15.1 Lipids 15.2 Fatty Acids · Lipids Lipids are • biomolecules that contain fatty acids or a steroid nucleus. • soluble in organic solvents, but not in water. • named for

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Chapter 15 Lipids

15.1 Lipids 15.2 Fatty Acids

Copyright © 2009 by Pearson Education, Inc.

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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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Types of Lipids

The types of lipids containing fatty acids are •  waxes •  fats and oils (triacylglycerols) •  glycerophospholipids •  prostaglandins

The types of lipids that do not contain fatty acids are •  steroids

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Classes of Lipids

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Fatty Acids

Fatty acids are •  long-chain carboxylic acids. •  typically 12-18 carbon atoms. •  insoluble in water. •  saturated or unsaturated.

Olive oil contains 84% unsaturated fatty acids and 16% saturated fatty acids.


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Saturated and Unsaturated Fatty Acids

Fatty acids are •  saturated with

all single C–C bonds.

•  unsaturated with one or more double C=C bonds.

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Properties of Saturated Fatty Acids

Saturated fatty acids •  contain only single C–C bonds.

•  are closely packed.

•  have strong attractions between chains.

•  have high melting points.

•  are solids at room temperature.

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Properties of Unsaturated Fatty Acids

Unsaturated fatty acids •  contain one or more cis double C=C

bonds. •  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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Melting Points of Some Saturated Fatty Acids


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Melting Points of Some Unsaturated Fatty Acids

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Learning Check

Assign the melting points of –17 °C, 13 °C, and 69 °C to the correct fatty acid. Explain.

stearic acid (18 C) saturated oleic acid (18 C) one double bond linoleic acid (18 C) two double bonds

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Solution

Stearic acid is saturated and would have a higher melting point than the unsaturated fatty acids. Because linoleic has two double bonds, it would have a lower mp than oleic acid, which has one double bond.

stearic acid mp 69 °C saturated oleic acid mp 13 °C

linoleic acid mp -17 °C most unsaturated

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Chapter 15 Lipids

15.3 Waxes, Fats, and Oils

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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.

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Fats and Oils: Triacylglycerols

Fats and oils are •  also called triacylglycerols. •  esters of glycerol. •  produced by esterification. •  Formed when the hydroxyl

groups of glycerol react with the carboxyl groups of fatty acids.


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

Triacylglycerols


Ester Bonds

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Formation of a Triacylglycerol

Glycerol + three fatty acids triacylglycerol

+ 3H2O

+

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Learning Check

What are the fatty acids in the following triacylglycerol?

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Solution

Stearic acid

Oleic acid

Myristic acid

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Melting Points of Fats and Oils

A fat •  is usually solid at room temperature. •  is prevalent in meats, whole milk, butter, and cheese. An oil •  is usually liquid at room temperature. •  is prevalent in plants such as olive and safflower.

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Oils with Unsaturated Fatty Acids

Oils •  have more unsaturated fats. •  have cis double bonds that cause “kinks” in the fatty

acid chains. •  with “kinks” in the chains do not allow the

triacylglycerol molecules to pack closely. •  have lower melting points than saturated fatty acids. •  are liquids at room temperature.

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Diagram of Triacylglycerol with Unsaturated Fatty Acids

Unsaturated fatty acid chains with kinks cannot pack closely.

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Percent Saturated and Unsaturated Fatty Acids In Fats and Oils

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Chapter 15 Lipids

15.4 Chemical Properties of Triacylglycerols


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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)

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What products are obtained from the complete hydrogenation of glyceryl trioleate?

1. Glycerol and 3 oleic acids 2. Glyceryltristearate 3. Glycerol and 3 stearic acids

Learning Check

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What products are obtained from the complete hydrogenation of glyceryl trioleate?

