Biomembranes

Question 1

Mention the kinds of lipids found in membranes?

Answer 1

• Phosphoglycerides
• Sphingolipids
• Cholestrol

Question 2

What is the structural role of phospholipids and sphingolipids based on?

Answer 2

Their amphipathic nature

Question 3

Why are phospholipids aphipathic?

Answer 3

Phospholipids are called amphipathic compounds, since they contain both hydrophobic (lipophilic) and hydrophilic (lipophobic) components.

Question 4

p-lipids are insoluble in water - but how can they be made into highly water-soluble micelles?

Answer 4

A suspension of p-lipids aggregates can be readily rearranged into highly water-soluble micelles by a short burst of sonication.

Question 5

Describe the micelles and bilayers structures that lipids assemble into

Answer 5

Lipids assemble into micelles and bilayers, structures in which the hydrophobic tails’ (fatty acids) are oriented away from the aqueous environment and the polar “heads” toward the aqueous solution.

Question 6

The fatty acid tails are the ………………. portion?

Answer 6

Hydrophobic portion

Question 7

The head group: charged phosphate residue + nitrogenous base (or sugar), is ………….. in character?

Answer 7

Polar

Question 8

“That membranes contain a continuous hydrocarbon phase contributed by the lipid components of the membrane.” Who proposed this hypothesis?

Answer 8

H. Davson and J. Danielli

Question 9

Who proposed the first hypothesis of the structure of biological membrane?

Question 10

Who modified and refined the first hypothesis of the structure of biological membrane?

Answer 10

JQ Robertson, into the unit-membrane hypothesis.

Question 11

What’s the second hypothesis?

Answer 11

That membranes consist of a bilayer of mixed polar lipids, with their hydrocarbon chains oriented inward to form a continuous HC phase and their hydrophilic heads oriented outward.

Each surface was thought to be coated with mono-molecular layer of protein molecules.

Question 12

In a bilayer; individual lipid molecules can move laterally. What is the importance of that?

Answer 12

Provides the bilayer with:
- Fluidity
- Flexibility
- A characteristically high electrical resistance and relative impermeability to high polar molecules.

Question 13

What accounts for the high content of α-helix of membrane proteins (in the lipid bilayer model)?

Answer 13

The membrane proteins are globular (polypeptides that have a 3-dimensional structure) to account for their high content of α-helix.

Question 14

How are the membranes described in the globular or subunit model?

Answer 14

Membranes were viewed as consisting of sheets of recurring lipoprotein subunits

Question 15

What’s a limitation of the globular or subunit model?

Answer 15

Globular models have failed to account satisfactorily for many properties of membranes.

Question 16

Who postulated the fluid-mosaic model?

Answer 16

SJ Singer and G.L. Nicolson

Question 17

When was the fluid-mosaic model postulated?

Answer 17

1972

Question 18

What is the most satisfactory model of membrane structure?

Answer 18

The fluid-mosaic model

Question 19

Describe the fluid-mosaic model?

Answer 19

• The membrane consist of a bimolecular lipid layer with protein inserted in it or bound to either surface.
• Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called trans membrane proteins, while others are embedded in either the outer or inner leaflet of the lipid bilayer.
• Loosely bound to the other or inner surface of the membrane are the peripheral proteins. Many of the proteins and all the glycolipids have externally exposed oligosaccharide chains.

Question 20

What are the accounts satisfactorily for many features and properties of biomembrane?

Answer 20

It provides for membranes with widely different protein contents, depending on the number of different protein molecules per unit area of membrane.

It accounts for the varying thickness of different types of membrane.

The asymmetry of natural membranes, since it permits proteins of different types to be arranged on the two surfaces of the lipid bilayer.

The electrical properties and permeability of membranes.

The observation that some protein components of cell membranes move in the plane of the membrane at a rather high rate.