Chapter 15: lipids

Chapter 15: Lipids

- unlike carbohydrates and proteins, lipids are not easily characterized by their structural features
- lipids encompass a range of molecular structures and, as a result, an extraordinary range of
biochemical functions:
- as molecules of dietary fat certain lipids provide a major source of metabolic energy
- as components of biological membranes, lipids provide an insoluble partition between a cell
and its watery environment
- as hormones, lipids regulate a wide spectrum of cellular activities
- lipids are relatively nonpolar cmpds and can be separated from more polar cellular substances
by their solubility in nonpolar organic solvents such as diethyl ether or chloroform
- in fact the word lipid comes from the Greek word “lipos” meaning “fat”
- within the lipid family there are distinct structures that distinguish the different types of lipids:
the two main types are those that can be hydrolyzed and those with the steroid nucleus
- we will mainly be looking at lipids that can be hydrolyzed – esters such as waxes, fats, oils, and
phospholipids
Fatty Acids
- waxes, triacylglycerols (fats/oils), and phospholipids all yield fatty acids when hydrolyzed
- fatty acid: a long-chain carboxylic acid containing an even number of C atoms
- there are over 100 different fatty acids found in lipids in amounts that vary with the organism
- those with 14 -22 C are prevalent in the fats in mammals with 16 and 18 C the most abundant
- the variables found in the various fatty acids can be summarized as:
a. #C in the chain
b. # double bonds (the degree of unsaturation)
c. the location(s) of the double bond(s)
**you do not need to know the structures of the fatty acids in Table 15.1, p. 517**

- formulas of fatty acids:
- saturated fatty acids – example palmitic acid CH3(CH2)14COOH
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COOH
- this can also be drawn in a line-bond structural formula:
- for unsaturated fatty acids, the double bonds in nature are mostly found to have the cis
configuration
- example cis- oleic acid (C-18):
- fatty acids can be monounsaturated or polyunsaturated
- human body can synthesize most fatty acids from other fatty acids or from carbohydrates but
cannot synthesize polyunsaturated fatty acids
- some of these fatty acids, such as linoleic and linolenic acid, are required by the body so they
must be present in the diet and are termed essential fatty acids (EFAs)
- there are many natural sources of these EFAs; nuts, soybean, corn, fish oil
- linoleic acid is sometimes referred to as an -6 fatty acid and linolenic acid as -3 because the
first double bond occurs 6 and 3 carbons in from the end opposite the carboxylic acid group:
Physical Properties of Fatty Acids
- melting point: solid  liq
- sat’d fatty acids – the uniform structure of the C chains allows the molecules to fit closely
together in a regular pattern:
- the close pattern allows very many weak interactions to work together – much like a zipper
- the C – H bonds in each fatty acid are only very slightly polar but because of the length of the
chains and their regular structures many weak attraction can take place
- as a result, a significant amount of energy (heat) is required to separate the chains and melt the
fatty acid
– as the length of the chain increases more interactions occur between chains leading to higher
melting points
- as a result, saturated fatty acids have high melting points and are usually solids at room temp
Unsat’d fatty acids: the cis double bonds cause the C chain to “kink” and thus have an irregular
shape
- as a result, molecules cannot fit together in a regular pattern – so fewer interactions occur
between C chains:
- as a result the melting pts of unsat’d fatty acids are lower than those of sat’d fatty acids since it
takes less energy to separate the chains
- as a result most unsat’d fatty acids are liquids at room temp
- also, the more double bonds, the more disorder, the lower the melting pt
**we are not covering Prostaglandins**

Waxes
- an ester of a saturated fatty acid and a long chain alcohol, each containing from 14-30 C
- nonpolar – protective coatings on plant leaves and stems