2. Glyceryltristearate

Solution

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OlestraTM, 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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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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Hydrogenation and Trans Fatty Acids

Most naturally occurring fatty acids have cis double bonds. •  During hydrogenation, some cis double bonds are converted to

trans double bonds. •  In the body, trans fatty acids behave like saturated fatty acids. •  It is estimated that 2-4% of our total Calories is in the form of

trans fatty acid. •  Several studies reported that trans fatty acids raise LDL-

cholesterol and lower HDL-cholesterol.

Hydrogenation of Unsaturated Fats

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Learning Check

(1) True or (2) false:

A. There are more unsaturated fats in vegetable oils. B. Vegetable oils have higher melting points than fats. C. Hydrogenation of oils converts some cis-double bonds

to trans-double bonds. D. Animal fats have more saturated fats.

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Solution

(1) True or (2) false:

A. T There are more unsaturated fats in vegetable oils. B. F Vegetable oils have higher melting points than fats. C. T Hydrogenation of oils converts some cis-double

bonds to trans- double bonds. D. T Animal fats have more saturated fats.

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Hydrolysis

In hydrolysis, •  triacylglycerols split into glycerol and three fatty acids. •  an acid or enzyme catalyst is required.

+

+

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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

+ 3NaOH

3+ “soap”

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What products are obtained from the complete hydrolysis of glyceryl trioleate?

1. Glycerol and 3 oleic acids 2. Glyceryl tristearate 3. Glycerol and 3 stearic acids

Learning Check

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What products are obtained from the complete hydrolysis of glyceryl trioleate?

1. Glycerol and 3 oleic acids

Solution

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Learning Check

Write the product of the following reaction:

+ 3

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Solution

C

O

( C H 2 ) 1 4 C H 3

C ( C H 2 ) 1 4 C H 3

O

C

O

( C H 2 ) 1 4 C H 3

C H 2 O

C H

C H 2

O

O

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Chapter 15 Lipids

15.5 Glycerophospholipids

Glycerol

PO4 Amino alcohol

Fatty acid

Fatty acid

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Glycerophospholipids are •  the most abundant lipids in cell membranes. •  composed of glycerol, two fatty acids, phosphate,

and an amino alcohol.

Glycerol

PO4 Amino alcohol

Glycerophospholipids

Fatty acid

Fatty acid

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Polarity of Glycerophospholipids

A glycerophospholipid has •  two nonpolar fatty acid chains. •  a phosphate group and a polar amino alcohol. CH3 │+ + HO−CH2−CH2−N−CH3 HO−CH2−CH2−NH3

│ Choline CH3 Ethanolamine + NH3 │ Amino alcohols HO−CH2−CH−COO− Serine

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Structure and Polarity of a Glycerophospholipid

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Lecithin and cephalin are glycerophospholipids •  abundant in brain and nerve tissues. •  found in egg yolk, wheat germ, and yeast.

Lecithin and Cephalin


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Learning Check

Identify each as A. fatty acid B. triacylglycerol C. amino alcohol D. glycerophospholipid

1. glyceryl trioleate 2. cephalin 3. choline 4. palmitic acid

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Solution

Identify each as A. fatty acid B. triacylglycerol C. amino alcohol D. glycerophospholipid

1. glyceryl trioleate B. triacylglycerol 2. cephalin D. Glycerophospholipid 3. choline C. amino alcohol 4. palmitic acid A. fatty acid

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Chapter 15 Lipids

15.6 Steroids: Cholesterol and Steroid

Hormones


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Steroid Nucleus

A steroid nucleus consists of

•  3 cyclohexane rings. •  1 cyclopentane ring. •  no fatty acids.

steroid nucleus

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Cholesterol

Cholesterol •  is the most abundant steroid in the body. •  has methyl CH3- groups, alkyl chain, and -OH

attached to the steroid nucleus.

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Cholesterol in the Body

Cholesterol •  is obtained from meats, milk, and

eggs. •  is synthesized in the liver. •  is needed for cell membranes, brain

and nerve tissue, steroid hormones, and vitamin D.

