Practice Questions - Week 5 - Lipids and biomembranes

Question 1

Which plasma membrane component can be either found on its surface or embedded in the membrane structure?

a) protein

b) cholesterol

c) carbohydrate

d) phospholipid

Answer 1

a)

(Membrane proteins can be embedded, or attached to the surface of the membrane (via covalent bonds to lipids, or via ionic bonds and H-bonds (peripheral membrane proteins)). Cholesterol and phospholipids are part of the lipid bilayer; carbohydrates are exposed at the surface of the membrane)

Question 2

The properties of a lipid bilayer are determined by the structures of its lipid molecules. Predict the properties of the lipid bilayers that would result if the following were true:

(Match the following)

Left side:

a. Phospholipids had only one hydrocarbon chain instead of two

b. All of the hydrocarbon chains were saturated

c. All of the hydrocarbon chains were unsaturated

d. The hydrocarbon chains were shorter than normal (e.g. about 10 carbon atoms long)

Right side:

1. The shape of the molecule would be a cone and would form micelles instead of bilayers
2. Bilayer would be more rigid, because the lipids would pack better
3. Bilayer would be more fluid because the lipids would pack together less well
4. Bilayers would be less hydrophobic and the forces driving formation of the bilayer reduced

Answer 2

a-1
b-2
c-3
d-4

Question 3

Glycolipids are not found on the cytoplasmic face of membranes in living cells. True or False?

Answer 3

True

(Glyclipids are found on the outer leaflet of the cell membrane, exposed at the cell surface)

Question 4

Predict which one of the following organisms will have the highest % of unsaturated fatty acid chains in their membranes.

antarctic fish

desert iguana

polar bear

human being

Answer 4

antarctic fish

(Antarctic fish, they are cold-blooded, live in freezing waters. To maintain fluidity in their membranes under these extreme conditions, they require a higher proportion of unsaturated FA chains, which make the membranes more fluid than saturated FA chains)

Question 5

Lipid rafts are not thicker than other parts of the bilayer. True or False?

Answer 5

False

(Lipid rafts are thicker because they are enriched with sphingolipids and cholesterol. FA chains of sphingolipids are normally longer (C20-26) and have fewer double bonds, which increases the length and van der Waals interactions. Furthermore, cholesterol with its rigid 4-rings structure increases the rigidity of these domains which further increase the length of the bilayer)

Question 6

Which of the following lipid molecules possess a different fundamental structural make-up from the others?

fatty acids

triacylglycerides

cholesterol

glycolipids

Answer 6

cholesterol

(cholesterol - does not have a long FA chain as part of its makeup)

Question 7

Fatty acids with an odd number of carbons are rare, compared to those with an even number. True or false?

Answer 7

True

Question 8

Unsaturated fatty acids usually have trans double bonds. True or False?

Answer 8

False

(Double bonds in fatty acids normally have a Cis conformation)

Question 9

Which of the following lipids is not found in biological membranes?

triacylglicerides

phospholipids

sphingolipids

cholesterol

Answer 9

triacylglicerides

Question 10

Which of the following groups is not present in sphingomyelin?

sphingosine

sugar

phosphate

fatty acid

Answer 10

sugar

Question 11

Cell membranes typically display asymmetry. What does this mean?

a) Membrane lipids only have cis double bonds, never trans

b) Only one stereoisomer is ever seen at the chiral C2 of the glycerol residue in a membrane lipid

c) The fatty acyl chain on C1 of a membrane lipid is usually different from that on C3

d) the two leaflets in the membrane bilayer are composed of different lipids and proteins

Answer 11

d)

Question 12

Cholesterol tends to increase membrane fluidity. True or False?

Answer 12

False

Question 13

Membrane proteins embedded in the membrane bilayer always involves ___________ with lipids

a) electrostatic interactions

b) hydrophobic interactions

c) covalent linkages

d) all of the above

Answer 13

b)

Question 14

Biological waxes are composed of

a) glycerol and one fatty acid

b) glycerol and two fatty acids

c) fatty acid and a long-chain alcohol

d) fatty acid and cholesterol

Answer 14

c)

Question 15

Which of the following is a metabolic precursor of protaglandins and leukotrienes?

a) vitamin A

b) sphingomyelin

c) cholesterol

d) arachadonic acid

Answer 15

d)

Question 16

Which of the following statements are true about steroid hormones? Why?

a) are derivatives of cholesterol

b) are less polar than cholesterol

c) are paracrine hormones

d) trigger gene expression

e) are endocrine hormones

Answer 16

a, d, e

Steroid hormones are derivatives of cholesterol, but more polar than cholesterol. They function as endocrine hormones (i.e. secreted in the blood stream and act in tissues that are distinct from those where they are produced). Steroids bind to tissue-specific nuclear receptors where they trigger gene expression.