11 - Lipids

Question 1

Lipids

Answer 1

diverse group of naturally occurring molecules that are soluble in non-polar organic solvents such as chloroform (insoluble/poorly water soluble)

Question 2

Fatty acids

Answer 2

• Simple lipids
• Building blocks for complex lipids (e.g. fats and phospholipids)
• Source of energy (ATP) for many tissues. Heart and skeletal muscle ‘prefer’ fatty acids to glucose
• Important for human diet:
- Essential fatty acids
- Unsaturated v saturated fats
- Trans (‘hydrogenated’) fats

Question 3

General structure of a fatty acid

Answer 3

Hydrophillic carboxyl head group

Hydrophobic hydrocarbon tail

Question 4

Amphipathic

Answer 4

2 different chemical characteristics

Question 5

Saturated

Answer 5

No C=C double bond (e.g. palmitic acid)

Question 6

Monounsaturated

Answer 6

Only 1 C=C double bond (e.g. oleic acid)

Question 7

Polyunsaturated

Answer 7

More than 1 C=C double bond (e.g. linoleic acid)

Question 8

Fatty acid nomenclature

Answer 8

1. How many carbons
2. Is it saturated: saturated = anoic, monounsaturated = enoic, polyunsaturated = dienoic (2 x C=C), trienoic (3 x C=C)
3. position and type of double bond: cis or trans

Question 9

Essential fatty acids (EFAs)

Answer 9

• Unsaturated fatty acids that animals cannot synthesize but which are need to synthesize other fatty acids
• Animals do not have enzymes that insert C=C into the hydrocarbon chain beyond carbon 9
• Symptoms of fatty acid deficiency include a variety of skin problems such as eczema
• Two classes of EFAs:
- omega-3 type: alpha-linolenic acid (ALA)
- omega-6 type: linoleic acid (LA)

Question 10

Why are plants a source of short chain EFAs in the diet?

Answer 10

because they possess the enzymes needed for their synthesis (e.g. delta 12 and delta 15 desaturases)

Question 11

Fish oils

Answer 11

good source of longer chain omega 3 fatty acids – fish accumulate by eating microalgae (or eating fish that have eaten microalgae)

Question 12

Triacylglycerols

Answer 12

• Carboxylic acid triesters of glycerol (a 3C trialcohol)
• The main dietary source of fatty acids (in fats and oils)
• The storage form of fatty acids
− an efficient energy store (much better than carbohydrates)
− a major source of biochemical energy

Question 13

Variation in fatty acids

Answer 13

• The properties of the triacylglycerol (e.g. melting point) depend on its fatty acid composition:
‒ Vegetable oils: almost entirely unsaturated fatty acids
‒ Animal fats: larger percentage of saturated fatty acids
• Bend in tail of unsaturated fatty acid prevents molecules packing tightly together – increasing fluidity – lowering melting temperature (e.g. sunflower oil v lard)

Question 14

Fats

Answer 14

• Mainly saturated fatty acids
• Tightly packed molecules
• High melting point

Question 15

Oils

Answer 15

• Mainly unsaturated fatty acids
• Loosely packed molecules
• Low melting point

Question 16

Hydrogenation

Answer 16

• Industrial processing of unsaturated oils and fats to make them saturated (converts liquid oils to solid or semi-solid fats)
• Incomplete hydrogenation can isomerize cis double bonds into trans double bonds, to produce ‘trans fats’ implicated in circulatory diseases

Question 17

Hydrogenation

Answer 17

• Industrial processing of unsaturated oils and fats to make them saturated (converts liquid oils to solid or semi-solid fats)
• Incomplete hydrogenation can isomerize cis double bonds into trans double bonds, to produce ‘trans fats’ implicated in circulatory diseases

Question 18

Why is hydrogenation useful?

Answer 18

Partially hydrogenated vegetable oils:
‒ are cheaper than animal source fats
‒ are available in a wide range of consistencies
‒ have desirable characteristics (e.g. longer shelf life)

Question 19

Cis and trans double bonds

Answer 19

• Cis C=C causes a bend but a trans C=C bond does not
• Fats containing fatty acids with cis double bonds are less solid than ones containing trans double bonds
• Naturally occurring unsaturated fatty acids are usually cis

Question 20

Phospholipids and membranes

Answer 20

• Phospholipids have two ends with different affinities for water (amphipathic)
• In water, phospholipids spontaneously form lipid bilayers
• These structures cluster the hydrophobic regions toward the inside and leave the hydrophilic regions exposed to the water environment