Fats and Oils: Triacylglycerols
- both fats and oils have the same general chemical structure
- fats – solids at room temp – generally from animal sources
- oils – liquids at room temp – generally from plant sources
- are naturally occurring mixtures of triesters formed from the reaction of 3 fatty acid molecules
and a glycerol molecule:
- an example of this esterification reaction is the reaction between glycerol and 3 molecules of
palmitic acid:
- a simple triacylglycerol is a triester formed from glycerol and 3 identical fatty acids
- a mixed triacylglycerol is a triester formed from glycerol and different fatty acids:
- in nature simple triacylglycerols are rare, most naturally occurring are mixed
- fats and oils that are biologically important are complex mixtures of mixtures of mixed
triacylglycerols
- triacylglycerol mixtures from animals contains more sat’d fatty acid components than those
from plants
-fats – have a high % sat’d fatty acids
- oils – have a high % unsat’d fatty acids
- the degree of unsaturation can depend on the climate the plant grows in and for “lard” can
depend on the diet of the animal
Melting Points of Fats and Oils
- as for fatty acids themselves, triacylglycerols containing sat’d fatty acids have higher melting
pts than those with more unsat’s fatty acids:
-fats have a more regular structure and can pack together better and hence have more
interactions between molecules that require more energy to separate (higher melting pt)
- oils with their cis- fatty acids have more tangling of the molecules, can’t pack together as well,
and hence are easier to separate (lower melting pt)
Chemical Reactions of Triacylglycerols
- same kinds of reactions as alkenes and esters: hydrogenation of double bonds and acid and base
hydrolysis of ester bonds
Hydrogenation
- review of hydrogenation of an alkene:
- increases the degree of saturation when some double bonds are hydrogenated
- if an oil is partially hydrogenated, some of the double bonds are converted to single bonds
- with this change there is a corresponding increase in the melting pt of the substance
- when vegetable oils are partially hydrogenated then a semi-solid is produced from the liquid –
many food products are produced in this way
- the peanut oil in many popular brands of peanut butter has been partially hydrogenated to
convert the oil into a solid that does not separate of the mixture
Health Note (p.527)
- the high temps used in hydrogenation have been found to incr the # of trans double bonds by
isomerization in those double bonds that don’t get saturated:

- some studies have shown that trans double bonds can increase blood cholesterol levels and that
can lead to an incr in arterial plaque that can incr risk of heart attack
- trans fats also do not oxidize as fast as cis fats so shelf life of products containing them is
longer
Hydrolysis
- can take place using an acid catalyst or in our body with the use of enzymes (lipases):
Saponification
- is an hydrolysis reaction carried out in a basic solution
- products are glycerol and fatty acid salts (soap):
- see Environmental Note Ch. 13 (p.461) for cleaning action of soaps
- sodium and potassium salts are soluble in water because of full charges (hydrophilic end)
- long nonpolar chains (hydrophobic end) dissolve in nonpolar grease and form micelles:
Glycerophospholipids
- are triesters of glycerol in which 2 –OH groups are esterified with fatty acids and the 3rd is
esterified with phosphoric acid, which is in turn esterified with an amino alcohol:
- these serve as:
1. major components of cell membranes
2. make up much of the myelin sheath that protects nerve cells
3. in body fluids they combine with (emulsify) less polar triacylglycerols and cholesterol to make
them more soluble as they are transported in the body
Example of a lecithin:
**you do not need to know the structures of any particular glycerophospholipids **

