Cell Membrane Structure

So I have another online quiz coming up which I'm nervous about even though it's only worth like 2%... I was about to keep frantically reading through all of the textbook chapters that are related to what we've covered in the lectures, then I realised that that really isn't an efficient use of my time. Instead I'm going to do a mixture of answering textbook questions and making questions out of the "Lecture Objectives" dot points.

For this particular post, I'm going to try my hand at answering some chapter questions. Please tell me if I've gotten something wrong so that a) I can learn from it and b) I don't confuse other people :)

Q1: "Although lipid molecules are free to diffuse in the plane of the bilayer, they cannot flip-flop across the bilayer unless enzyme catalysts called phospholipid translocators are present in the membrane." (True/False, explain why)

This statement is true. The "flip-flop" of lipids from one side of the bilayer to the other is energetically unfavourable, and thus enzymes (in this case phospholipid translocators, a.k.a. flippases, are required).

Q2: "Whereas all the carbohydrate in the plasma membrane faces outward on the external surface of the cell, all the carbohydrate on internal membranes faces toward the cytosol." (True/False, explain why)

This statement is false. The carbohydrates always face away from the cytosol, no matter whether the plasma membrane is an internal or external one. This is due to their addition in the lumen of the Golgi apparatus.

Q3: "Although membrane domains with different protein compositions are well known, there are at present no examples of membrane domains that differ in lipid concentration." (True/False, explain why)

False. Areas with high protein concentrations would have to have lower lipid concentrations, as there is less room for the lipids there. Also, the different kinds of lipids can be found in different concentrations in the two different layers of the membrane.

Q4: When a lipid bilayer is torn, why does it not seal itself by forming a "hemi-micelle" cap at the edges?

I would assume that this would have to do with the shape of phospholipids. The two tails of phospholipids give them a more cylindrical structure, as opposed to the more "conical" structure of single-tailed lipids. Hence, while the latter can form micelles, phospholipids used in membranes cannot.

Q5: Margarine is made from vegetable oil by a chemical process. Do you suppose this process converts saturated fatty acids to unsaturated ones, or vice versa? Explain your answer.

I think that this process converts unsaturated fatty acids to saturated ones. Saturated fatty acids, due to lacking "kinks" in their chains that cis-unsaturated acids have, can pack together more closely. This property may account for the greater density of margarine as compared to vegetable oil.

Q6: If a lipid raft is typically 70 nm in diameter and each lipid molecule has a diameter of 0.5nm, about how many lipid molecules would there be in a lipid raft composed entirely of lipid? At a ratio of 50 lipid molecules per protein molecule (50% protein by mass), how many proteins would be in a typical raft? (Neglect the loss of lipid from the raft that would be required to accommodate the protein.)

Note: I have been informed that my original answer to this question was incorrect- I think I overcomplicated the question the first time. This is my re-attempted version- hopefully it's correct this time! (Also, to the people who are using this site to help with homework, please have a go at doing it yourself before looking on here! You'll learn more by actually doing the problems, and you might find other mistakes that I've made- I am only human, after all ;))

First things first, we need to calculate the area of the lipid raft and the area of the lipid molecules. To do this, we can use the formula for area of a circle, which is πr². Remember, the radius is half the diameter; hence the radius of the lipid raft is 35nm and the radius of a lipid molecule is 0.25nm. This gives us the following equations:

Area of the lipid raft = π * 35²
Area of a single lipid molecule = π * 0.25²

To get the number of lipid molecules, you need to divide the area of the lipid raft by the area of a single molecule:

No. of lipid molecules = (π * 35²) / (π * 0.25²)
No. of lipid molecules = (35²) / (0.25²)
No. of lipid molecules = 19 600

Hence, there are 19600 molecules... in a monolayer of the lipid raft. Remember, lipid rafts come in bilayers, so we need to double our answer to get the number of lipid molecules altogether:

No. of lipid molecules in the bilayer = 19600 * 2 = 39 200 lipid molecules.

Next, we need to find out how many protein molecules there are. If there are 50 lipid molecules per protein molecule, then we simply need to divide the number of lipid molecules by 50 to get the number of proteins.

