midterm ch28

5/20/2018 Midterm Ch28

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Chapter

Outline

Chapter 28

Introduction to General, Organic, and Biochemistry, 10e

John Wiley & Sons, Inc

Morris Hein, Scott Pattison, and Susan Arena

LipidsPolar bears

have a largereserve

of lipids.


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Chapter

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28.1Lipids: Hydrophobic Molecules

28.2 Classification of Lipids

28.3 Simple Lipids

28.4 Fats in Metabolism

28.5 Compound Lipids

28.6 Steroids

28.7 Hydrophobic Lipids and Biology

Chapter 28 Summary

Course Outline

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Lipids: Hydrophobic Molecules

Lipids are water-insolublesubstances that have several

important biological functions that are related to how

lipids are classified.

Lipids interact weakly with water molecules because they

are composed primarily of nonpolar alkyl groups.

Lipids are classified as hydrophobic(water fearing) todesignate their inability to interact effectively with water

or their strong tendency to move away from water.


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Fatty acidsare common components of lipids.

As fatty acids get larger, the water solubility of the

fatty acid decreases dramatically as shown on thenext slide . . .


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Your Turn!

What characteristic do all lipids have in common?


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Your Turn!

What characteristic do all lipids have in common?

All lipids are insoluble in water.


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Your Turn!

Which fatty acid would you predict to have the lowest

melting point?

CH3CH2CH2C

O

OH CH3CH2CH2CH2CH2C

O

OH


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Your Turn!

Which fatty acid would you predict to have the lowest

melting point?

CH3CH2CH2C

O

OH CH3CH2CH2CH2CH2C

O

OH

m.p. = -7.9oC m.p. = -3.4

oC

Both fatty acids are saturated. The fatty acid with the

fewest number of carbon atoms should have the

lowest melting point.


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Classification of Lipids

Lipids molecules are relatively large and nonpolar. Yet,

within this broad description, lipid structures vary

markedly. Lipids can be classified in four categories

which recognize major structural similarities.

Simple lipids

Compound lipids

Steroids

Miscellaneous lipids


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The simple lipidsinclude fats,oils, and waxes.These

simple lipids are derivatives of lipid-like substances call

fatty acids.

Fatty acidsare long-chain carboxylic acids (generally

greater than about 12 carbons) that have no solubility in

water.

The hydrophilic-COOH group is referred to as apolar

head and the hydrophobic hydrocarbon portion is

referred to as a nonpolar tail.

Simple Lipids


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Simple Lipids

Fats and waxes are solids due to a higher composition of

saturated fatty acids while oils are liquids due to a

higher composition of unsaturated fatty acids.

Table 28.1 on the next slide lists properties of saturated

and unsaturated fatty acids separately. Notice how

unsaturated fatty acids generally have lower melting

points than saturated fatty acids.

Lipids prepared from fatty acids with lower melting points

tend to be liquids instead of solids at room temperature.


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Simple Lipids


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Simple Lipids

Unsaturated fatty acids exist as cis andtransisomers. The

cisisomer is more prevalent in nature.

The cis and transisomers of oleic acid are shown on thenext slide.

Oleic acid is an unsaturated fatty acid. The cisisomer has a

bent structure which prevents close stacking resulting ina compound that resists solidification.


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Simple Lipids

Ch


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Your Turn!

Draw the cisand transisomers of palmitoleic acid.

CH3(CH2)5CH=CH(CH2)7COOH.

Ch t


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Your Turn!

CC

H

H

C

C

H

H

trans-isomer

cis-isomer

Draw the cisand transisomers of palmitoleic acid.

CH3(CH2)5CH=CH(CH2)7COOH.

Ch t


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Simple Lipids

Certain fatty acids, as well as other lipids, are biochemical

precursors of several classes of hormones.

For example, -6

(omega-6) and -3 fatty acidsare usedto make hormones, the most common of which are the

eicosanoids.