•  clogs arteries when high levels form plaque.

An artery clogged by cholesterol plaque

A normal, open artery.

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Cholesterol in Foods

Cholesterol •  is considered

elevated if plasma cholesterol exceeds 200 mg/dL.

•  is synthesized in the liver and obtained from foods.

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Learning Check

Match the components of the cholesterol molecule with the following: ___ carbon chain ___hydroxyl group ___ steroid nucleus ___methyl group

A C

B

D

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Solution

Match the components of the cholesterol molecule with the following: _D carbon chain _A_hydroxyl group _C steroid nucleus _B_methyl group

A C

B

D

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Lipoproteins

Lipoproteins •  combine lipids

with proteins and phospholipids.

•  are soluble in water because the surface consists of polar lipids.

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Types of Lipoproteins

Lipoproteins •  differ in density, composition, and function. •  include low-density lipoproteins (LDLs) and high-

density lipoproteins (HDLs).

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Transport of Lipoproteins in the Body

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Steroid Hormones

Steroid hormones are •  chemical messengers in cells. •  sex hormones.

- androgens in males (testosterone) - estrogens in females (estradiol)

•  Adrenocortical hormones from adrenal glands. - mineralocorticoids (electrolyte balance) - glucocorticoids (regulate glucose level)

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Steroid Hormones

Steroid hormones •  are produced from

cholesterol.

•  include sex hormones such as androgens (testosterone) in males and estrogens (estradiol) in females.


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Adrenal Corticosteroids

Steroid hormones called adrenal corticosteroids •  are produced by the adrenal glands located on the top

of each kidney. •  include aldosterone, which regulates electrolytes and

water balance by the kidneys. •  include cortisone, a glucocorticoid, which increases

blood glucose level and stimulates the synthesis of glycogen in the liver.

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Anabolic Steroids

Anabolic steroids •  are derivatives of testosterone. •  are used illegally to increase muscle mass. •  have side effects including fluid retention, hair growth,

sleep disturbance, and liver damage.


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Adrenal Corticosteroids


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Learning Check

Identify each lipid as a (1) fatty acid, (2) steroid, or (3) triacylglycerol.

A. cholesterol B. stearic acid C. glyceryl tristearate D. estradiol E. contains no fatty acids

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Solution

Identify each lipid as a (1) fatty acid, (2) steroid, or (3) triacylglycerol. A. 2 cholesterol B. 1 stearic acid C. 3 glyceryl tristearate D. 2 estradiol E. 2 contains no fatty acids

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Chapter 15 Lipids

15.7 Cell Membranes

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Cell Membranes

Cell membranes •  separate cellular contents from the external

environment. •  consist of a lipid bilayer made of two rows of

phospholipids. •  have an inner portion made of the nonpolar tails of

phospholipids with the polar heads at the outer and inner surfaces.

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Fluid Mosaic Model of Cell Membranes

The lipid bilayer

•  contains proteins, carbohydrates, and cholesterol.

•  has unsaturated fatty acids that make cell membranes fluid-like rather than rigid.

•  has proteins and carbohydrates on the surface that communicate with hormones and neurotransmitters.

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Fluid Mosaic Model of Cell Membranes

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Transport Through Cell Membranes

The transport of substances through cell membranes

involves

•  diffusion (passive transport), which moves particles from a higher to a lower concentration.

•  facilitated transport, which uses protein channels to increase the rate of diffusion.

•  active transport, which moves ions against a concentration gradient.

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Transport Pathways Through Cell Membranes


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Learning Check

The transport of particles across a cell membrane from

high concentration to low concentration is called

1. facilitated transport.

2. diffusion.

3. active transport.

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Solution

The transport of particles across a cell membrane from high concentration to low concentration is called

2. diffusion.

15.1 Lipids 15.2 Fatty Acids · Lipids Lipids are • biomolecules that contain fatty acids or a steroid nucleus. • soluble in organic solvents, but not in water. • named for

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