General Characteristics
IB Biology
Allen
Topic 3: The Chemistry of life:
Lipids
Types of Lipids
• Lipids with fatty acids
Triglycerides (Fats and oils)
Phospholipids
Waxes
• Lipids without fatty acids
Steroids
Triglycerides
• Triglycerides are
synthesized via
condensation
reactions.
• Glycerol & fatty acids
are joined with ester
bonds (a condensation
reaction involving
hydroxyl and carboxyl
groups)
• All Lipids are hydrophobic. Consist of carbon, hydrogen
and oxygen
• Contain fewer polar O-H bonds and more non-polar C-H
bonds than carbohydrates.
• includes fats and oils, waxes, phospholipids, steroids, and
some other related compounds.
• Primarily energy sources and structural compounds and
signaling molecules.
• Large number of bonded hydrogens - therefore release a
larger amount of energy than other organic compounds. Fats
yield 9kcal/gm, carbohydrates 4kcal/gm
Triglycerides
Triglycerides (fats and oils)
Hydroxyl
groups
• Composed of three fatty acids
joined to a glycerol molecule.
• Glycerol is a three carbon
molecule with hydroxyl
groups on each carbon.
• Fatty acids - long
hydrocarbon chains with
terminal carboxyl (COOH)
group.
…Types of Lipids
• The main distinction between fats and oils is whether they’re
solid or liquid at room temperature
• Fats
– mostly from animal sources
– triglycerides containing saturated fatty acids (e.g. butter is
solid at room temperature)
• Oils
– mostly from plant sources
– triglycerides with unsaturated fatty acids (e.g. corn oil is
liquid at room temperature)
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Fatty Acids
Structure of Fatty Acids
Saturated and Unsaturated Fatty Acids
Saturated Fatty acids
• all fatty acids have long
hydrocarbon chains with terminal
carboxyl (COOH) group
• The “tail” of a fatty acid is a long
hydrocarbon chain, making it
hydrophobic.
• Typically 12-18 carbon atoms
(even number)
• Saturated fatty acids have no
double bonds between their
carbon atoms
• Unsaturated fatty acids have
double bonds in the carbon chain
•
Saturated and Unsaturated Fatty Acids
Saturated and Unsaturated Fatty Acids
Unsaturated Fatty acids
• Oils have some double bonds between some of the carbons in the
hydrocarbon tail, causing bends or “kinks” in the shape of the
molecules.
• Because some of the carbons share double bonds, they’re not
bonded to as many hydrogen as they could if they weren’t double
bonded to each other. These oils are called unsaturated fats.
• Because of the kinks in the hydrocarbon tails, unsaturated fats
can’t pack as closely together, making them liquid at room
temperature.
• unsaturated fats are “healthier”
than the saturated ones.
• Hydrogenated vegetable oil (as in
shortening and commercial peanut butters
where a solid consistency is sought) started
out as “good” unsaturated oil. However,
this commercial product has had all the
double bonds artificially broken and
hydrogen artificially added (in a chemistry
lab-type setting) to turn it into saturated fat
that bears no resemblance to the original oil
from which it came (so it will be solid at
room temperature).
Cis and Trans Bonds
• In unsaturated fatty acids, there are two ways the
pieces of the hydrocarbon tail can be arranged around
a C=C double bond.
• In cis bonds, the two hydrogens on either side of the
double bond are either both “up” or both “down,”
(both are on the same side of the molecule).
• In trans bonds, the two
hydrogens are on opposite
sides of the double bond,
One is “up” and one is
“down”.
•
•
The terms saturated, mono-unsaturated, and poly-unsaturated refer to the
number of hydrogen attached to the hydrocarbon tails of the fatty acids as
compared to the number of double bonds between carbon atoms in the tail.
Saturated fats have all single bonds between the carbons in their fatty acid
tails, thus all the carbons are also bonded to the maximum number of
hydrogen possible…maximum possible amount of hydrogen saturated
fats.
The hydrocarbon chains in these fatty acids are, thus, fairly straight and can
pack closely together. There are strong attractions between the fatty acids in
triglycerides with saturated fatty acids. These fats are solid at room
temperature.
