10: Membrane structure

Question 1

What are the major lipid classes of the animal plasma membrane?

Answer 1

Phospholipids
Cholesterol
Glycolipids

Question 2

Name two subclasses of phospholipids and what characterizes them

Answer 2

Phosphoglycerides:

• Main phospholipid of cell memb.
• 3C glycerol backbone
• 2 long-chain fatty acids tails
• 1 P group bound to a head gr.

Sphingolipids

• Built from sphingosine, not glycerol
• Has amino gr. and 2 -OH
• P-linked head gr.

Question 3

Characterize a phospholipid

Answer 3

• Polar head group containing a P gr.
• 2 hydrophobic hydrocarbon tails
  
  • Usually 1 saturated and 1 unsaturated

Question 4

What is cholesterol and what are its roles in the plasma membrane?

Answer 4

One of the three major lipid components of the plasma membrane.

Sterol, has ring structure bound to a -OH gr. in one end and a nonpolar hydrocarbon tail at the other end.

In membrane: oriented with -OH gr. close to the polar head of the adjacent phospholipids.

Roles:
- Tightens packing of the lipids => increased
stiffness.
- Decreases permeability to small polar molecules

Question 5

What are the characteristics of a glycolipid and what is its role in the plasma membrane?

Answer 5

Made from sphingosine
Tends to self-associate into lipid raft phases
Sugar molecules have been added in the Golgi

In membrane:
- Only found in the extracellular monolayer facing
away from the cytosol.
- Asymmetric distribution because of the sugar
groups (exposed on the cell surface)

Question 6

What are the roles of inositol phospholipids in cell signaling?

Answer 6

Extracellular signals => phosphorylation of head groups => docking sites for cytosolic signaling proteins

Cleaved by phospholipases to generate small intracellular signaling molecules

=> signaling cascades

Question 7

What is the difference between a glycoprotein and a preoteoglycan?

Answer 7

Glycoprotein:
Mono-, oligo-, or polysaccharides covalently bound to membrane proteins.

Proteoglycan:
Protein glycosylated by one or more glycosaminoglycans (GAGs) - often long chains.
E.g., heparin and hyaluronic acid

Question 8

Which reagents can solubilize membrane proteins?

Answer 8

Must disrupt hydrophobic interactions and destroy the lipid bilayer.

Detergents - amphiphilic. When present in high enough concentrations they aggregate to form micelles.
Hydrophobic ends bind to hydrophobic ends of membrane proteins and displace lipid molecules.

Depend on temperature, pH, and salt concentration.
SDS = strong detergent

Question 9

What is a liposome? Can you identify some advantages/disadvantages?

Answer 9

Spherical vesicle that has at least one lipid bilayer.

Advantages:

• Increase stabilization via encapsulation
• Increase efficiacy and safety of drug delivery
• Biocompatible/-degradable

Disadvantages:
  - Low solubility
  - Generally short half-life
  - High production cost
  - Leakage and fusion of encapsulated 
    drug/molecules

Question 10

Name some techniques for studying cell membranes in the lab (e.g. separation techniques)

Answer 10

Separation of cell components:

• Sequential centrifugation
• Density gradient centrifugation
• Chromatography

Question 11

What is bacteriorhodopsin?

Answer 11

Light-driven proton (H+) pump: out of the cell
=> gradient used for ATP-synthesis

7 transmembrane α-helices
1 light-absorbing group (chromophore): retinal

Has lipid interactions => stabilization

Question 12

How do membrane proteins move within the membrane?

Answer 12

Do not “flip-flop” across the bilayer (similar to lipids).
Rotate about an axis perpendicular to the plane of the bilayer (rotational diffusion).
Some can move laterally in the membrane (lateral diffusion).