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Chem 150
Unit 5 - Biological Molecules I
Lipids
Like organic molecules, biological molecules are grouped
into families. There are four major families of biological
molecules, including proteins, nucleic acids,
carbohydrates, and lipids. The lipids are the subject of this
unit. Of these four families, the lipids are the structurally the
most diverse. This is because unlike members of the other
three families, members of this families do no share a
common structural feature, but rather share a common
physical property; the are hydrophobic.

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Introduction
Lipids are hydrophobic, nonpolar molelcules.
• They are soluble in nonpolar solvent.
• They are insoluble in polar solvents, such as water
They are isolated from the other biological molecules by
extracting them with nonpolar solvents.

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Introduction
The types of lipids that we will look at include.
• Fatty Acids
• In the carboxylic acid family
• Waxes
• Fatty Acids + Alcohols
• Triglycerides
• 3 Fatty acids + glycerol
• Phospholipids and glycolipids
• 2 fatty acids + glycerol + phosphate + X
• Steroids
• Derivatives of cholesterol
• Eicosanoids
• Derivatives of the Fatty acid arachidonic acid
• Membranes
• Formed from phospholipids and glycolipids

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Fatty Acids
Fatty acids contain a carboxylic acid group
• This should make them quite polar
However, they also contain a long hydrocarbon tail
• Which overall, makes them nonpolar.
nonpolar polar

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Fatty Acids
Fatty acids typically contain between 12 and 20 carbons
• The number is usually always even.
• The nonpolar tails interact with London forces.
nonpolar polar

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Fatty Acids
Melting points for saturated fatty acids:
No. of Carbons
Melting
Temperature
{°C}

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Fatty Acids
Some fatty acids contain double bonds
• unsaturated
• monounsaturated
• polyunsaturated
• polyunsaturated

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Fatty Acids
Normally the double bonds are cis
• This lowers the melting points for fatty acids containing
double bonds.
No. of Double Bonds
Melting
Temperature
{°C}

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Fatty Acids
The cis double bonds produce kinks, which disrupt the
London forces by preventing the tails from packing close to
one another.

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Fatty Acids
As acids, the carboxylic acid group in fatty acids can react
with a base to produce a carboxylate ion
• By donating its proton (H+) to the base the fatty acid
becomes negatively charged.
• We will talk more about acids and bases in Unit 6

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Fatty Acids
The negative charge makes the polar head portion of the the
fatty acid even more more polar and hydrophilic.

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Fatty Acids
The salts of fatty acids are also called soaps, and are
considered amphipathic, meaning they have a part that is
very hydrophobic along with a part that is very hydrophilic.

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Biochemical Compounds &
Their Interactions with Water (Unit 3)
When placed in water,
amphipathic molecules,
form structures, such as
micelles, which attempt to
address the conflict.

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Fatty Acids
The salts of fatty acids are also called soaps, and are
considered amphipathic, meaning they have a part that is
very hydrophobic along with a part that is very hydrophilic.

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Waxes
Waxes are made by combining fatty acids with long chain
alcohols.
• In Unit 2 we discussed how carboxylic acids react with
alcohols to from esters.

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Waxes
Waxes are esters.
14-36 carbons 16-30 carbons

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Waxes
When two more molecules combine to form a larger
molecule, the word residue is used to indicate which
molecule that part of the the larger molecule came from.
came from the alcohol
came from the fatty acid

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Waxes
Waxes are very hydrophobic and are used by plants and
animals for protective, water-proof coatings

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Reactions Involving Water (Unit 4)
Hydrolysis
• Hydrolysis can also be catalyzed using a base (OH-):.
• Because one of the products of the hydrolysis is a carboxylic acid, in base
catalyzed hydrolysis the base undergoes a second acid/base reaction with the
carboxylic acid to produce a carboxylate ion.
• The base catalyzed hydrolysis of esters is also called saponification
• We will be discussing acids and bases in Unit 6

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Triglycerides
Triglycerides are a storage form of fatty acids in mammals.
• Often when blood tests are done, they measure your
triglycyeride levels.
• High triglyceride levels in the blood are a risk indicator for
artherosclerosis.
*American Heart Association

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Triglycerides
Triglycerides are a combination of three 3 fatty acid
molecules with a glycerol molecule.

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Triglycerides
Glycerol, which is also called glycerin,
is an alcohol with three hydroxyl
groups.
• As with the waxes, the fatty acids
can react with the hydroxyl groups to
form esters.
• Since there are three hydroxyl
groups, three fatty acids can react to
form three esters.

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Triglycerides
Just as with fatty acids, where the presence of cis double
bonds lower the melting points, triglycerides made from
unsaturated fatty acids have lower melting points than those
made from saturated fatty acids.
• Triglycerides from animals tend to have a higher proportion
of saturated fatty acids.
• Most are solids at room temperature and are called fats.
• Examples include: butter, lard and bacon grease
• Triglycerides from plants tend to have a higher proportion
of unsaturated fatty acids.
• Most are liquids at room temperature and are called oils.
• Examples include: corn oil, canola oil, peanut oil and olive oil.

