Structure of lipids

Question 1

Describe the basic properties of lipids

Answer 1

1. Heterogeneous group of molecules
2. Multiple hydrocarbon groups
3. Few oxygen or polar groups
4. Much more soluble in organic solvents than water (insoluble in water)
5. Soluble in non-polar solvents such as chloroform, hydrocarbons, benzene etc.

Question 2

Give three reasons why lipids are important

Answer 2

1. Major energy store
2. Membrane structures
3. Hormones and signalling compounds

Question 3

What are the two types of lipids and give example of each

Answer 3

1. Complex lipids- phospholipids

2. Simple lipids- cholesterol prostacyclin

Question 4

What are complex lipids

Answer 4

1. Most common in nature
2. Energy storage and membrane lipids
3. Major part is made of fatty acids

Question 5

What are simple lipids

Answer 5

1. Smaller proportion but have specific functions

Question 6

What are the two main parts of a fatty acid

Answer 6

1. Carboxyl group

2. Long hydrocarbon chain

Question 7

Describe the properties of the carboxyl group in fatty acids

Answer 7

1. Typical pKa of carboxyl group is about 4.5
2. Ionised at most physiological pHs
3. At 4.5 half of carboxyl groups will be O- and half OH

Question 8

Describe properties of hydrocarbon chain in fatty acids

Answer 8

1. Saturated or unsaturated
2. Most double bonds are CIS which introduces a bend in the chain
3. Stereospecificity of enzymes which create double bonds

Question 9

What is ususally common about number of carbon atoms in fatty acids

Answer 9

1. Normally 14< and 20>

2. Normally have an even number of carbon atoms as usually biosynthesised by concatenation of C2 units

Question 10

Describe the nomenclature for fatty acids

Answer 10

1. Number of carbons in chain with ‘oic acid’ or ‘oate’ in ionised form
2. If there are double bonds these are indicated with a number.

Question 11

Where is the double bond in cis-9-hexadecanoate

Answer 11

1. the double bond is between the 9th and 10th carbons starting the numbering from the carboxyl carbon

Question 12

Give the systematic name and formula for palmitate/palmitic acid and stearate

Answer 12

1. Hexadecanoate/ hexadecanoic acid CH3(CH2)14COOH

2. Octadecanoate/octadecanoate acid CH3(CH2)16COOH

Question 13

What are two major complex lipids

Answer 13

1. Phospholipids

2. Triacylglycerols

Question 14

Describe the structure of triacylglycerols

Answer 14

1. carbon backbone with 3 usually different fatty acids attached
2. Fatty acid triesters of glycerol
3. Nonpolar and water-soluble

Question 15

Describe the difference between fats and oils

Answer 15

1. Fat- 3 fatty acids are saturated, higher melting points3

2. Oil- fatty acids unsaturated, don’t pack closely together so less energy needed to melt

Question 16

Describe the structure of phospholipids

Answer 16

1. Have phosphate group instead of one of the fatty acids
2. X- attached to phosphate group
3. X- can be a number of different groups- more precise name E.g. amino acid serine
4. Hydrophilic head and hydrophobic tails

Question 17

Describe the effect of different chain lengths on interaction and fluidity

Answer 17

1. Chain length has significant effect on extent of inter-chain interaction
2. Longer chain- more interaction

Question 18

Describe how membranes adapt to environmental chagnes

Answer 18

1. Needs to be able to adapt the composition of membranes as environment changes- fluidity
2. As temp increases- longer chains and less double bonds

Question 19

What does this symbolism mean 18:2 ∆9,11

Answer 19

1. 18 carbon chain length

2. 2 double bonds at carbon 9 and 11

Question 20

What does Amphipathic (schizophrenic) molecules mean

Answer 20

1. Have both hydrophilic and hydrophobic part

Question 21

Give 5 examples of amphiphatic molecules

Answer 21

1. Free fatty acids
2. Phospholipids
3. Glycolipids
4. Sphingolipids
5. Ceramides

Question 22

Are tricylglycerols amphiphatic

Answer 22

1. No

Question 23

What happens when amphiphatic molecules are added to water

Answer 23

1. When added to water form a monolayer with hydrophobic hydrocarbon chains projecting from the water.
2. Vigorous shaking can produces micelles- driven by entropy not free energy