These hormones are derived from fatty acids with 20carbon atoms, either the -6 (arachidonic acid) or the

-3 (eicosapentaenoic acid).

Ch t


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Simple Lipids

Omega () is the last letter in the Greek alphabet.

Correspondingly, the last carbon atom in a carbon chain

of a compound is often referred to as the omega carbon.

In reference to unsaturated carboxylic acids, omega plus a

number (e.g., -3) indicates the location of the first

carboncarbon double bond, counting from the omega

carbon.

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Simple Lipids

So for example arachidonic acid is an -6 fatty acid

because the first double bond from the last carbon atom

is on the sixth carbon atom.

COOH

CH312

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arachidonic acid

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Simple Lipids

Examples of eicosanoids are:

Thromboxanes

Prostacyclins Prostaglandins

Leukotrienenes

The synthesis of eicosanoids from arachidonic acid isshown on the next slide . . .

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Simple Lipids

Eicosanoids are hormones and coordinate various cellular

responses.

Some are involved with blood clotting as they can causeplatelet aggregation while others trigger an increase in

body temperature.

Some eicosanoids coordinate HCl and mucous secretionby the stomach lining and constriction of the bronchial

tubes in the lungs.

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Simple Lipids

Some eicosanoids stimulate and attract white cells,

while other eicosanoids cause the white cells to

disperse.

Eicosanoids can also cause vasodilation as well as

vasoconstriction.

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Many drugs control one or more of the physiological

effects produced by the eicosanoids.

For example, aspirin is a nonsteroidal anti-inflammatorydrug (andNSAID)that blocks the oxidation of

arachidonic acid which in turn prevents the formation

of prostaglandins and thromboxanes.

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Simple Lipids

Chapter


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Simple Lipids

Fats and oilsare triester derivatives of glycerol and three

fatty acid molecules as shown below.

Because there are three ester groups per glycerol, theselipids are called triacylglycerolsor triglycerides. The

fatty acids have carbon chains with 14-18 carbon atoms.

Chapter


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Triglycerides are prepared by the reaction of a glycerol

molecule and three fatty acid molecules. The fatty acid

molecules can be saturated or unsaturated.

Simple Lipids

Chapter


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Simple Lipids

Waxes are estersof high-molar-mass fatty acids and high-

molar-mass alcohols.

They have the general formula shown below in which thealcohol (ROH) contributes up to about 30 carbons.

Chapter


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Simple Lipids

Waxes are very large molecules with almost no polar

groups. They represent one of the most hydrophobic

classes of lipids.

Their extreme water insolubility allows waxes to serve a

protective function. Leaves, feathers, fruit, and fur are

often naturally coated with wax.

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An important biological function of a wax is to act as a

protective coating.

The shine on these leaves is due

to a thick protective wax coating.

Simple Lipids

Chapter


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Write the formula for a wax formed from palmitic acid

[CH3(CH2)14COOH] and 1-hexacosanol

[CH3(CH2)24CH2OH].

Your Turn!

Chapter


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Your Turn!

O

O

palmitic acid portion

1-hexacosanol portion

Write the formula for a wax formed from palmitic acid

[CH3(CH2)14COOH] and 1-hexacosanol

[CH3(CH2)24CH2OH].

As with all waxes, this wax is an ester.

Chapter


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What is the correct classification of the fatty acid shown

below?

CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH

-3

-6

Your Turn!

Chapter


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What is the correct classification of the fatty acid shown

below?

CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH

-3

-6

Your Turn!

The double bond is attached to the sixth carbon from the

end of the molecule furthest from the carboxyl group.

This is an -6 fatty acid.

Chapter


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Fats in Metabolism

Fatsare an important food source for humans and

normally account for about 2550% of our caloric

intake. Fats are an especially good source of metabolic

energy.

Most metabolic energy is derived from carbon oxidation.

When oxidized to carbon dioxide and water, fats supply

about 40 kJ per gram (9.5 kcal/g), which is more thantwicethe amount obtained from carbohydrates or

proteins.

