13.E: Exercises

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Additional Exercises

The melting point of elaidic acid is 52°C. a. What trend is observed when comparing the melting points of elaidic acid, oleic acid, and stearic acid? Explain.
b. Would you expect the melting point of palmitelaidic acid to be lower or higher than that of elaidic acid? Explain.
Draw a typical lecithin molecule that incorporates glycerol, palmitic acid, oleic acid, phosphoric acid, and choline. Circle all the ester bonds.
Explain whether each compound would be expected to diffuse through the lipid bilayer of a cell membrane.
a. potassium chloride; b. CH₃CH₂CH₂CH₂CH₂CH₃; c. fructose
How does the structure of cholic acid differ from that of cholesterol? Which compound would you expect to be more polar? Why?
a. What fatty acid is the precursor for the prostaglandins? b. Identify three biological effects of prostaglandins.

Answers

a. Stearic acid has the highest melting point, followed by elaidic acid, and then oleic acid with the lowest melting point. Elaidic acid is a trans fatty acid, and the carbon chains can pack together almost as tightly as those of the saturated stearic acid. Oleic acid is a cis fatty acid, and the bend in the hydrocarbon chain keeps these carbon chains from packing as closely together; fewer interactions lead to a much lower melting point.
b. The melting point of palmitelaidic acid should be lower than that of elaidic acid because it has a shorter carbon chain (16, as compared to 18 for elaidic acid). The shorter the carbon chain, the lower the melting point due to a decrease in intermolecular interactions.
a. KCl - NO, the ionic compound would not be able to cross the inner non-polar portion of the lipid bilayer.; b. CH₃CH₂CH₂CH₂CH₂CH₃- Yes, this non-polar compound would be able to cross the inner non-polar portion of the lipid bilayer.; c. fructose NO, this relatively large, polar sugar would not be able to cross the inner non-polar portion of the lipid bilayer.
Cholic acid is missing the alkene (C=C) but has two additional alcohol (OH) groups and one additional carboxylic acid (COOH) group, compared to cholesterol. The very polar bonds of the hydroxyl and carboxyl groups make cholic acid much more polar overall.
a. arachidonic acid; b. induce smooth muscle contraction, lower blood pressure, and contribute to the inflammatory response