11 - Lipids Flashcards
Lipids
diverse group of naturally occurring molecules that are soluble in non-polar organic solvents such as chloroform (insoluble/poorly water soluble)
Fatty acids
• 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
General structure of a fatty acid
Hydrophillic carboxyl head group
Hydrophobic hydrocarbon tail
Amphipathic
2 different chemical characteristics
Saturated
No C=C double bond (e.g. palmitic acid)
Monounsaturated
Only 1 C=C double bond (e.g. oleic acid)
Polyunsaturated
More than 1 C=C double bond (e.g. linoleic acid)
Fatty acid nomenclature
- How many carbons
- Is it saturated: saturated = anoic, monounsaturated = enoic, polyunsaturated = dienoic (2 x C=C), trienoic (3 x C=C)
- position and type of double bond: cis or trans
Essential fatty acids (EFAs)
• 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)
Why are plants a source of short chain EFAs in the diet?
because they possess the enzymes needed for their synthesis (e.g. delta 12 and delta 15 desaturases)
Fish oils
good source of longer chain omega 3 fatty acids – fish accumulate by eating microalgae (or eating fish that have eaten microalgae)
Triacylglycerols
• 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
Variation in fatty acids
• 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)
Fats
- Mainly saturated fatty acids
- Tightly packed molecules
- High melting point
Oils
- Mainly unsaturated fatty acids
- Loosely packed molecules
- Low melting point
Hydrogenation
- 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
Hydrogenation
- 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
Why is hydrogenation useful?
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)
Cis and trans double bonds
- 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
Phospholipids and membranes
- 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