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Triglycerides
Triglycerides as primarily used as a form of stored energy.
• This is why when you eat more than you need to meet your
energy requirements, the excess energy is stored in the
form of fat.
• Fat can store almost twice as much energy per gram as
carbohydrates and proteins
• In mammals the fats are stored in the adipose tissue.
• Adipose tissue also functions to protect organs from shock
and cold.

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Triglycerides
Reactions that involve triglycerides include:
• Hydrogenation
• Oxidation
• Base-catalyzed hydrolysis (saponification)

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Triglycerides
Saturated vs Unsaturated Fats

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Triglycerides
Saturated vs Unsaturated Fats

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Triglycerides
Saturated vs Unsaturated Fats

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Triglycerides
Saturated vs Unsaturated Fats

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Triglycerides
Saponification of triglycerides
• Saponification is the base-catalyzed hydrolysis of the ester
bonds in a triglyceride.
• This cleaves the esters back into carboxylic acids (fatty
acids) and an alcohol (glycerol).
• Because the reaction is base-catalyzed, the base also
reacts with the carboxylic acids to from carboxylate ions

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Reactions With Water (Unit 4)
Hydrolysis example:
• The base catalyzed hydrolysis of fats produces soap and
glycerol

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Phospholipids and Glycolipids
Phospholipids and Glycolipids are the stuff that biological
membranes are made of.
• Like the soaps, these molecules are highly aphipathic, and
when mixed with water spontaneously form membranes
that are described as lipid bilayers.

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Phospholipids and Glycolipids
Soaps form
Micelles
Phospholipids form
Lipid Bilayers

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Phospholipids and Glycolipids
Phospholipids and Glycolipids are the stuff that biological
membranes are made of.
• Like the soaps, these molecules are highly aphipathic, and
when mixed with water spontaneously form membranes
that are described as lipid bilayers.

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Phospholipids and Glycolipids
Phosphospholipids
• There a are two types of phospholipids
• Glycerophospholipids

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Phospholipids and Glycolipids
Phosphospholipids
• There a are two types of phospholipids
• Sphingolipids

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Phospholipids and Glycolipids
Phosphospholipids
• The Glycerophospholipids have a structure similar to
triglycerides, with one of the fatty acids replaced with a
phosphate.
There is usually
an additional
alcohol attached
to the other side
of the phosphate

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Phospholipids and Glycolipids
Phosphospholipids
• The Glycerophospholipids have a structure similar to
triglycerides, with one of the fatty acids replaced with a
phosphate.
phosphoester
bonds

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Phospholipids and Glycolipids
Phosphospholipids
• Phospholipids are used commercially as emulsifying
agents.
• An emulsifying agent stabilizes an emulsion.
• An emulsion is a colloidal suspension of one liquid in another.
‣ An example is mayonnaise, which is a colloidal suspension of oil and
water.
• Lecithin, which is another name for the phospholipid
phosphotidylcholine, is used as an emulsifying agent in
mayonnaise and other prepared foods.

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Steroids
Steroids are a type of lipid that is not derived form a fatty
acid.
• They are based instead on a system of five cycloalkane
rings that are fused together.

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Steroids
Steroids are a type of lipid that is not derived form a fatty
acid.
• They are based instead on a system of five cycloalkane
rings that are fused together.

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Steroids
Cholesterol is the steroid that used as the starting point for
the synthesis of other steroids.
Note the fused ring
system

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Steroids
• Cholesterol is only found in animals
• Besides being used to synthesize the other steroids,
cholesterol is dissolved in membranes to keep them fluid.
• Plants use the alternative strategy of using polyunsaturated fatty
acids to make their phospholipids.

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Lipoproteins
Lipoproteins are used to transport
the water insoluble lipids such as
triglycerides, phospholipids and
cholesterol, in the blood.
• Lipoproteins contain lipids and
proteins.
• They include:
• Chylomicrons transport primarily
triglycerides from the digestive track.
• LDLs (low density lipoproteins)
transport cholesterol, triglycerides and
phospholipids from the liver to other
tissues.
• HDLs (high density lipoproteins)
transport cholesterol and
phospholipids back to the liver.

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Lipoproteins
The HDL and LDL levels in the blood can be used to assess
ones risk for atherosclerosis.
• High levels of HDL is considered good
• This is why HDL is sometimes referred to as “good cholesterol”
• > 40 mg/dL is good.
• High levels of LDL is considered bad
• This is why LDL is sometimes referred to as “bad cholesterol”
• > 100 mg/dL is bad.

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Membranes
Fluid mosaic model

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Membranes
Transport across membranes

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The End