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Fats in Metabolism

Fats are what our bodies prefer when storing energy

reserves. These reserves are in the form of

triacylglycereolsin fatty tissue.

On average, this tissue stores about two to three weeks

worth of energy.

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Your Turn!

The carbohydrate glucose (C6H12O6) and the fatty acid

capric acid (C9H19COOH) have similar molecular

weights. Recall that most metabolic energy is derived

from carbon oxidation. Using these two molecules asexamples, why does fat supply more energythan an

equivalent amount of carbohydrate?

Chapter


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Your Turn!

The carbohydrate glucose (C6H12O6) and the fatty acid

capric acid (C9H19COOH) have similar molecular

weights. Recall that most metabolic energy is derived

from carbon oxidation. Using these two molecules asexamples, why does fat supply more energythan an

equivalent amount of carbohydrate?

Fats are about 75% carbon by mass as compared to about40% for carbohydrates. In addition fats have a lower

degree of carbon oxidation when compared to

carbohydrates which results in higher energy yields

when fats are oxidized.

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There are three broad categories of compound lipids.

Phospholipids

Sphingolipids Glycolipids

Compound Lipids

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Compound Lipids

Thephospholipids

are a group of compounds that yield

one or more fatty acid molecules, a phosphate group,

and usually a nitrogenous base upon hydrolysis.

In contrast to the triacylglycerols, phospholipids have a

hydrophilic end that interacts with water.

Chapter

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Compound Lipids

Phospholipids are one of the most important membrane

components.

They are also involved in the metabolism of other lipidsand nonlipids.

Three categories of phospholipids arephosphatidic acids,

phosphatidylcholines, andphosphatidylethanolamines.

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Compound Lipids

Phosphatidic acidsare glyceryl esters of fatty acids and

phosphoric acid. The phosphatidic acids are important

intermediates in the synthesis of triacylglycerols and

other phospholipids.

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Compound Lipids

Phosphatidylcholines(lecithins) are glyceryl esters of fatty

acids, phosphoric acid, and choline.

Chapter

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Compound Lipids

The single most important biological function for

phosphatidylcholine is as a membrane componentwhich

makes up between 10 and 20% of many membranes.

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Compound Lipids

Another important constituent of biological membranes is

thephosphatidylethanolamines(cephalins). These

lipids are glyceryl esters of fatty acids, phosphoric acid,

and ethanolamine (HOCH2CH2NH2).

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Compound Lipids

Sphingolipidsare another type of compound lipid that,

when hydrolyzed, yield:

a hydrophilic group (either phosphate and choline or acarbohydrate)

a long-chain fatty acid (1826 carbons)

sphingosine(an unsaturated amino alcohol)

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Compound Lipids

The similarities between sphingosine and glycerol are

shown in the structures below.

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Compound Lipids

Sphingolipids are common membrane components because

they have both hydrophobic and hydrophilic character.

Sphingomyelinsare found in the myelin sheath membranesthat surround nerves.

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Compound Lipids

Glycolipidsare a third type of compound lipid. These

compounds that contain a carbohydrate group.

The two most important classes of glycolipids arecerebrosidesand gangliosides. These substances are

found mainly in cell membranes of nerve and brain

tissue.

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Compound Lipids

A cerebrosidemay contain either D-galactose or D-

glucose. The following formula of a galactocerebroside

shows the typical structure of cerebrosides.

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Your Turn!

What is the classification of the following lipid?

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Your Turn!


This lipid is an example of a phospholipid and more

specifically a phosphatidylethanolamine.

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Your Turn!


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Your Turn!


This moleculeis a

glycolipid.

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Steroids

Steroids

are compounds that have the steroid nucleus,

which consists of four fused carbocyclic rings. This

nucleus contains 17 carbon atoms in one five-membered

ring and three six-membered rings.

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Steroids

Modifications of this nucleus that occur in the various

steroid compounds include, for example, added side

alkyl chains, hydroxyl groups, carbonyl groups, and ring

double bonds.

