1 chapter 17: lipids lipids are biomolecules that contain fatty acids or a steroid nucleus. ...

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Chapter 17: Lipids

Lipids are Biomolecules that contain fatty acids or a steroid

nucleus. Soluble in organic solvents but not in water. Named for the Greek word lipos, which means “fat.” Extracted from cells using organic solvents.

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Structures of Lipids

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Fatty Acid: Long Chain COOH’s

The formulas for fatty acids are written as Condensed formulas. Line-bond formulas. For example caprylic acid with 8 carbon atoms. CH3—(CH2)6—COOH

CH3—CH2—CH2—CH2—CH2—CH2—CH2—COOH

OH

O

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Saturated fatty acids have Single C–C bonds. Molecules that fit closely together

in a regular pattern. Strong attractions between fatty

acid chains. High melting points that make

them solids at room temperature.

Saturated Fatty Acids

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Some Saturated Fatty Acids

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TABLE 17.1

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Unsaturated Fatty Acids

Unsaturated fatty acids Have one or more double C=C bond Typically contain cis double bonds.

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Properties of Unsaturated Fatty AcidsUnsaturated fatty acids Have “kinks” in the fatty

acid chains. Do not pack closely.

Have few attractions between chains.

Have low melting points. Are liquids at room

temperature.

“kinks” in chain

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Unsaturated Fatty Acids

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TABLE 17.1

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Comparing Melting Points of Some Fatty Acids

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Prostaglandins

Prostaglandins have 20 carbon atoms in their fatty acid chains. An OH on carbon 11 and 15. A trans double bond at carbon 13.

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C 11C 15C 13

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Prostaglandins in the Body

Prostaglandins are Produced by injured

tissues. Involved in pain,

fever, and inflammation.

Not produced when anti-inflammatory drugs such as aspirin inhibit their synthesis.

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Omega-6 and Omega 3- Fatty Acids Fatty acids In vegetable oils are mostly omega-6 with

the first C=C at C6.

linoleic acid

CH3─(CH2)4─CH=CH─CH2─CH=CH─(CH2)7─COOH

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In fish oils are mostly omega-3 with the

first C=C at C3.

linolenic acid CH3─CH2─(CH=CH─CH2)3─(CH2)6─COOH

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Some Omega-6 and Omega-3 Fatty Acids

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Structures of Lipids

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Waxes

Waxes are Esters of saturated fatty acids and long-chain alcohols. Coatings that prevent loss of water by leaves of plants.

TABLE 17.2

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Structures of Lipids

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In a triacylglycerol, Glycerol forms ester bonds with three fatty acids.

Fats and Oils: Triacylglycerols

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Formation of a Triacylglycerol

glycerol + three fatty acids triacylglycerol

OHCH2

OH

OHCH2

CHO

(CH2)14CH3CHO

O

(CH2)14CH3CHO

O

(CH2)14CH3CHO

+ 3H2O

O

O

C (CH2)14CH3

CH O

O

C (CH2)14CH3

CH2 O

O

C (CH2)14CH3

CH2

+

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Comparing Melting Points of Some Fatty Acids

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Melting Points of Fats and Oils

A triacylglycerol that is a fat Is solid at room temperature. Is prevalent in meats, whole milk, butter, and cheese.

A triacylglycerol that is an oil Is liquid at room temperature. Is prevalent in plants such as olive and safflower.

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Diagram of Triacylglycerol with Unsaturated Fatty Acids

Unsaturated fatty acid chains have kinks that do not allow close packing.

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Saturated and Unsaturated Fatty Acids In Fats and Oils

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The chemical reactions of triacylglycerols are similar

to those of alkenes and esters.

In hydrogenation, double bonds in unsaturated fatty acids react with H2 in the presence of a Ni or Pt catalyst.

In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme.

Chemical Properties of Triacylglycerols

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Hydrogenation of Oils

The hydrogenation of oils Adds hydrogen (H2) to the carbon atoms of double bonds. Converts double bonds to single bonds. Increases the melting point. Produces solids such as margarine and shortening.

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Hydrogenation

Ni + 3H2

glyceryl tripalmitoleate

(tripalmitolean)

glyceryl tripalmitate

(tripalmitin)

O

(CH2)14CH3C

O

(CH2)14CH3C

O

(CH2)14CH3C

O

O

OCH2

CH2

CH

CH(CH2)7CH3(CH2)5CH

O

C

CH(CH2)7CH3(CH2)5CH

O

C

CH(CH2)7CH3(CH2)5CH

O

C

O

O

OCH2

CH2

CH

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Comparing Melting Points of Some Fatty Acids

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Olestra, A Fat Substitute

Olestra is Used in foods as an artificial fat. Sucrose linked by ester bonds to several long-chain

fatty chains. Not broken down in the intestinal tract.

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Oxidation of Unsaturated Fats

Unsaturated fatty acids oxidize to short-chain aldehydes and short-chain carboxylic acids

Example: Heavy perspiration

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Cis and Trans Fatty Acids

Unsaturated fatty acids can be Cis with bulky groups on same side of C=C. CH3─ (CH2)5 (CH2)7─ COOH cis

C=C H H

Trans have bulky groups on opposite sides of C=C. CH3─ (CH2)5 H

C=C trans

H (CH2)7─ COOH

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Trans Fatty Acids and Hydrogenation Trans fatty acids

Are formed during hydrogenation when cis double bonds are converted to trans double bonds.

In the body behave like saturated fatty acids.

Are estimated to make up 2-4% of our total Calories.

Are reported in several studies reported to raise LDL-cholesterol (bad) and lower HDL-cholesterol. (good)

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Foods that contain Trans Fats

Milk Bread Fried foods Ground beef Baked goods Stick margerine, soft margerine Butter Cookies, crackers

Food Label reads: “Partially hydrogenated oil”

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Hydrolysis

In hydrolysis, Triacylglycerols split into glycerol and three fatty acids. An acid or enzyme catalyst is required.

OCH2

OCH

OCH2

OHCH2

OHCH

OHCH2

O

(CH2)14CH3CHO

H2OO

(CH2)14CH3C

O

(CH2)14CH3C

O

(CH2)14CH3C

H+

+3

+ 3

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Saponification and Soap

Saponification Is the reaction of a fat with a strong base. Splits triacylglycerols into glycerol and the salts of

fatty acids. Is the process of forming “soaps” (salts of fatty

acids). With KOH gives softer soaps.

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Saponification

O

O

C (CH2)14CH3

CH O

O

C (CH2)14CH3

CH2 O

O

C (CH2)14CH3

CH2

+ 3NaOH

Na+ -O

O

C (CH2)14CH33

OH

CH OH

CH2 OH

CH2

+

“soap”