L4 - Lipids and Membranes

Question 1

Define: Lipids
Examples:

Answer 1

Lipids are water insoluble (hydrophobic) and are soluble in organic solvents.
e.g. Triacylglycerols, Glycerophospholipids, Steroids and cholesterol

Question 2

Functions and Properties of lipids:

Answer 2

1. Hydrophobic
2. Form droplets in water
3. Energy storage molecules.

Question 3

Membrane lipids:

Answer 3

It is polar and amphipathic. (Has polar head group and rest of molecule is hydrophobic).
Most common types are phospholipids (have phosphate and glycerol backbone).
Most are glycerol-based. (Glycerophospholipids).

Question 4

Polar head groups in phospholipids:

Answer 4

1. Choine (most common type)
2. Ethanolamine
3. Serine
4. Inositol

Question 5

Acyl/FA chain:

Answer 5

Can vary how long they are.
Can be saturated and unsaturated.
Double bond effects physical properties of cell membrane.

Question 6

Phospholipid names:

Answer 6

1. Phosphatidylcholine - Lecithin
2. Phosphatidylethanolamine - Cephalin
3. Phosphatidylserine - Cephalin
4. Phosphatidylinositol - involved in cell signalling.

Question 7

Sphingosine based phospholipids:

Structure:

Answer 7

Not glycerol based like others.
Similar backbone to glycerol.

Sphingosine structure:
Backbone similar to glycerol except amino group (NH3) on middle C. Has permanent hydrocarbon chain on 1C and C2 on chain has double bond.

Question 8

Sphingomyelin:

Structure:

Answer 8

It is a type of sphingosine based phospholipid.
Myelin sheaths of neurons.

Structure:
Sphingosine + fatty acid + phosphate + polar head group (choline)
Fatty acid added to C2 by bonding with NH3 group forming an amide group.
Phosphate and choline group added to C3.

Question 9

Glycolipids based on sphingosine:
Cerebroside:
(Structure, Head group, Found in)

Answer 9

Structure:
No phosphate
Glycerol. Permanent FA on C1. FA on C2 (11C long). Galactose on C3 which is a carbohydrate.

Head group:
Single sugar head group (monosaccharide) - Glucose or galactose.

Found in:
Brain cell membranes

Question 10

Glycolipids based on sphingosine:
Ganglioside:
(Structure, Head group, E.g.)

Answer 10

Structure:
No phosphate
Glycerol. Permanent FA on C1. FA on C2 (13C long). Oligosaccharide on C3 which is a carbohydrate. Can label cells on outside.

Head group:
Oligosaccharide head group - Sugars joined together and maybe modified as well.

E.g.
ABO blood group determinants (antigen of ABO)

Question 11

Membrane lipids summary:
Phospholipids:

Glycolipids/Glcosphingolipids:

Answer 11

Phospholipids

1. Glycerol based - Glycerophospholipids
2. Sphingosine based - Sphingomyelin

Glycolipids/Glcosphingolipids
1. Sphingosine based - no phosphate, carbohydrate polar head group e.g. cerebroside, ganglioside.

Question 12

FA components of membrane:
Saturated:

Mono-unsaturated:

Polyunsaturated:

Answer 12

Saturated: 
Palmitic acid C16 
(Hexadecanoic acid)
Most common in animals, plants and microorganisms.
Named from PALM oil
MP 62-63°C

Stearic acid C18
(Octadecanoic acid)
Waxy
MP 69-70°C

Mono-unsaturated:
Oleic acid C18 - 18:1 (Δ^9) 
(9-Octadecenoic acid)
Named from OLIVE oil
MP 13-14°C

Polyunsaturated:
Linoleic acid C18 - 18:2 (Δ^9,12) 
(9,12-Octadecadienoic acid)
Essential in diet (cannot be synthesised).
MP -9°C

Linolenic acid C18 - 18:3 (Δ^9,12,15)
(9,12,15-Octadecatrienoic acid)
Essential in diet (cannot be synthesised).
MP -17°C

Arachidonic acid C20 - 20:4 (Δ^5,8,11,14)

Question 13

Membrane formation:

Answer 13

Energetically favourable.
Spontaneous reaction.
Polar, hydrophilic face aqueous environment outside and inside of cell.
Tucks hydrophobic groups inside.
In the cell, new membrane formed in ER. Carried by vesicles to where it is needed.

Question 14

Vesicle formation:

Answer 14

It is energetically favourable. Planar phospholipid bilayer edges exposed to water - hydrophobic. Causes it to form sealed compartment.
Minimum size: 25nm

Question 15

Fluidity of membrane:

Answer 15

Lipids can float (diffuse) with a layer - lateral diffusion where movement occurs side by side. Can rotate so spin from head group.
Movement between two layers (flip-flop) rare but can be helped by ‘flippase’ enzymes.

Question 16

Regulation of membrane fluidity:

Answer 16

Gel to Liquid crystal with heat and cool to go back to gel.
Gel is where head group tightly packed. Tails regular (rigid) and membrane thicker.
Liquid crystal is where head group is loosely packed. Tails disordered and membrane thinner.

Membrane must be in liquid form to function.

Longer chain FA increase transition temperature.
Higher melting point. Decrease fluidity. Increase VDW interactions as long chain.

Unsaturated FA decrease transition temperature.Increases fluidity. Kinks due to cis transformation causing looser packing of FA. Reduce VDW interactions.

Bacteria can respond to changes in environment by changing proportion of FA in membrane. Animals can maintain stable body temp so not require to change lengths/configuration of FA.

Question 17

Cholesterol as Fluidity regulator:

Answer 17

Key regulator in animal cells.
At high temps, it stiffen membrane by filling space between unsaturated FA so less fluid and more rigid.
At low temps, it increases fluidity by preventing FA packing closely.

Acts as a fluidity buffer. Blur the membrane transition temperature - less heat absorbed with cholesterol.

Question 18

Cell membrane is selectively permeable:

Answer 18

Hydrophobic e.g. O2, short chain FA 
to
Small uncharged polar e.g. H20, C02, urea
to 
large uncharged polar e.g. glucose, sucrose
to 
ions e.g. H+, Na+
to 
charged polar molecules e.g. AA, ATP

Large uncharged polar molecules, ions and charged polar molecules require protein to be transported across.
Permeability of lipid bilayer is higher for molecules that are uncharged, non-polar, and small.

Question 19

Functions of membrane proteins:

Answer 19

1. Transporters
2. Anchors
3. Receptors
4. Enzymes