Steroids are closely related in structure but are highly

diverse in function . . .

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Steroids

For example:

Cholesterol, the most abundant steroid in the body, is

widely distributed in all cells and serves as a majormembrane component.

Bile saltsaid in the digestion of fats.

Ergosterol, a yeast steroid, is converted to vitamin D by

ultraviolet radiation.

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Steroids

Digitalisand related substances called cardiac glycosides

are potent heart drugs.

The adrenal cortex hormonesare involved inmetabolism.

The sex hormones.

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Steroids

Cholesterolis the parent steroid compound from which the

steroid hormones are synthesized. In this process

cholesterol is converted toprogesterone, a compound

that helps control the menstrual cycle and pregnancy.

This hormone is also the parent compound from which

testosterone and the adrenal corticosteroids are

produced.

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Steroids

Cholesterol is also used to build cell membranes, many of

which contain about 25% by mass of this steroid.

ChapterOutline


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Your Turn!

Identify the functional groups in cortisone.

O

CH3

O

CH3 C

CH2OH

O

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Your Turn!

Identify the functional groups in cortisone.

O

CH3

O

CH3C

CH2OH

O

hydroxyl group

carbonyl groupcarbonyl group

carbonyl group carbon-carbon

double bond

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Hydrophobic Lipids and Biology

The hydrophobic nature of lipidshas many important

biological consequences. The water insolubility of

lipids results in:

lipid aggregation that causes atherosclerosis

lipid aggregation that formsbiological membranes

A lipid is in a hostile environment when it is surrounded

by water. Lipid molecules aggregate to minimize their

contact with water when in water.

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The hydrophilic part of lipid molecules is attracted to water

and forms an interface with it, but the hydrophobic part

distances itself from water molecules.

Complex lipids such as phospholipids and sphingolipids

have two hydrophobic alkyl groups.

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Complex lipids form liposomesin aqueous mixtures.

Liposomes are bounded by two layers of lipid.

The hydrophobic alkyl chains are covered by hydrophilicgroups on both the liposomes inside and outside.

Liposomes have a water core as shown below.

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Atherosclerosisis a metabolic disease that leads to

deposits of cholesterol and other lipids on the inner

walls of the arteries. Blood pressure increases as the

heart works harder to pump sufficient blood through thenarrowed passages, which may eventually lead to heart

attack.

Plaque formation begins because of a lipids naturaltendency to aggregate.

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Improper transport of cholesterol through the blood

contributes to atherosclerosis.

Cholesterol (and other lipids) must be packaged fortransport because lipids aggregate in the aqueous

bloodstream.

Lipids are packaged in particles called lipoproteins.

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The main types of lipoproteins are VLDL(very-low-

density lipoprotein), LDL(low-density lipoprotein)

and HDL(high-density lipoprotein).

The lipid distribution system through the bloodstream

using these lipoproteins is shown on the following

slide . . .

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Biological membranesare thin, semipermeable cellular

barriers. The general function of these barriers is to

exclude dangerous chemicals from the cell while

allowing nutrients to enter.

Because almost all the dangerous chemicals, nutrients, and

special molecules are water soluble, the membranes can

act as effective barriers only if they impede themovement of hydrophilic(water-soluble) molecules.

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The hydrophobic interior of the membrane provides the

necessary barrier while the hydrophilic exterior

interacts with the aqueous environment.

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Membrane lipids naturally aggregate to form lipid bilayers.

A lipid bilayer

is composed of two adjoining layers of

lipid molecules aligned so that their hydrophobic

portions form the bilayer interior while their hydrophilic

portions form the bilayer exterior.

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All known cells in todays world need a membrane that is

more complicated than a simple lipid bilayer.

A membrane must function as more than just a barrier.

Tasks such as passing molecules from one side of a

bilayer to the other are an essential part of life.

Proteinsin the fluid bilayer solve this dilemma. Theyallow specific molecular transport through the

hydrophobic interior.

