Membranes

Question 1

What are the 5 roles of a cell membrane?

Answer 1

• Define boundaries of cell/organelles
• Separate cellular and extracellular environment
• Control movement of molecules & signal transduction
• Separate aqueous regions of different compositions within cells

Question 2

Which components have a higher intracellular concentration?

Answer 2

• K+

- H+

Question 3

Which components have a higher extracellular concentration?

Answer 3

• Na+
• Mg2+
• Ca2+
• Cl-

Question 4

Briefly describe the fluid mosaic model

Answer 4

• Sea of phospholipids which is movable and flexible

- Protein molecules which are slightly more fixed but are also movable and can cross the bi-layer

Question 5

Describe the phospholipid structure

Answer 5

Hydrophilic head composed of: choline (or others), phosphate and glycerol
2 Hydrophobic hydrocarbon tails: At least one tail always contains at least 1 C=C bond which determines fluidity

Question 6

What 3 components can form the hydrophilic head? Which one produces a negative head?

Answer 6

Ethanolamine, Serine (-), Choline

Question 7

What other molecule can choline form a hydrophilic head with?

Answer 7

Phosphate and sphingosine (in place of glycerol)

Produces sphingomyelin

Question 8

What is the length range of hydrocarbon tails and how does this affect fluidity?

Answer 8

14-24 hydrocarbons long

Shorter tailed hydrocarbons are often more fluid

Question 9

What is the primary role of sphingomyelin?

Answer 9

Forms the myelin sheath

Question 10

What 3 characteristics define the properties of the plasma membrane?

Answer 10

• Head components
• Saturation
• Length

Question 11

What is the effect of sterols on the phospholipid membrane?

Answer 11

• Attach themselves to the head of a phospholipid, with sterol rings interacting with hydrocarbon tail making it stiffer (ambiphilic molecules)
• Allow lipid-soluble molecules to cross the membrane
• Cholesterol increases the viscosity of membranes by limiting flexion, rotation and lateral diffusion

Question 12

What is cell membrane viscosity believed to affect?

Answer 12

Cell signalling

Question 13

What is cell thickness thought to influence?

Answer 13

Trafficking of proteins across cells

Question 14

How does the composition of the interior and exterior cell membrane differ?

Answer 14

• Outside composed mostly of choline and sphingomyelin, interior mostly serine
• Causes difference in charge

Question 15

Describe lipid microdomains

Answer 15

• Groups of lipids aggregate together for a short time period causing a microdomain to regulate a cell function, particularly cellular vent
• Regulates release of hormones through interactions with proteins

Question 16

Name 6 types of transmembrane protein

Answer 16

1. Single-pass transmembrane (alpha-helical)
2. Multiple-pass transmembrane (alpha-helical)
3. Beta-sheet barrel
4. Associated with one leaflet of membrane (cytosolic) with amphiphilic alpha-helix
5. Covalently bonded to a hydrocarbon
6. Lipid-anchored

Question 17

Describe Glycophorin A

Answer 17

Amphiphilic alpha helical transmembrane protein, passing once with hydrophobic membrane spanning section

Question 18

Describe bacteriorhodopsin

Answer 18

• transmembrane protein with 7 helical transmembrane regions
• first membrane-bound protein to have its structure determined
• oritentates itself to stay within the hydrophobic region

Question 19

Describe aquaporin

Answer 19

• alpha helical, composed of 2 smaller proteins that span only half of the membrane
• acts as gates for the water channel in the kidney

Question 20

Where are beta-barrel transmembrane proteins commonly found and what is their function?

Answer 20

Found in: mitochondria, chloroplasts and bacteria

Allow controlled movement of hydrophilic molecules

Question 21

How are lipid-anchored proteins attached to the phospholipid membrane?

Answer 21

• Protein is covalently attached to lipid on extracellular side of the membrane by GPI anchor composed of inositol
• Can be attached via amide linkage ( myristoyl anchor) or thioester linkage (palmitoyl anchor/farnesyl anchor)

Question 22

How can the fluidity of the plasma membrane be demonstrated?

Answer 22

• Attach fluorescently tagged protein to plasma membrane (GFP)
• Bleach area and measure recover time
• Allows the measure of lateral mobility by diffusion constant