Chp 11: Short Answer

Question 0

List major components of membranes

Answer 0

Phospholipids, sterols, integral/peripheral proteins

Question 1

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?

Answer 1

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

Question 2

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?

Answer 2

Peripheral membrane proteins, ionic and H bonds btwn their charged/polar side chains and the charged head groups of phospholipids

Question 3

What kind of proteins constitute the insoluble 60% and what forces hold them to the membrane?

Answer 3

Integral membrane proteins (hydrophobic inxns btwn nonpolar side chains and hydrophobic core) and peripheral membrane proteins that are held by lipid anchors

Question 4

Principal features of the fluid mosaic model:

Answer 4

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

Question 5

How do amphipathic compounds contribute to the structure of biological membranes?

Answer 5

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.

Question 6

When fatty acids are in water, they form:

Answer 6

Micelles

Question 7

When membrane phospholipids are dissolved in water, they form:

Answer 7

Bilayers

Question 8

Why are micelles and bilayers energetically favored?

Answer 8

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.

Question 9

What are the forces that contribute to the formation of the micelles and bilayers?

Answer 9

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

Question 10

Why are phosphoglycerides able to form lipid bilayers while triacylglycerols are not?

Answer 10

The whole triacylglycerol molecule is hydrophobic while phosphoglycerides are amphipathic; being amphipathic means that you spontaneously form the lipid bilayer

Question 11

What forces drive lipid bilayer formation?

Answer 11

Stabilized by energy gain of keeping hydrophobic areas away from water

Question 12

How can you test which proteins are exposed at the outer surface and which are transmembrane?

Answer 12

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

Question 13

How can you test if phosphatidylethanolamine and phosphatidylserine are symmetrically disposed in the two faces of the bilayer?

Answer 13

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

Question 14

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.

Answer 14

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

Question 15

Difference btwn integral and peripheral proteins?

Answer 15

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

Question 16

What forces/bonds hold anchor integral membrane proteins?

Answer 16

Hydrophobic inxns between hydrophobic domain of protein and hydrophobic core

Question 17

What forces hold a peripheral membrane protein to membrane?

Answer 17

Ionic/charged polar inxns between charged aa side chains and the hydrophilic head group of lipid bilayer

Question 18

How do you solubilize peripheral vs integral membrane proteins?

Answer 18

Integral: detergents to convert the membrane into micelles

Peripheral: buffers with high/low salt concentration or pH or divalent cations

Question 19

Describe the location of types of aa in an integral membrane protein and the ability of that protein to move within the membrane

Answer 19

Hydrophobic aa in hydrophobic core; charged aa’s at the hydrophilic surface. Protein can diffuse laterally, but can’t move across the lipid bilayer

Question 20

If hydrophobic interior were 3nm thick, whats the minimum # of aa’s in a transmembrane alpha helix?

Answer 20

Alpha helices are 5.4 angstrom/turn; 3.6 residues/turn. One residue= 1.5 angstrom or .15 nm. Need 20 residues

Question 21

What is a hydropathy plot?

Answer 21

Graphical representation of average hydropathy values of contiguous aa’s. X axis= residue number; Y-axis= hydropathy index

Question 22

Why is a hydropathy plot useful and what are its limitations?

Answer 22

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

Question 23

What temp would you expect Ecoli membrane to have higher ratio of saturated/unsaturated fats? Why?

Answer 23

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

Question 24

2 ways a plant can adjust cell membrane components to keep them as fluid as possible on a cold winter morning?

Answer 24

Increase unsaturated fatty acids, use shorter chain fatty acids

Question 25

If you increase tolerance to cold, you decrease heat tolerance. Why is this a thing?

Answer 25

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

Question 26

What is “transition temp” of a membrane? List two characteristics and whether they increase or decrease this temp

Answer 26

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)

Question 27

Glycosphingolipids and cholesterol cluster together in membrane regions to form:

Answer 27

Rafts

Question 28

Rafts are more___ than the surrounding phospholipid rich membrane due to the high concentration of ___ fatty acids

Answer 28

Ordered; saturated

Question 29

Proteins can be anchored to rafts through ____ and ____. Functions of these proteins include ____ and ____.

Answer 29

Myristoyl, palmitoyl, receptor, signalling

Question 30

Compare and contrast integrins, cadherins, and selectins

Answer 30

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

Question 31

Between simple diffusion, facilitated diffusion, and active transport, which can be saturated by substrate?

Answer 31

Facilitated diffusion and active transport

Question 32

How much energy does it take to transport an uncharged substrate in if its starting inside concentration is 10-fold greater than outside?

Answer 32

5.7 kJ/mole

Question 33

Why can nonpolar compounds diffuse across a membrane w/o help?

Answer 33

Its energetically favorable for them to enter the hydrophobic core

Question 34

Why does being amphipathic account for the impermeability of biological membranes to polar compounds and ions?

Answer 34

The energy cost of sending a polar molecule through a nonpolar region and leaving water is incredibly high

Question 35

Compare/contrast structure and activity of membrane transport protein and a typical enzyme.

Answer 35

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

Question 36

Compare/contrast antiport and symport. Which one is NaK ATPase?

Answer 36

Both are cotransporters, symport = same direction. Antiport= opposite directions. NaK ATPase is antiport that moves 2 K in for every 3 Na out

Question 37

3 differences between ion channels and ion transporters

Answer 37

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)