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If the protein helps transport without using energy, the

process is called facilitated diffusion.

Energy-requiring transport is termed active transport.

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A complete cellular membrane must have both lipid and

protein. A typical membrane includes about 60%

protein, 25% phospholipid, 10% cholesterol, and 5%

sphingolipid.

The fluid lipid bilayer is studded with many solid

proteins. The proteins form a random pattern on the

outer surface of the oily lipid. This general membraneis called the fluid-mosaic modelwhich is shown on the

next slide . . .

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Which statement best describes the composition of a

biological membrane?

Hydrophobic exterior and hydrophilic interior

Hydrophilic exterior and hydrophobic interior

Hydrophilic exterior and interior

Hydrophobic exterior and interior

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Which statement best describes the composition of a

biological membrane?

Hydrophobic exterior and hydrophilic interior

Hydrophilic exterior and hydrophobic interior

Hydrophilic exterior and interior

Hydrophobic exterior and interior

Biological membranes have hydrophilic exteriors and

hydrophilic interiors.

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Chapter 28 Summary

Lipids are water-insoluble, oily, or greasy biochemicals.

All lipids are relatively large and nonpolar, yet lipid

structures may differ markedly.

Simple lipids are esters of fatty acids and alcohols.

Fats and oils are esters of fatty acids and glycerol.

Waxes are esters of fatty acids and high-molar-mass

alcohols.

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Chapter 28 Summary

Compound lipids are composed of fatty acid esters and

other components.

Phospholipids yield a phosphate and a nitrogen-

containing base as well as glycerol and fatty acids upon

hydrolysis.

Sphingolipids yield an unsaturated amino alcohol(sphingosine) and a carbohydrate or phosphate and

nitrogen base in addition to a fatty acid upon hydrolysis.

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Chapter 28 Summary

Glycolipids yield sphingosine and a carbohydrate as wellas a fatty acid upon hydrolysis.

Steroids posses the steroid nucleus containing 17 carbon

atoms.

Miscellaneous lipids include lipoproteins.

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Chapter 28 Summary

Fatty acids, which form part of most lipids, arecarboxylic acids with long, hydrophobic carbon chains.

Common fatty acids are straight-chain compounds with

an even number of carbons.

Unsaturated (double bondcontaining) fatty acids are

commonly cisisomers.

Omega-3 and omega-6 fatty acids are essential in human

diets. The eicosanoids are formed by oxidizing either

omega-3 or omega-6 fatty acids.

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Chapter 28 Summary

Fats and oils are esters of glycerol and long carbon chain

fatty acids.

Waxes are esters of fatty acids and a high-molar-massalcohol (up to about 30 carbons long).

Fats are a rich source of metabolic energy. They contain

a high percentage of carbon, whose oxidation releasesenergy.

Fats are our primary energy-storage molecules.

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Chapter 28 Summary

The phospholipids are a group of compounds that yieldone or more fatty acid molecules, a phosphate group,

and a nitrogenous base upon hydrolysis.

Phospholipids combine hydrophobic and hydrophilic

properties, essential features of membrane lipids.

Sphingolipids are compounds that, when hydrolyzed,yield a hydrophilic group, a fatty acid, and sphingosine.

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Chapter 28 Summary

Glycolipids are sphingolipids that contain carbohydrates.

Steroids are compounds that have the steroid nucleus.

The combined hydrophobic and hydrophilic character of

some lipids causes them to aggregate into specific

structures when placed in water.

Atherosclerosis is a metabolic disease in which fatty

material (plaque) is deposited on the inner walls of

arteries.

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Chapter 28 Summary

Improper transport of cholesterol through the bloodsystem contributes to atherosclerosis. Lipoproteins

(specific protein-lipid aggregates) transport lipids

through the blood.

A lipid bilayer is composed of two adjoining layers of

compound lipid molecules.

Proteins aid passage of molecules through a lipid

bilayer.

midterm ch28

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