The Membrane Bilayer : Main biophysical properties

Question 1

What is the function of Biological membranes?

Answer 1

• Continuous, highly selective permeability barrier
• Control of the enclosed chemical environment
• communication
• recognition
• signal generation

Question 2

What is the composition of membrane?

Answer 2

• 40% lipid
• 60% protein
• 1-10% carbohydrate

Question 3

Describe properties of lipid bilayer

Answer 3

• Forms a bilayer spontaneously in water
• van der Walls forces between hydrophobic tails
• electrostatic and hydrogen bonding between hydrophilic heads
• interaction between hydrophilic groups and water

Question 4

What are amphipathic molecules?

Answer 4

They contain both hydrophilic and hydrophobic moiety

Question 5

Describe the phospholipid molecule

Answer 5

Head group :

• range of polar head groups
• eg. choline, amines, sugars

Fatty acid chains :

• mostly C16-18 long
• Cis double bonds - kinks

Question 6

What is sphingomyelin?

Answer 6

only phospholipid not based on glycerol backbone

Question 7

What is the function of glycolipids?

Answer 7

Form H bonds with water to stabilize membrane structure

Question 8

State the characteristics of glycolipids

Answer 8

• sugar containing lipid
• cerebrosides : head group sugar monomer
• gangliosdes : head group oligosaccharide

Question 9

What is the important role played by cholesterol in plasma membrane?

Answer 9

stabilises plasma membrane

Question 10

State the characteristics of cholesterol

Answer 10

• polar head group
• rigid planar steroid ring sturcture
• non-polar hydrocarbon tail

Question 11

What is the function of cholesterol in maintaining fluidity of membrane?

Answer 11

High temp

• reduces fluidity
• reducing phospholipid chain motion

Low temp

• increases fluidity
• reduces phospholipid packing

Question 12

What is meant by fluid membrane?

Answer 12

phospholipid bilayer with protein molecules floating around freely within it

Question 13

What are the different membranes that have specialised functions?

Answer 13

• Microvilli : absorption of nutrients
• Neural membrane : electrica signal conduction
• Goblet cell membrane : secrete mucus

Question 14

How does unsaturated double bonds in f.a side chain increases membrane fluidity?

Answer 14

They disrupt the hexagonal pacaging of phospholipids

Question 15

Why does flip flop uses a lot of energy?

Answer 15

Movement of lipid molecules from one half of the bilayer to the other involves passing through hydrophobic and hydrophilic regions

Question 16

Why small polar molecules like water can diffuse through lipid bilayer but smaller substance like H+ cannot?

Answer 16

Non polar molecules

• Hydrophobic and non polar molecules diffuse across bilayer at rate porpotional to size

Polar molecules

• charged nolecules complex water around them
• become too large to cross

Water

• water in middle of extremes
• diffuse at rate proportional to size and polarity

Question 17

List the type of mobilities of lipid in lipid bilayer

Answer 17

• Intra chain motion - kink formation in fatty acid chains
• fast axial rotaion
• fast lateral diffusion
• Flip flop