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CH 150: Introduction to Biochemistry Lipid Characterization Introduction: Lipids represent one of the four classes of biological molecules. These molecules are all based on the presence of fatty acids, which are long carbon tails (16-20+ C long) with a carboxylic acid group at one end. The carbon tails may be fully saturated (no double bonds) or may be mono- or poly-unsaturated (containing one or more carbon-carbon double bonds). When combined with the 3-carbon alcohol glycerol, the resulting compounds are triglycerides. This is the form in which most lipids are transported in the blood. The phospholipids are modified triglycerides, in which one of the fatty acid groups is replaced with a phosphate group modified with a polar head group. Cholesterol is a polycyclic compound that is often esterified to fatty acids. These cholesterol esters are associated with lipo-proteins and transported in the blood as high-density lipoproteins (HDL) or low density lipoproteins (LDL), the difference being in the amount of cholesterol-esterified fatty acids present in the complex. HDL has less cholesterol and fatty acids present, while LDL contains more cholesterol and fatty acids. In this experiment you will be observing different kinds of lipids and noting their different physical and chemical properties. You will then compare these properties to their structures and try to draw conclusions about common properties and molecular structure. Procedure: 1.
Observe the samples of lipids provided: pure fatty
acids (stearic acid, oleic acid, linoleic acid), triglycerides (canola oil,
coconut oil, olive oil, soy bean oil, lard, shortening) and cholesterol.
Complete the data table to compare the physical properties (state and
color) of each of the fatty acids and draw the structures of each sample.
For the triglycerides, complete the table as before, but also indicate
whether saturated or unsaturated fatty acids are more abundant in each.
For cholesterol, describe its physical properties and draw its structure.
You will need to consult additional resources to determine the structures of
each compound. Do this BEFORE lab.
2. Determine the degree of unsaturation of each of the 5 % fatty acid solutions provided (5 % stearic acid 5 % oleic acid, 5% linoleic acid, lard, coconut oil, olive oil, soybean oil, canola oil, or cholesterol) in methylene chloride. Place 10 drops of the solution of the lipids (all but cholesterol) into individual clean, dry test tubes. Move to the fume hood. Add a bromine solution drop wise until the color of the bromine persists; record the number of drops necessary to achieve the color change.
BE CAREFUL!
Methylene chloride solutions tend to spurt from droppers!
Be quick but careful when transferring you samples to your
test tubes from the reagent vials. 3. Perform the Liebermann-Barchard test on the solutions of cholesterol, coconut oil, and lard. Place 10 drops of the lipid in a clean dry test tube and add 3 drops of acetic anhydride and one drop of concentrated sulfuric acid. The formation of a green-blue color after a few minutes is a positive test for cholesterol. Be sure to record your observations.
Results and Discussion:
3. The American Heart Association recommends that one's diet be low in all fats, especially saturated fats. Rank the following lipids in order of decreasing desirability of use for cooking (best at the top, worst at the bottom): lard, soybean oil, coconut oil, canola oil, olive oil
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