- these glycerophospholipids contain both polar (actually ionic) and nonpolar regions, that allow
them to interact with both polar and nonpolar substances
- so we can view the molecule as having a polar (ionic) “head”, the choline + phosphate, and 2
nonpolar tails, the 2 fatty acid carbon chains
- another simplified representation for this structure is as follows:
- this dual polarity is similar to that encountered with soaps and is responsible for the ability of
soap to dissolve in both water and grease as we saw previously
Steroids
- nonsaponifiable lipids (can’t be hydrolyzed)
- based on a fused ring system referred to as the steroid nucleus:
- numerous steroids have been isolated from plants, animals, and human beings
- the locations of double bonds within the rings and the nature and locations of substituents
distinguish one steroid from another
Cholesterol
- the most abundant steroid in the human body
- the –ol ending because it is an alcohol
- within body – found in cell membranes (up to 25% by mass), nerve tissue, and brain tissue
(10% by mass)
- cholesterol plays a vital biochemical role in chemical synthesis – it is the starting material for
the synthesis of numerous steroid hormones, vitamin D, and bile salts
- its presence is essential to life
- the human body synthesizes, mainly in the liver, about 1 gram of cholesterol each day – an
amount sufficient to meet the body’s biosynthetic needs
- therefore cholesterol is not really necessary in the diet
- when we eat food containing cholesterol (meat, milk, cheese) the amount synthesized by the
body is reduced, but the reduction is less than the amount ingested
- therefore the total body cholesterol level increases with dietary cholesterol level
- medical science considers high blood cholesterol (plaque buildup), along with high blood
pressure and smoking, as major risk factors in cardiovascular disease
Lipoproteins: Transporting Lipids
- lipids must be transported in the bloodstream – stored, used for energy, to make hormones
- most lipids as we’ve seen are nonpolar and hence insoluble in the aqueous bloodstream
- they can be made more soluble by combining them with phospholipids and proteins to form
water-soluble complexes called lipoproteins:
- these are spherical particles – the outer surface has polar proteins and phospholipids that
surround hundreds of nonpolar molecules of triacylglycerols and cholesteryl esters (the
esterification of cholesterol and a fatty acid)
Types of lipoproteins
- they differ in density, lipid composition, and function:
Chylomicrons
Very Low Density Lipoproteins (VLDL) - in liver
- both of the above transport triacyglycerols, phospholipids, and cholesterol to tissue for storage
or muscle for energy
LDL – transport cholesterol to tissue for synthesis of cell membranes, steroid hormones, and bile
salts
- when level of LDLs exceeds amount of cholesterol needed by tissues, cholesterol is deposited
in arteries  restricts blood flow  myocardial infarction
- elevated LDL levels known as “bad cholesterol”
HDL – remove excess cholesterol from tissues and carry it to liver where it is converted to bile
salts and eliminated
- when HDL levels are high cholesterol not needed by tissues is carries to the liver for
elimination rather than deposited – therefore called “good cholesterol”
- it is found that a person on a high fat diet reabsorbs cholesterol from the bile salts causing less
cholesterol to be eliminated
- also, higher levels of sat’d fats in the diet stimulate the synthesis of cholesterol by the liver
Current suggested cholesterol levels:
Total cholesterol: < 200 mg/DL
LDL < 130 mg/DL
HDL > 40 mg/DL
- “statin” type drugs limit the production of cholesterol - Lipitor
** we are not covering steroid hormones or adrenal corticosteroids**

Cell Membranes
- all cells are surrounded by a plasma membrane that confines their contents
- the membrane separates the aqueous interior of a cell from the aqueous environment
surrounding the cell
- up to 80% of the mass the membrane is lipid material
- the fundamental structure of a cell membrane is a lipid bilayer predominantly made up of
various phospholipids
- a lipid bilayer is a 2-layer-thick structure of lipid molecules in which the nonpolar tails of the
lipids are in the middle and the polar heads are on the outside surface
- it is held together by dipole-dipole interactions, not by covalent bonds
- most of the phospholipids in the bilayer contain unsat’d fatty acids – due to the kinks at the
double bonds the phospholipids do not fit closely together
- as a result the bilayer is not a rigid, fixed structure but rather dynamic and fluid-like
-  this model of biological membranes is referred to as the Fluid Mosaic Model
- in the lipid bilayer there are also proteins, carbohydrates, and cholesterol molecules
- proteins- responsible for moving substances such as nutrients and electrolytes across the
membrane
- carbohydrates – outside of cell – cell recognition and communication with chemical
messengers such as hormones and neurotransmitters
- cholesterol – regulates membrane fluidity – compact shape lets it slip between side chains and
restrict movement making bilayer more rigid – in animals is 20-25% of bilayer

Source: http://www.chemistry.lakeheadu.ca/personal/martin/1010%20w10%20Ch15%20Notes.pdf