L31 Structure and Function of Plasma Membrane

Question 1

Plasma membranes have been described as having ‘a trilaminar staining pattern’. Explain this.

Answer 1

When stained with osmium there appears to be 2 dark lines separated by a lightly stained central zone.

Resembles train tracks.

Hydrophilic heads form dark layers and the middle is the hydrophobic tails (which do not stain).

Question 2

How much of a cell membrane is made of lipid molecules?

Answer 2

30-80%

Question 3

What lipids are found in cell membranes?

Answer 3

Phospholipids, glycolipids, sterols

Question 4

How long are phospholipid fatty acid ‘tails’?

Answer 4

14-24 carbon atoms long

Question 5

When surrounded by water, cone-shaped (single chain) lipid molecules aggregate to form a __?__

Answer 5

micelle

Question 6

When surrounded by water, cylindrical-shaped (double tailed) lipid molecules aggregate to form a __?__

Answer 6

bilayer

Question 7

What is a glycerol based phospholipid known as?

Answer 7

Phosphoglyceride

Question 8

What are the main phosphoglycerides in the plasma membrane?

Answer 8

Phosphatidylcholine

Phosphatidylethanolamine

Phosphatidylserine (-)

Phosphatidylinositol

Question 9

Which of these molecules has an overall negative charge?

a) Phosphatidylcholine
b) Phosphatidylethanolamine
c) Phosphatidylserine
d) Phosphatidylinositol

Answer 9

c) Phosphatidylserine

Question 10

What is the main sphingolipid in the plasma membrane?

Answer 10

Sphingomyelin

Question 11

What type of molecule forms the head of a phospholipid?

Answer 11

A polar molecule

e.g. choline, ethanolamine, serine, inisitol

Question 12

In a phospholipid, the fatty acid chains need to attach to __?__

Answer 12

glycerol or sphingosine

Question 13

How are glycolipids formed?

Answer 13

Addition of CHO group(s) to lipids.

Glycerol-based: glycolipid

Sphingosine-based: sphingolipid

Combo of glycerol and sphingosine: glycosphingolipid

Question 14

What are the most common glycosphingolipids?

Answer 14

• Cerebrosides

- Gangliosides

Question 15

In what cells are glycosphingolipids prominent?

Answer 15

Nerve cells.

Think of the names of the most common glycosphingolipids:

• Cerebroside
• Ganglioside

Question 16

Phosphatidylserine is a negatively charged phosphoglyceride in the plasma membrane. The other common phosphoglycerides are uncharged. What purpose does phosphatidylserine serve?

Answer 16

Phosphatidylserine is located on the inside of the membrane. During apoptosis the plasma membrane modifies a lot and internal proteins become exposed at the surface. This will now be on the outside and allow macrophages to recognise the apoptosis bodies.

Question 17

Eukaryotic plasma membranes contain large amounts of cholesterol. What is the role of cholesterol in the PM?

Answer 17

• Maintains stability and integrity of PM
• Decreases permeability of PM
• Decreases fluidity

Question 18

How does the plasma membrane’s composition affect fluidity?

Answer 18

• shorter chain length on phospholipid reduces the tendency of the tails to interact with one another
• Cis-double bonds produce kinks in the hydrocarbon chains that make them difficult to pack together

Question 19

How does temperature affect the fluidity of the plasma membrane?

Answer 19

Movement of phospholipids decreases as the temperature drops and increases as it rises.

Question 20

In what ways can a phospholipid move within the membrane?

Answer 20

They can rotate, diffuse laterally (within their monolayer), and - rarely - they can flip into the other side of the bilayer.

Question 21

What are integral membrane proteins?

Answer 21

• Embedded within bilayer
• Hydrophobic segments have affinity for interior of bilayer
• Hydrophilic regions extend into aqueous phase

Question 22

What are peripheral membrane proteins?

Answer 22

• Lack hydrophobic segment, therefore sit on the periphery, linked to polar heads of the phospholipids.
• Glycosylphosphatidylinositol (GPI) anchors

Question 23

What are lipid-anchored membrane proteins?

Answer 23

• Hydrophilic, sit on surface of membrane attached to a lipid molecule in the bilayer.