AS Biology
Biological Molecules-Lipids
Biology & Disease
Lipids are fats and oils. Fats are solid at 20°C whereas oils are liquid at 20°C. Both are insoluble in
water.
Lipids are important biological molecules. What important roles do lipids perform in living organisms?
The structure of lipids
Lipids are large molecules made from the elements __________. ____________,
and_____________ (like carbohydrates, only containing much less oxygen).
They are formed when 3 fatty acid molecules join with a glycerol molecule
to form a triglyceride.
Glycerol
Fatty Acids
A glycerol molecule contains a chain of 3
carbon atoms with attached hydrogen atoms
and hydroxyl (OH) groups. The glycerol
molecule is the same in all fats and lipids.
There are three OH groups, all capable of
joining with a fatty acid in a condensation
reaction. This is its structure:
A single fatty acid molecule contains an acid
(carboxyl) group – COOH, attached to a
hydrocarbon chain. The hydrocarbon chain
can vary in length (up to 24 carbon atoms) and
structure causing different lipids to have
different properties.
The structure of a fatty acid is:
The formation of a triglyceride
FRI
AS Biology
Biology & Disease
Triglycerides can be represented in different ways:
Triglycerides are non-polar. This means there is no uneven distribution of charge within the
molecule. This means they do not form hydrogen bonds with water molecules and therefore do not
dissolve in water.
The ‘R’ group or hydrocarbon tail
The R group of a fatty acid can be saturated or unsaturated. What does this mean?
Saturated- A saturated fatty acid is
saturated with hydrogen- every bond in the
hydrocarbon tail is a single bond- there are no
double carbon bonds (C= C). This makes
saturated fatty acids straight and they have
a high melting point. Most animal fats are
saturated.
Unsaturated- An unsaturated fatty acid is
one in which there is at least one double C=C
bond somewhere in the hydrocarbon tail. The
C=C bond causes the tail to bend. Unsaturated
fatty acids have much lower melting points
than saturated fatty acids. Most plant fats
are unsaturated. A tail with many double C=C
bonds is polyunsaturated.
Questions
1. Describe a chemical test that could test
for the presence of fat. What is the name of
this test?
2. Describe the functions of fats in living
organisms.
3. What type of bonds are formed when a
glycerol molecule joins with three fatty acid
molecules?
FRI
4. What is the name given to the chemical
reaction which breaks down triglycerides into
fatty acids and glycerol?
5. What is the chemical difference between a
saturated and an unsaturated fat?