Biological Molecules
Most molecules found in living organisms fall into three main groups: carbohydrate’s, proteins,
and lipids. All of these molecules contain carbon, hydrogen and oxygen.
Carbohydrates
• They are molecules made up of small basic units called simple sugar molecules or you
can talk of sugars or monosaccharides. (An example of simple sugars is glucose)
• Simple sugar molecules(monosaccharides) can link together to form larger molecules
called disaccharides and polysaccharides.
• When two monosaccharides combine, they form disaccharides and when two
disaccharides combine, they form polysaccharides
• A good example of disaccharides is maltose and Sucrose
• A good example of polysaccharides are Starch and cellulose in plants and glycogen in
animals
Proteins
• Proteins are formed from long chains of amino acids
• When amino acids are joined together a protein is formed
• Amino acids can be arranged in any order, resulting in hundreds of thousands of different
proteins
o Different proteins have different amino acid sequences resulting in them
having different shapes; the shape of a protein determines its function
o Examples of proteins include enzymes, haemoglobin, ligaments and keratin
Lipids
• A lipid is what we commonly call a fat or oil.
• Fats are solids at room temperature and oils are liquids at room temperature
• Both fats and oils are made from basic units called fatty acids and glycerol
• There are three fatty acids in each lipid and one glycerol
• Lipids are important in forming cell membranes
Structures of large biological molecules forming from smaller molecules
Tests for food molecules
● Starch - add iodine solution to the sample. A colour change to blue-black indicates
the presence of starch.
● Reducing sugars - Glucose is a reducing sugar. Glucose and other reducing sugars are
detected using Benedict’s solution. The pale blue Benedicts solution is added to a
prepared sample that contains glucose and heated to around 95 degrees celcius.
If it changes colour or forms a precipitate, this indicates a presence of glucose.
A green colour means there is only small amounts of glucose in the solution.
A medium amount of glucose produces a yellow colour
A significant amount of glucose produces a precipitate that is orange in colour
● Proteins - The sample solution is placed in a test tube with an equal amount of sodium
hydroxide solution. Add a few drops of dilute copper II sulphate solution, if the
sample turns purple, a protein is present in the sample. If it remains blue, no protein is
present. This test is known as the biuret test.
● Fats and oils - place sample in a test tube and add ethanol. Shake tube to dissolve any
fats and oils. Add water and shake gently. If the sample contains fats or oils, it will turn
cloudy white. This test is known as the emulsion test.
● Vitamin C - Add DCPIP solution to a test tube, this is a blue colour. Add the sample
and shake gently. If the DCPIP becomes colourless, vitamin C is present.
The structure of the DNA
• DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for the
growth and development of all organisms
• It consists of two strands of DNA wound around each other in what is called a double
helix
• The DNA strand contains four different bases. Bases are chemicals or base nitrogen-
containing molecule that forms part of the nucleotide building blocks of the DNA
structure. There are four different bases, Adenine (A), Cytosine (C), Thymine (T) and
Guanine (G)
• The bases on each strand pair up with each other, holding the two strands of DNA in the
double helix
• The bases always pair up in the same way:
• Adenine always pairs with Thymine (A-T)
• Cytosine always pairs with Guanine (C-G)
