Class IX IGCSE
Biological molecules
Carbon is an element present in all biological molecules and carbon can form a ring structure, hence
biological molecules are very large.
When molecules are made up of monomers held together by a chemical bond they are called polymers
Carbohydrates
These may be simple sugars, or complicated materials like starch and cellulose. All
carbohydrates contain carbon, hydrogen and oxygen only.
Sugars with single rings are called a monosaccharide, like glucose and
fructose and sugars with 2 carbon rings are called disaccharide eg
maltose and sucrose.
Monosaccharides and disaccharides are readily soluble in water
Glucose is the form in which carbohydrates are transported around
the human body. Glucose dissolve in blood plasma which delivers to
every cell.
A carbohydrate that is
stored as energy store in
plant cells.
In plants the glucose
molecules are linked in a
slightly different way to make
starch. Starch is found in the
plant cell and it breaks down
to form glucose.
A carbohydrate that is
used as an energy store
in animal cells
Glucose molecule
can link up together
to form larger
molecules. In animals
the larger molecule is
glycogen.
Plant cell wall are made up
of long interconnected
cellulose fibres
Test for starch
A solution of iodine in potassium iodide
turns the orange-brown solution to blueblack when mixed with starch
Benedict's test for reducing sugars
Most of the sugars can be detected using Benedict's solution.
Benedict's solution is blue and it changes colour when heated with
reducing sugars such as glucose.
Fats and oils are made from carbon, hydrogen,
and oxygen.
Fat is made of 3 molecule of fatty acid , joined
with one molecule of glycerol.
They form a part of cell membrane or the
nuclear membrane.
Fats also form the source of energy in the
cytoplasm.
Mammals often have a layer of cells containing
fat droplets beneath the skin, this is both an
energy resource and heat insulating layer
Fats
Fats and oils do not dissolve in water however, they do
dissolve in ethanol. We can use this test to detect their
presence in food.
The food is cut into small pieces and
shaken with ethanol. Any fats in the
food will dissolve in the ethanol. Then
the ethanol is poured into clean tube
containing water which gives a milky
appearance. This mixture is called
emulsion.
Proteins
Some proteins form part of structures in the cell like the
cell membranes, mitochondria, ribosomes and
cytoplasm.
Enzymes are proteins. Proteins contain carbon,
hydrogen, oxygen, nitrogen, and sometimes sulphur.
They are made up of long chains of acids called amino
acids.
There are 20 different amino acids. Each kind of
protein has different amino acids linked in a
precise order. If the amino acid is changed. There
are almost infinite number of different proteins
that can be made.
Functions
 Proteins have different functions. All
enzymes are proteins.
 Antibodies which help to protect our body
from pathogens are proteins,
 Haemoglobin the red pigment that transports
oxygen in the mammalian cells is a protein.
 Proteins are present in cell membranes.
 In Humans hair and fingernails are made up
of a protein called keratin.
A protein molecule is made up of long
chains of amino acids arranged in a
special sequence.
Why there are larger number of proteins
than polysaccharides ?
Proteins are biomolecules made up of 20
types of amino acids linked by peptide
bonds. Each type of protein has a unique
sequence of amino acids, and many
thousands of different proteins are known.
Polysaccharides are carbohydrates made up
of long chains of the same type of sugar
molecule, called monosaccharides, linked by
a chemical bond. The prefix "poly" means
"many", and the root word "saccharide"
means "sugar".
Biuret test for proteins
The compounds with proteins undergo this test. Proteins are
polypeptides of amino acids linked together by chemical bonds. The
blue color biuret reagent turns violet when mixed with proteins.
Test for vitamin C
In a test tube take DCPIP solution (Blue dye). DCPIP
solution is used to test Vitamin C. Add Lemon juice in
it. The solution will turn colourless. Repeat the
experiment with orange juice.
Nutri present
Elements
Examples
Sub-units
Carbohydrates
Carbon, hydrogen, oxygen
Starch, glycogen, cellulose,
sucrose
Glucose
Fats/Oils
Carbon, Hydrogen Oxygen
(but low oxygen content
than carbohydrates
Vegetable oils eg olive oil,
animal fats eg. Cod liver oil
and waxes
Fatty acids and Glycerol
Protein
Carbon, Hydrogen Oxygen,
nitrogen and sometimes
sulphur and phosphorus
Enzymes, muscle,
haemoglobin, cell
membranes
Amino acids (about 20
different forms
Nucleic acids
Nucleic acids are large, complex molecules that carry genetic information and are essential for the
growth, development, and reproduction of all living organisms.
A DNA molecule is made up of long chains of nucleotides formed into 2 strands
A nucleotide is a 5-carbon sugar molecule joined to a phosphate group (-PO3) and an organic base.
The nucleotide are joined by their phosphate groups to form a long
chain. The phosphate and the sugar molecule are the same all the
way down the chain. However, the bases can be any one of the four
listed.
Now the DNA is made up of two strands held together by chemical
bonds between the bases.
A always pairs with T , G always pairs with C and the sequence i.e.
Cytosine, Cytosine, Guanine, Adenine, Thymine is a part of the
genetic code.
The double strand is twisted to form a helix.
Chargaff found out that Adenine = Thymine
Cytosine = Guanine
C
A
T
T
C
T
Watson and crick
Schematic diagram of a part of a DNA molecule
G