Lipids and Membranes

Question 1

1) Define lipids and give examples

Answer 1

• molecules that are hydrophobic in cells but soluble in organic solvents
  e. g. TAGs [neutral fats, polar amphipathic], glycerophospholipids, steroids, cholesterol

Question 2

2) What is the structure of glycerophospholipids?

Answer 2

• Based on glycerol, most common lipid

- Instead of a 3rd fatty acid (in TAG), there is a phosphate group attached to a polar head group (e.g. amino alcohol)

Question 3

3) What are different polar head groups in glycerophospholipids (the molecule attached to the phosphate group)?

Answer 3

• ethanolamine
• serine
• inositol
• choline

Question 4

4) What does the ‘phosphatidyl’ section of the glycerophospholipid include?

Answer 4

Phosphatidyl: the fatty acid tail, glycerol and phosphate group (not the polar head group)

[the name of the head group is added at the end]

Question 5

5) What would a glycerophospholipid with a choline polar head group be called?

Answer 5

phosphatidylcholine

Question 6

6) What are some other lipids based on, instead of glycerol, and describe the structure?

Answer 6

Sphingosine

• Backbone like glycerol
• permanent hydrocarbon chain (1 fatty acid tail)

Question 7

7) What is the structure of sphingomyelin?

Answer 7

• sphingosine with its permanent hydrocarbon chain + another fatty acid tail + a phosphate group bonded to choline (polar head group)

Question 8

8) What are glycolipids/glycosphingolipids and name the two types?

Answer 8

• sphingosine based, no phosphate group
• with a carbohydrate (glyco-) polar head group
• two fatty acid tails
• two types are Cerebrosides and Gangliosides

Question 9

9) Describe the structure and function of cerebrosides and gangliosides

Answer 9

• Cerebrosides: monosaccharide [single sugar] head group (glucose, galactose)
• important in brain cell membranes
• Gangliosides: oligosaccharide group
• ABO blood group determinant, in the membrane of red blood cells

Question 10

10) What is the structure of cholesterol and what is its function?

Answer 10

• rigid structure
• has a polar head group, rigid steroid ring structure and a non polar hydrocarbon tail
• inserts between phospholipids and modulates membrane fluidity (fluidity buffer)
• also blurs the membrane transition temperature…

Question 11

11) Name 2 saturated fatty acid membrane components

Answer 11

• Palmitic Acid C16:0 - most common, from palm oil
• Stearic Acid C18:0

[higher melting points than MUFA and PUFA)

Question 12

12) Name a mono-unsaturated fatty acid membrane component (one double bond)

Answer 12

• Oleic Acid C18:1 - from olive oil

Question 13

13) Name 3 polyunsaturated fatty acid membrane components

Answer 13

• Linoleic Acid C18:2
• Linolenic Acid C18:3
• Arachidonic Acid C20:4

Question 14

14) What form must cell membrane stay in, for the cell to function? And what kind of phospholipid movement can occur?

Answer 14

• Liquid crystal form
• Lipids can diffuse and move around within a layer
• Movement between layers (flip-flop, catalysed by flippase) is rare as this would involve a hydrophobic group in a polar area

Question 15

15) Why are polar groups hydrophilic?

Answer 15

Polar groups interact well with water and so mix well

Question 16

16) Describe the differences between the liquid crystal form and gel form of the cell membrane and how each one is attained

Answer 16

• Gel form: head groups tightly packed, tails regular, thicker membrane (formed when liquid crystal form is cooled)
• Liquid Crystal form: head groups loosely packed, tails disordered, thinner membrane (formed when gel form is heated)

Question 17

17) How does chain length and saturation affect the transition temperature between the gel and liquid crystal form?

Answer 17

• Longer chain FAs increase the transition temp : more VDWs and a decreased membrane fluidity
• Unsaturated FAs reduce the transition temp : membrane is more fluid as the double bond causes a kink in the hydrocarbon tail (looser fatty acid tail packing) so lowers the point of transition

Question 18

18) How do kinks affect VDW interactions?

Answer 18

Reduce VDW interactions

Question 19

19) How does bacteria affect the transition temperature?

Answer 19

Bacteria respond to changes in their environment which causes them to vary the proportion of unsaturated fatty acids in their cell membranes
- Reindeer: temperature is colder so bacteria membranes have more unsaturated FAs to reduce transition temperature, to maintain liquid crystal form

Question 20

20) How does changing the temperature affect cholesterol regulation of fluidity?

Answer 20

• Higher temp: membrane stiffens as cholesterol fills the space between unsat FA
• Lower temp: cholesterol increases fluidity by preventing FA packing too closely

Question 21

21) Give 5 types of molecules that the cell membrane is selectively permeable to

Answer 21

• Hydrophobic molecules (pass straight across)
• Small/large uncharged, polar molecules require transporters to pass through membrane
  [if small enough, polar can pass through)
• Ions require transporters
• Charged polar molecules require transporters

Question 22

22) Name 4 important roles proteins play in membrane function

Answer 22

• Transporters
• Anchors (holding cytoskeleton)
• Receptors
• Enzymes (between cytosol and extracellular space)