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Unit 3. Lipids
1. Omega-3 and omega-6 fatty acid. Fish oils are rich sources of omega-3 and polyunsaturated fatty acids and omega-6 fatty
acids are relatively abundant in corn and sunflower oils. Write the key convention and classify the following fatty acids as
omega-3, omega-6, or neither:
(a) Linolenic acid Omega-3 18:3 (Δ9,12,15) (b) Linoleic acid Omega-6 18:2 (Δ9,12) (c) Arachidonic acid Omega-6 20:4 (Δ5,8,11,14) (d) Oleic acid Neither 18:1 (Δ^9 ) (e) cis-8,11,14-eicosatrienoic acid Omega-6 20:3 (Δ8,11,14) (f) Eicosapentaenoic acid Omega-3 20:5 (Δ5,8,11,14,17) (g) Docosahexaenoic acid Omega-3 22:6 (Δ4,7,10,13,16,19) (h) Stearic acid Neither 18:
2. Catalytic Hydrogenation of Vegetable Oils. Catalytic hydrogenation, used in the food industry, converts double bonds in
the fatty acids of the oil triacylglycerols to –CH 2 –CH 2 –. How does this affect the physical properties of the oils?
Catalytic hydrogenation breaks double bonds, which lead to increasing in melting point so that they cannot easily melted. Catalytic hydrogenation combats the oxidative cleavage - causes fats to become rancid, which could help to store food for longer. Catalytic hydrogenation causes fats to harden so that they are usually solid at room temperature (such as butter)
3. Fat/oil and Cardiovascular Disease. Why does the American Heart Association recommend the use of canola oil or olive
oil rather than coconut oil in cooking?
Both canola oil and olive oil not only contain lower percentage of saturated fat but also contain higher polyunsaturated fat and monounsaturated fat. Compared to a tablespoon of olive oil, a table spoon of coconut oil contains about six times the amount of saturated fat. High saturated fat intake has been tied to increased levels of LDL cholesterol (low density lipoprotein cholesterol) which will lead to risks of heart diseases.
4. Alkali Lability of Triacylglycerols. A common procedure for cleaning the grease trap in clothes or dishes is to add a
product that contains sodium hydroxide. Explain why this works.
Grease trap in clothes or dishes are very common in our daily life, their main characteristic is hydrophobic, therefore they cannot be easily dissolved in water. So only water cannot wash away stains that was caused by grease. Cleaners that contain sodium hydroxide (either as a solid or already dissolved in water) will convert the fats to soap, substance that can easily dissolve in water so that this reaction will help to clean out all the dirt made from grease wastes. Soap appears to dissolve in water because amphipathic free fatty acids salts form micelles, with hydrophobic fatty acid tails toward the center and carboxylate groups facing outward toward the water. Fat-soluble particles can then dissolve inside micelles in the soap-water solution and wash away. Water-soluble compounds can freely dissolve in the water
5. Proportion of Unsaturated fatty acid in Bacterial Membrane. Suggest a reason why the cell membranes of bacteria
grown at 20°C tend to have a higher proportion of unsaturated fatty acids than the membranes of bacteria of the same species
8. Hydrophobic and Hydrophilic Components of Membrane Lipids. A common structural feature of membrane lipids is
their amphipathic nature. For example, in phosphatidylcholine, the two fatty acid chains are hydrophobic and the
phosphocholine head group is hydrophilic. For each of the following membrane lipids, name the components that serve as the
hydrophobic and hydrophilic units:
Hydrophobic Hydrophilic (a) phosphatidylethanolamine 2 fatty acids Phosphoethanolamine (b) sphingomyelin Ceramide (sphingosine + 1 fatty acid) Phosphocholine (c) galactosylcerebroside Ceramide D-galactose (d) ganglioside Ceramide Oligosaccharide (e) cholesterol Hydrocarbon ring skeleton Hydroxyl group
9. Ninhydrin to Detect Lipids on TLC Plates. Ninhydrin reacts specifically with primary amines to form a purplish-blue
product. A thin-layer chromatogram of rat liver phospholipids is sprayed with ninhydrin, and the color is allowed to develop.
Which phospholipids can be detected in this way?
Phosphatidylethanolamine and phosphatidylserine. They are the only phospholipids that have primary amine groups that can react with ninhydrin.
Bonus (optional) Draw a flowchart that summarizes unit 3
G ly^ cFat ty er ol aci d Fat ty aci d PO 4 Alc oho l S p h^ i nFatt g o s^ i n e y acid Mo no or disa cch a Gl yc erFat ty ol aci d Fat ty aci d (SO 4 Mo no ) or dis acc ha S p h i n gFatt o s i n e y PO acidCho line 4 Glyc^ erol (^) F a t t y a c i d F a t t y a c i d F a t t y a c i d Omega- Omega- Saturated Lauric/myristic/palmitic/stearic acid Polysaturated Monosaturated (Oleic acid omega-9) Composed of UV Vitamin D 3 Steroid hormone: Testoterone, Estradiol, Cortisol, Aldosterone, Predisolone, Predisone Bile acids: Taurocholic acid (made in liver, act as detergent in the intestine, emulsifying dietary fats to make them more accessible to digestive enzymes.) Cholesterol: major sterol in animal tissues, amphipathic 7-dehydrocholesterol isomer Partially hydrogenation Liquid: unsaturated (vegetable oil) Solid: long-chains, saturated (animal fat, butter) cis trans Unsaturated Sterols Subgroup of steroids Precursors for a variety of products Cell surface antigen Determinants of blood groups Sphingolipids Galactolipids (sulfolipids) Glycerophospholipids Steroids Phospholipids Glycolipids Archaebacterial tetraether lipids Structural Membrane lipids Polar heads Nonpolar tails Signals, Cofactors & Pigments Lipid Not polymers of repeating monomer Composed of C-H bonds Waxes Long-chain (C14-36); high melting point ( 60- 100°C) Beewax: triacontanoylpamitate (palmitic acid + 1-triacontanol) Triacylglycerol Fats & oils Storage
