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Introduction to the Study of Lipids

Lipids

Classification of a compound as a lipid is based on its solubility behavior rather than the presence of a common functional group.

Lipids are compounds which dissolve in nonpolar or low polarity solvents such as toluene, dioxane, or

carbon tetrachloride.

Lipids

Note:

Other than triacylglycerols, most lipids are actually amphipathic:

One part of the molecule is hydrophobic. One part of the molecule is hydrophilic.

However, the hydrophobic part of a lipid molecule is usually much larger than the hydrophilic part.

Therefore, most lipids are insoluble (or very slightly soluble) in water.

Varieties of Lipids

• Triacylglycerols—animal fats and plant oils— sources of energy and a storage form of energy not required for immediate use.

• Glycerophospholipids, sphingolipids, and cholesterol are the primary structural components of the membranes that surround all cells and organelles.

• Steroid hormones and other hormone-like lipids act as chemical messengers, initiating or altering activity in specific target cells.

• The lipid-soluble (fat-soluble) vitamins - required for a variety of physiological functions.

• Bile salts - needed for the digestion of lipids in the intestinal tract.

Classification of Lipids

Hydrolyzable Lipids

Non-Hydrolyzable Lipids

Contain one or more straight chain fatty acids connected to

another structure through ester, amide, or glycoside linkages.

Do not contain hydrolyzable linkages.

Classification of Lipids

Fatty Acids

long hydrocarbon chain

carboxylic acid functional group

Fatty Acids

Biologically Important Fatty AcidsFatty acids may be saturated or unsaturated:

Saturated fatty acids: contain no C=C bonds. Palmitic (C-16) and stearic (C-18) acids are the most common.

Unsaturated fatty acids: contain one or more C=C bonds

(Almost all naturally occurring unsaturated fatty acids contain cis double bonds.)

(monounsaturated and polyunsaturated fatty acids).

Oleic (C-18, one C=C) - the most common.

Linoleic and linolenic acids - essential fatty acids for animals Synthesized only by plants. They must be supplied in the diet.

(The position of the double bonds in unsaturated fatty acids is sometimes indicated by an omega number (ω-). The ω-1 carbon is the methyl group farthest from the carbonyl carbon.) Linoleic and linolenic acids are ω-6 and ω-3 fatty acids.

Fatty Acids - Physical Properties

Fatty Acids - Structures

Waxes

Most waxes are fatty acid esters with long chain alcohols having only one –OH group.

The alcohols in waxes usually contain an even number of carbon atoms ranging from 14 to 36.

Beeswax Carnauba wax

Lanolins:

OCH3-(CH2)n-C-O

OCH3-(CH2)24-C-O-(CH2)29-CH3

OCH3-(CH2)14-C-O-(CH2)29-CH3

Classification of Lipids

Triacylglycerols (Triglycerides)

Make up about 90% of our dietary lipid intake.

Animal Fats, Fish oils, Plant Oils

Triacylglycerols are triesters of glycerol:

Naturally occurring triacylglycerols contain a variety of fatty acid components. Any fat or oil is a mixture of many different triacylglycerols, few of which are “simple” (R1=R2=R3).

The percentage of unsaturated fatty acids varies with the source:

Animal sources other than fish: 40-60% Fish (high percentage of polyunsaturation): 75-80% Plant sources: 85-90%

Triacylglycerols Defined by Their “R” Groups

Triacylglycerols - Fatty Acid Composition

Triacylglycerols - Physical Properties

Triacylglycerols - Chemical Properties

Hydrolysis

Acidic Conditions

Basic Conditions

+

+

+-

-

-

Soaps

Soaps are produced by a process called “saponification” (technically, “basic hydrolysis of fats”)

H2C

C

H2C

OH

O

O

C

C

O

O

C

O

C

C

O

O

C

O

-O

-O

-O

Na+

Na+

Na+

Hot aq NaOH

H2C

C

H2C

OHH

OH

OH

C

O

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH3-ONa+

Soaps

Soaps are the sodium or potassium salts of long-chain fatty acids.

hydrophilic region

hydrophobic region

How Do Soaps Work?

micelle micelle+grease

Synthetic Detergents

S

O

O-ONa+

O

S

O

-ONa+

O

S

O

-ONa+

O

Sodium Dodecyl Sulfate (SDS)

Sodium Alkyl Benzene Sulfonate (ABS)

Sodium Dodecyl Benzene Sulfonate (LAS)

N+CH2

N+

Synthetic Detergents

Na+

O

O

O

O

OS

O

O

-O

O

O

O

HO

Behavior of Soaps in Water

Triacylglycerols - Chemical Properties

Reduction

Triacylglycerols - Catalytic Hydrogenation

Catalytic hydrogenation - the addition of H2 to alkene double bonds in the presence of a metal catalyst (Ni or Pt).

This process is the basis for the conversion of plant oils into margarine and other products.

Some but not all of the double bonds in vegetable oils are hydrogenated in order to convert them into more solid, more palatable forms.

Effectively, one can adjust the melting point of any lipid component of a food.

Butter and other animal triacylglycerols contain a high percentage of saturated fatty acids which are implicated in the development of atherosclerosis.

Vegetable oils contain a much higher percentage of unsaturated fatty acids and almost no cholesterol.

Triacylglycerols in the Diet

What’s good about unsaturated triglycerides?

Decreasing the double bond content of vegetable oils also increases their shelf life because oxidation of the double bonds causes rancidity.

The catalysts used for hydrogenation reactions have a serious drawback. They also catalyze an unwanted reaction: the isomerization of some of the remaining cis double bonds into trans double bonds. Evidence suggests that trans double bonds raise blood cholesterol levels more than cis double bonds and perhaps more than saturated fatty acids.

What’s bad about unsaturated triglycerides?

Triacylglycerol Digestion

Triacylglycerols in the diet are too large to diffuse through the intestinal membranes.

Prior to absorption, triacylglycerols are first digested in the small intestine (basic pH) with enzymes called lipases with the help of bile salts.

The digestion in the intestines produces a mixture of primarily monoacylglycerols and fatty acids with some diacylglycerols and glycerol.

The intestinal cells then rebuild the triacylglycerols and combine them with proteins into particles called chylomicrons.

Chylomicrons are transported via the lymphatic system to the bloodstream where they are carried to various tissues.

In the tissues the triacylglycerols are separated from the proteins, hydrolyzed to fatty acids and glycerol, and metabolized to produce energy.

Fatty acids not needed for energy are reconverted to triacylglycerols and stored in adipose cells (adipocytes or fat cells) as fat droplets.

Triacylglycerol Digestion

Triglycerides as an Energy Source

Triacylglycerols are the primary energy storage form in animals.

Triacylglycerols: 9.2 kcal/g dry weight Glycogen: 4.0 kcal/g dry weight

In addition, glycogen is stored in a highly hydrated form whereas triacylglycerol is stored in an anhydrous form. Taking this into account 1 g of triacylglycerol yields 7 times more energy than 1 g of hydrated glycogen.

Normal liver and muscle glycogen levels can fill your energy needs for approximately 12 hours.

Adipose tissue triacylglycerols can fill your energy needs for several weeks to a couple of months.