones
- Neurotransmitters
- attractants/repellants (chemotaxis)
- photons
- gustants/odorants
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Tyeps of energy transduction mediated by membrane
- electron transport
- oxidative phosphorylation
- photosynthesis
- light absorption
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Give an example of light absorption by a membrane.
light absorption by the purple membrane of halobacterium halobrium (salt-loving) that energizes locomotion
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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
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Membrane composition of the interior and exterior are generally ___.
very different
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Membranes have a ___ carbohydrate component, ___ and ___, always on the ___ membrane surface.
small, glycolipids, glycoproteins, non-cytoplasmic
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Protein to lipid ratios vary ___ to ___.
- 1:4 (myelin sheath - not very active)
- 4:1 (mitochondrial inner membrane - active)
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Lipids are ___ and form a ___.
amphiphilic, closed bilayer
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List functions of protein in a membrane.
- Receptors
- Enzymes
- Transport
- Energy transducers
- Structure
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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
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In general, membrane components move laterally and ___.
cannot move from one side of a membrane to another.
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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.
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Carbohydrates are always on ___ side of a cellular membrane.
a single
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What are the three types of membrane lipids?
- Glycerophospholipids
- Sphingolipids
- Cholesterol
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What groups does a glycerophospholopid have?
- Two fatty acid: R-C=O
- Glycerol
- Head group: P(O2)OR
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How many carbons do fatty acids typically have?
C14-C20, with C16 and C18 the most common
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An unsaturated fatty acid is always ___ and no double bonds are ever ___; double bond is always on ___ of glycerol.
cis, conjugated; C2
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Fatty acids establish a certain ___ in membrane for membrane ___.
fluidity, proteins
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In bacteria, an increase in temperature leads to a ___ in fluidity.
increase (butter/fatty acids)
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Glycerophospholipds have a net ___ charge at pH 7.
negative
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Describe sphingolipids.
- Contain a sphingosine plus a fatty acid plus an R group
- H is R group => ceramide
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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
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What is a cerebroside?
A sphingolipid where glucose or galactose is in the beta-glycosidic linkage
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What is a globoside?
A sphingolipid with ceramide base and a neutral oligosaccharide in glycosidic linkage
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What is a ganglioside?
A sphingolipid with ceramide base and a negatively-charged oligosaccharide in glycosidic linkage
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Describe cholesterol.
Waxy steroid of fat; a membrane lipid found in eukaryotes
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How does adding cholesterol effect membrane Tm peak?
Adding cholesterol broadens the phase transition state, but does not change the Tm peak.
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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
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What is photobleach recovery?
The measurement of the lateral diffusion coefficient (D) of a membrane component
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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
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The distance travelled (d) is equal to ___
sqrt(4DT)
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Describe F/F electron microscopy.
Instantly freeze a membrane sample for electron microscopy
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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
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What is a carrier? Provide an example.
- Molecule/compound which binds to an ion shielding its charge from environment facilitating transport.
- Valinomycin (K+ specific)
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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+)
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Gramicidina is an ___ consisting of alternating ___ and ___ ___ a.a.'s.
oligopeptide, D, L, hydrophobic
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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.
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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?
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Membrane permeable reagents only access the ___ surface of a membrane.
outer
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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
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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
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What is reconstitution?
Formation of articfical membranes from isolated membrane components.
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What are examples of detergents used in reconstitution?
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What is "critical micelle concentration" (CMC)?
The concentration of surfactants above which micelles form.
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What molecules can pass thru a membrane via unmediated diffusion?
O2, CO2, H2O
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For H2O, which can pass thru despite its ___ due to its ___, there are ___ if a high transport rate is required.
polarity, small size, aquaporins
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How are transported solutes classfied?
- Number of species and directionality
- Net movement of charge
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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
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What are the kids of classifications by net movement of charge?
- Electrogenic: movement of charge
- Electroneutral: no movement of charge
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Describe delta-G in terms of solute concentrations and charge.
See equation on page 7 of notes on 7Nov2011
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Describe mediated passive diffusion.
A transport process moves a solute down its electrochemical gradient
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Describe active transport.
A transport process moves a solute up its electrochemical gradient at an energy cost
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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
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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)
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___ 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
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