Chp 11: Short Answer Flashcards
List major components of membranes
Phospholipids, sterols, integral/peripheral proteins
Plasma membrane of animal cell consists of 45% phospholipid, 55% protein. What is the mole ratio if avg molecular weight of phospholipids is 750 and avg MW of membrane proteins is 50,000?
In 100 grams of membrane, there are 45g/750 g/mol= .06 mol phospholipid and 55/50,000 g/mol= .0011 mol protein. So .06/.0011= 55 moles of lipid/ moles of protein
When mitochondrial membranes are treated with high salt, its observed that 40% total protein is solubilized. What kind of membrane proteins are in the soluble extract and what forces normally hold them to the membrane?
Peripheral membrane proteins, ionic and H bonds btwn their charged/polar side chains and the charged head groups of phospholipids
What kind of proteins constitute the insoluble 60% and what forces hold them to the membrane?
Integral membrane proteins (hydrophobic inxns btwn nonpolar side chains and hydrophobic core) and peripheral membrane proteins that are held by lipid anchors
Principal features of the fluid mosaic model:
1) lipid bilayer that lipids can move laterally but not across the bilayer 2) integral membrane proteins that can penetrate or span the bilayer, lateral mobility 3) peripheral membrane proteins, noncovalently associated with lipid head groups and are sometimes held down with a lipid anchor
How do amphipathic compounds contribute to the structure of biological membranes?
When added to water, amphipathic compounds tend to arrange in a way that exposes hydrophilic regions to the solvent and hides the hydrophobic regions. The lipid bilayer is an example of this.
When fatty acids are in water, they form:
Micelles
When membrane phospholipids are dissolved in water, they form:
Bilayers
Why are micelles and bilayers energetically favored?
Micelles are favored when the cross sectional area of head groups is larger than that of the lipid tails. Bilayers form when the cross sectional areas are equal.
What are the forces that contribute to the formation of the micelles and bilayers?
Stabilized by the energy gain from burying hydrophobic groups out of reach of water. Hydrophobic groups force the formation of organized water molecules around them; minimizing this organization by minimizing hydrophobic SA is entropically favored
Why are phosphoglycerides able to form lipid bilayers while triacylglycerols are not?
The whole triacylglycerol molecule is hydrophobic while phosphoglycerides are amphipathic; being amphipathic means that you spontaneously form the lipid bilayer
What forces drive lipid bilayer formation?
Stabilized by energy gain of keeping hydrophobic areas away from water
How can you test which proteins are exposed at the outer surface and which are transmembrane?
Have two radiolabelling agents, one of which can enter the cell and the other can’t. You can assess which proteins have just the non-permeating label and which ones have both
How can you test if phosphatidylethanolamine and phosphatidylserine are symmetrically disposed in the two faces of the bilayer?
Use two different radiolabels; one can only label the first while the other can only label the latter. Analyze the amount of labeling in each bilayer leaflet
One reagent labels phosphatidylethanolamine on both lipid bilayer leaflets, one labels only one side. What experiment can you run to determine which leaflet pa is found in.
If pa is in both leaflets in equal concentration, then the one reagent should have twice the concentration as the other. Deviations from this ratio indicate asymmetry
Difference btwn integral and peripheral proteins?
Integral membrane proteins are firmly associated with the membrane; hydrophobic domains interact with the hydrophobic core. Peripheral membrane proteins are more loosely associated, and do not penetrate into hydrophobic core
What forces/bonds hold anchor integral membrane proteins?
Hydrophobic inxns between hydrophobic domain of protein and hydrophobic core
What forces hold a peripheral membrane protein to membrane?
Ionic/charged polar inxns between charged aa side chains and the hydrophilic head group of lipid bilayer
How do you solubilize peripheral vs integral membrane proteins?
Integral: detergents to convert the membrane into micelles
Peripheral: buffers with high/low salt concentration or pH or divalent cations
Describe the location of types of aa in an integral membrane protein and the ability of that protein to move within the membrane
Hydrophobic aa in hydrophobic core; charged aa’s at the hydrophilic surface. Protein can diffuse laterally, but can’t move across the lipid bilayer
If hydrophobic interior were 3nm thick, whats the minimum # of aa’s in a transmembrane alpha helix?
Alpha helices are 5.4 angstrom/turn; 3.6 residues/turn. One residue= 1.5 angstrom or .15 nm. Need 20 residues
What is a hydropathy plot?
Graphical representation of average hydropathy values of contiguous aa’s. X axis= residue number; Y-axis= hydropathy index
Why is a hydropathy plot useful and what are its limitations?
Uses: pattern of areas of high hydrophobicity means transmembrane protein
Limitation: non-transmembrane proteins also have areas of hydrophobicity, so can give a false positive
What temp would you expect Ecoli membrane to have higher ratio of saturated/unsaturated fats? Why?
Higher temp has higher ratio because want to maintain fluidity. Temp increase also increases fluidity, so to maintain it you need to increase saturated lipids to lower fluidity back down
2 ways a plant can adjust cell membrane components to keep them as fluid as possible on a cold winter morning?
Increase unsaturated fatty acids, use shorter chain fatty acids
If you increase tolerance to cold, you decrease heat tolerance. Why is this a thing?
To be healthy at cold temp, need an abnormally high amount of unsaturated fatty acids. At high temp, you can’t get rid of enough unsaturated fatty acids and thus the fluidity of the membrane is too much. This causes cell death
What is “transition temp” of a membrane? List two characteristics and whether they increase or decrease this temp
Transition temp= temp at which structure changes from a solid to a liquid
Degree of unsaturation (more unsaturated lowers tt) and chain length (longer chain increases tt)
Glycosphingolipids and cholesterol cluster together in membrane regions to form:
Rafts
Rafts are more___ than the surrounding phospholipid rich membrane due to the high concentration of ___ fatty acids
Ordered; saturated
Proteins can be anchored to rafts through ____ and ____. Functions of these proteins include ____ and ____.
Myristoyl, palmitoyl, receptor, signalling
Compare and contrast integrins, cadherins, and selectins
All are involved in cell adhesion. Integrins adhere proteins to ECM. Cadherins adhere cells to other cells through cadherins. Selectins adhere cells to other cells through inxns with polysaccharides on the opposing cell surface
Between simple diffusion, facilitated diffusion, and active transport, which can be saturated by substrate?
Facilitated diffusion and active transport
How much energy does it take to transport an uncharged substrate in if its starting inside concentration is 10-fold greater than outside?
5.7 kJ/mole
Why can nonpolar compounds diffuse across a membrane w/o help?
Its energetically favorable for them to enter the hydrophobic core
Why does being amphipathic account for the impermeability of biological membranes to polar compounds and ions?
The energy cost of sending a polar molecule through a nonpolar region and leaving water is incredibly high
Compare/contrast structure and activity of membrane transport protein and a typical enzyme.
Similar: both have domains that are specific to bind their substrate via weak inxns; stereospecific and saturable.
MT spans the membrane with hydrophobic areas in the hydrophobic core. Enzymes keep hydrophobic aa’s on the interior
Compare/contrast antiport and symport. Which one is NaK ATPase?
Both are cotransporters, symport = same direction. Antiport= opposite directions. NaK ATPase is antiport that moves 2 K in for every 3 Na out
3 differences between ion channels and ion transporters
1) rate of mvmt through ion channels is much greater 2) ion transporters are saturable, ion channels are not 3) ion channels open/close in response to external stimuli (ie electric potential)