…Cis and Trans Bonds
• Naturally-occurring unsaturated vegetable oils have almost all
cis bonds, but using oil for frying causes some of the cis bonds
to convert to trans bonds. If oil is used only once like when
you fry an egg, only a few of the bonds do this so it’s not too
bad. However, if oil is constantly reused, like in fast food
French fry machines, more and more of the cis bonds are
changed to trans until significant numbers of fatty acids with
trans bonds build up.
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…Cis and Trans Bonds
Fatty acids with trans bonds are carcinogenic, or cancer-causing. The
levels of trans fatty acids in highly-processed, lipid-containing products
such as margarine are quite high.
• Cis fats exist naturally and, because the hydrogen atoms are crowded on
one side of the molecule, they bend, allowing other chemicals and
enzymes to bind to them.
• Trans fats do not exist naturally, with a very few exceptions. Because the
structure is uncrowded, they do not bend and so other molecules and
enzymes find it more difficult to bind to them.
• It is the very fact that they are straight that
allows trans fats to solidify at room temperature.
Natural, cis fats are curved and so can't pack into
a crystal formation at normal temperatures.
Trans fats, on the other hand, are straight and
CAN pack into a crystal formation, which allows
them to solidify at room temperature.
Stop & Think
•
How would the melting point of stearic acid
compare to the melting points of oleic acid and
linoleic acid? Assign the melting points of –17°C,
13°C, and 69°C to the correct fatty acid. Explain.
– stearic acid
– oleic acid
– linoleic acid
Stop & Think
Elaidic acid
Stop & Think
• Which two fatty acids are isomers?
• Identify each fatty acid as a
– trans unsaturated fatty acid
– cis unsaturated fatty
– saturated fatty acid
Oleic acid
(18 C) saturated
(18 C) one double bond
(18 C) two double bonds
Stearic acid
Membrane Structure
…Types of Lipids
Phospholipids - two fatty acids attached to
phosphate group
• phosphate heads are hydrophilic (water
soluble) but tails are hydrophobic (water
insoluble).
• spontaneously form micelles in water.
• very important because they form
biological membranes…
– Phospholipid bilayer
• Hydrophillic heads in contact with aqueous environment
• Hydrophobic tails reside in interior of bilayer
• Cholesterol is a structural component
– Proteins
• Integral
• Peripheral
• Lipid anchored
– Dynamic structure
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Stop & Think
• Sketch the arrangement of miscelles in an
oily environment
…Steroids (Sterols)
Cholesterol
• High concentrations of
cholesterol in the
bloodstream and a diet rich in
saturated fats have been
linked to the development of
atherosclerosis, a condition
in which fatty deposits, called
plaque, form on the inner
lining of blood vessels,
blocking the flow of blood to
tissues. Deprived of oxygen
and nutrients, the cells of the
tissues die.
…Steroids (Sterols)
…Cholesterol
• Cholesterol is not the bad guy. It’s how it is
packaged.
• LDL is the ‘bad’ cholesterol. It transports
cholesterol from liver to tissues. Excess is
dumped into arteries.
• HDL is the ‘good’ cholesterol. They 'sweep
up' the excess cholesterol and return it to the
liver for disposal.
Steroids (Sterols)
• Compact hydrophobic molecules containing
four fused hydrocarbon rings and several
different functional groups
Examples:
• Cholesterol-precursor
to sex hormones and
vitamin D
• Sex hormones
…Steroids (Sterols)
…Cholesterol
• Vitamin D is formed by the action of UV light in
sunlight on cholesterol molecules that have “risen” to
near the surface of the skin.
• Many medical journals state that people should not
shower immediately after being in the sun, but wait at
least ½ hour for the new Vitamin D to be absorbed
deeper into the skin.
• Our cell membranes contain a lot of cholesterol (in
between the phospholipids). Cholesterol makes the
membrane more rigid
Stop & Think
• Examine the cholesterol and phospholipid
molecules. Predict how the cholesterol molecules
are oriented among the phospholipids of a
membrane.
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