No. of proteins = 39 200 / 50 = 784 protein molecules.

Hopefully this answer is correct this time...!

Q7: Monomeric single-pass transmembrane proteins span a membrane with a single alpha helix that has characteristic chemical properties in the region of the bilayer. Which of the three 20-amino-acid sequences listed below is the most likely candidate for such a transmembrane segment? Explain the reasons for your choice.
A- ITLIYFGVMAGVIGTILLIS
B- ITPIYFGPMAGVIGTPLLIS
C- ITEIYFGRMAGVIGTDLLIS

For a transmembrane protein, you would expect to see some hydrophilic amino acids at either end, since they are in contact with aqueous solution, and some hydrophobic amino acids in the middle, where they are surrounded by hydrophobic lipids.

The textbook has helpfully given me "FAMILY VW" as a convenient mnemonic for the hydrophobic amino acids (phenylalanine, alanine, methionine, isoleucine, leucine, tyrosine, valine and tryptophan). Hence I need to choose the protein that has more of these letters in the middle and fewer of these letters at the end.

Here they are again, with the "FAMILY VW" letters highlighted in blue. As these are hydrophobic amino acids, I would expect to see more of them in the middle of the chains, and fewer of them at the end.

A- ITLIYFGVMAGVIGTILLIS
B- ITPIYFGPMAGVIGTPLLIS
C- ITEIYFGRMAGVIGTDLLIS

There probably isn't the clearest distinction between these three proteins, but the first one does have a few more hydrophobic amino acids in the middle (besides, the second and third both have the same number of hydrophobic and hydrophilic amino acids in the same places, and given that this is a multiple choice question, process of elimination leaves A). Hence I would choose A for my answer.

Q8: You are studying the binding of proteins to the cytoplasmic face of cultured neuroblastoma cells and have found a method that gives a good yield of inside-out vesicles from the plasma membrane. Unfortunately, your preparations are contaminated with variable amounts of right-side-out vesicles. Nothing you have tried avoids this problem. A friend suggests that you pass your vesicles over an affinity column made of lectin coupled to solid beads. What is the point of your friend's suggestion?

Lectin binds to carbohydrates, such as those that bind to proteins and lipids on the outer membrane of the cell (see my answer for Q2). Hence, an affinity column containing lectin could help to "weed out" the right-side-out vesicles by binding to carbohydrates on their cell surfaces.

Q9: Glycophorin, a protein in the plasma membrane of the red blood cell, normally exists as a homodimer that is held together entirely by interactions between its transmembrane domains. Since transmembrane domains are hydrophobic, how is it that they can associate with one another so specifically?

Transmembrane domains can associate with each other through their specific shapes, which result from their sequences of amino acids.

(I feel kind of bad for giving answers that are shorter than the questions, but moving on...)

Q10 (reworded due to lack of diagrams): There are several mechanisms by which membrane-binding proteins bend a membrane. Some of these cytosolic membrane-bending proteins would induce an invagination of the plasma membrane. Could similar kinds of cytosolic proteins induce a protrusion of the plasma membrane? Which ones? Explain how they might work.

I'm going to go through the different mechanisms that I've learned about, and then try and discuss whether or not I think they could cause protrusions.

Mechanism #1: Insertion of proteins or lipid anchors to increase the area of one side of the plasma membrane. Increasing the area of one side- but not the other- causes the membrane to curve around. This could work if there were proteins or lipid anchors in the extracellular matrix (actually I could probably use this as a cop-out answer for all of the mechanisms here). I'm not sure how this could work from the inside, however. One idea I have is that enzymes on the inside could remove proteins or lipid anchors that are on the inside of the membrane, but this would only work if there were enough proteins or lipid anchors on the outside of the membrane to cause a bending effect afterwards.

Mechanism #2: Forming a rigid scaffold that deforms the membrane or stabilises an already bent membrane. This could probably work from the inside, if you have a protein that was of a different shape.

Mechanism #2: Causing particular membrane lipids to cluster together so that their differently-sized head groups can induce curvature. If there were enough "large-headed" lipids on the extracellular layer, clustering a lot of "small-headed" lipids in the intracellular layer may cause a protrusion.