Organic Chemistry : Help with Organic Lipids

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Help With Organic Lipids

Which of the following lipids is polyunsaturated?

I. (9Z,12Z,15Z)-octadeca-9,12,15-trienoic acid

II. Octadecanoic acid

III. (9Z,12Z)-octadeca-9,12-dienoic acid

IV. (9Z)-9-Octadecenoic acid

Possible Answers:

I and IV

I and III

I, III, and IV

II and III

II only

Correct answer:

I and III

Explanation:

This question tests your knowledge of what an unsaturated lipid is, as well as your ability to obtain structural information from IUPAC names.

Firstly, a polyunsaturated lipid is a long carboxylic acid hydrocarbon chain that features multiple unsaturations, or double bonds. Remember from nomenclature that the stem "-en" indicates double bonds, or alkenes, while "-an" indicates alkanes, or fully saturated hydrocarbons. The relevant stems in the answer choices are italicized below:

I. (9Z,12Z,15Z)-octadeca-9,12,15-trienoic acid

II. Octadecanoic acid

III. (9Z,12Z)-octadeca-9,12-dienoic acid

IV. (9Z)-9-Octadecenoic acid

 

As you can see, "trien" and "dien" in I and III indicate these lipids are polyunsaturated, while "an" in II indicates a fully saturated lipid and "en" in IV indicates a monounsaturation. The lipids are drawn below as well, with the alkenes in polyunsaturated lipids circled in red, and the alkene in the monounsaturated lipid circled in green.

 

A4

Example Question #2 : Help With Organic Lipids

What is the name of the pictured fatty acid? 

Arachidonic acid

Possible Answers:

Arachidonic acid

Oleic acid

Eicosapentaenoic acid

Palmitoleic acid

Linoleic acid

Correct answer:

Arachidonic acid

Explanation:

The pictured structure represents arachidonic acid due to the 20-carbon carboxylic acid chain with characteristic unsaturated (double) bonds after carbons 5, 8, 11, and 14.

Example Question #3 : Help With Organic Lipids

Other than the traditional numerical system, the X marking the terminal carbon in the structure shown here can be labeled how?

Linoleic acid x

Possible Answers:

Omega-18

Gamma-18

Omega-1

Alpha-1

Alpha-18

Correct answer:

Omega-1

Explanation:

Fatty acids can be labeled numerically from 1-X, or they can be labeled in reverse starting with omega-1 as the terminal carbon.

Example Question #57 : Identifying Biochemical Molecules

Identify the common name of the fatty acid shown here.

Real oleic acid

Possible Answers:

Myristic acid

Palmitic ccid

Linoleic acid

Arachidonic acid

Oleic acid

Correct answer:

Oleic acid

Explanation:

Oleic acid is a fatty acid consisting of 18 carbon molecules and a single unsaturated (double) bond after carbon 9, as pictured.

Example Question #4 : Help With Organic Lipids

What type of lipid is shown below? 

Triglyceride

Possible Answers:

Phospholipid

Triglyceride

Fatty acid

Steroid

Phosphatidylcholine

Correct answer:

Triglyceride

Explanation:

A triglyceride consists of three fatty acid chains bound to a glycerol backbone via ester bonds, as shown by the pictured structure.

Example Question #5 : Help With Organic Lipids

Three fatty acid chains can be bound to a glycerol backbone via ester bonds to form a triglyceride. What type of chemical reaction is this?

Possible Answers:

Hydrolysis

Hoffman elimination

Condensation

Oxidation-reduction

Markovnikov

Correct answer:

Condensation

Explanation:

The reaction described is an esterification in which water is a product—as is characteristic of a condensation reaction—and in which an ester bond is formed to connect the fatty acid chain and the glycerol molecule.

Example Question #6 : Help With Organic Lipids

An unknown molecule was found to have a molecular formula of . This molecule can be identified as a __________.

Possible Answers:

steroid

triaclyglycerol

fatty acid

None of these

Correct answer:

None of these

Explanation:

This question is a little bit tricky. At first glance, we would jump to the conclusion that this molecule is a long hydrocarbon chain attached to a carboxylic acid and, therefore, a fatty acid. The premise is correct. However, our conclusion is false because, by convention, a fatty acid must contain a carbon chain of at least 12 carbons. The given formula does not match the description for a steroid or triacylglycerol. The correct answer is none of these.

Example Question #7 : Help With Organic Lipids

What is the primary biological function of a fatty acid?

Possible Answers:

All of these

To serve as a water barrier on the outside of leaves and feathers

To serve as a medium for energy storage

To serve as a medium for the derivation of other important lipids

Correct answer:

To serve as a medium for the derivation of other important lipids

Explanation:

The primary biological function of a fatty acid is to serve as a medium for the derivation of other important lipids such as waxes, triacylglycerols, phospholipids, and other necessary lipids.

Example Question #8 : Help With Organic Lipids

Before the advent of synthetic soap, hand soap was made from animal fat, namely triacylglycerols. What property of triacylglycerols make them an ideal component in hand soap?

Possible Answers:

None of these

The ability to engulf dirt/grease on skin

The ability to engulf and digest harmful bacteria on skin

The presence of hydrophobic tails and hydrophilic heads that work to dissolve into and remove dirt/grease on the skin

Correct answer:

The presence of hydrophobic tails and hydrophilic heads that work to dissolve into and remove dirt/grease on the skin

Explanation:

A triacylglycerol is derived from the hydrolysis of glycerols and fatty acids. Triacylglycerols have hydrophobic hydrocarbon tails and hydrophilic heads (amphipathic). The hydrophobic tails dissolve into dirt/grease on the skin while the hydrophilic tails work with running water to suspend and remove the dissolved impurities.

Example Question #8 : Help With Organic Lipids

 

Based on the formula and structure of this molecule, this molecule can be classified as a __________.

Possible Answers:

complex sugar

fatty acid

wax

protein

Correct answer:

wax

Explanation:

The correct answer is the lipid, wax. Lipids can often be identified based on their structure alone. The general structure of wax is an ester with two long carbon chains on each end of the ester, as seen in this problem. Additionally, fatty acids can be identified simply as long carbon chains with a carboxylic acid on one end, while the general structure of steroids can be seen as four connected hydrocarbon rings.

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