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Biological molecules
Chemical Elements
• Most of the molecules in living organisms fall into three categories:
carbohydrates, proteins and lipids
• These all contain carbon and so are described as organic molecules
Carbohydrates
• Long chains of simple sugars
• Glucose is a simple sugar ( a monosaccharide)
• They are used as a source of energy for the body.
• When 2 glucose molecules join together maltose is formed (a disaccharide)
• When lots of glucose molecules join together starch, glycogen or cellulose can form (a
polysaccharide)
Three types of carbohydrates
• Monosaccharides (i.e. glucose) are the unit, form of sugars. They are a single unit and they
cannot be broken down any further to make a more simple sugar.
• Disacchardies are literally two monosaccharides joined together.
• Polysaccharides are large chains of monosaccharides joined together
• Starch: energy store in plants
• Glycogen: energy store in animals
• Cellulose: structural carbohydrates in plants
Lipids (Fats & Oils)
• Most fats (lipids) in the body are made up of
triglycerides
• Their basic unit is 1 glycerol molecule chemically
bonded to 3 fatty acid chains
• The fatty acids vary in size and structure
• Lipids are divided into
• fats (solids at room temperature)
• oils (liquids at room temperature)
• Lipids have various purposes in the body:
• Source of energy. In fact, they have two times higher
energy content than carbohydrates!
• Heat insulation
• Myelin sheath formation
• Cell membrane formation
Fat molecule
Proteins
• Long chains of amino acids
• There are about 20 different amino acids
They all contain the same basic structure but
the ‘R’ group is different for each one
• When amino acids are joined together a protein is
formed
• The amino acids can be arranged in any order,
resulting in hundreds of thousands of different
proteins
• Even a small difference in the order of the amino
acids results in a different protein being formed
Different types
of amino acids
Proteins
• It is important to understand here that the final 3D structure of a
protein is derived from the specific interactions between amino
acids that are joined in the protein chain.
• The sequence of amino acids in the chain therefore creates the
final shape of the protein, and the shape is what gives the protein
its function.
• Proteins are extremely important to the body and serve many
different functions
• Growth
• Tissue repair
• Cell membrane formation
• Source of energy
Proteins
• Many of proteins are different shapes and the shape often has an important effect on the function
of the protein
• For example:
• Enzymes have an area in them known as the active site – this is important as this is the place where
another molecule fits into the enzyme in order for a reaction to take place. If the shape of the active site
does not match the shape of the molecule that fits into it, the reaction will not take place. Every enzyme
has a different shaped active site
• Antibodies are proteins produced by certain types of white blood cell to attach to antigens on the
surface of pathogens. The shape of the antibody must match the shape of the antigen so that it can
attach to it and signal it for destruction
Food Testing
Test for glucose (a reducing sugar)
• Add Benedict’s solution into sample solution in
test tube
• Heat at 60 – 70 °c in water bath for 5 minutes
• Take test tube out of water bath and observe
the colour
• A positive test will show a colour change
from blue to orange or brick red
Test for starch
• Add drops of iodine solution to the food sample
• A positive test will show a colour change
from orange-brown to blue-black
Test for protein
• Add drops of Biuret solution to the food sample
• A positive test will show a colour change from blue to violet / purple
Test for lipids
• Food sample is mixed with 2cm3 of ethanol and shaken
• The ethanol is added to an equal volume of cold water
• A positive test will show a cloudy emulsion forming
Test for vitamin C
• Add 1cm3 of DCPIP solution to a test tube
• Add a small amount of food sample (as a solution)
• A positive test will show the blue colour of the dye disappearing
DNA structure
• DNA, or deoxyribonucleic acid, is the molecule that
contains the instructions for growth and development of
all organisms
• It consists of two strands of DNA wound around each
other in what is called a double helix
Model of the structure of DNA
Nucleotides
• The individual units of DNA are called nucleotides
Base pairing
• All nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in
the base attached
• 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)
• The phosphate and sugar section of the nucleotides form the ‘backbone’ of the
DNA strand (like the sides of a ladder) and the base pairs of each strand connect
to form the rungs of the ladder
• It is this sequence of bases that holds the code for the formation of proteins
The DNA helix is made from two strands of DNA held together by hydrogen bonds
Part of a DNA molecule with four nucleotides
The drawing shows part of a DNA molecule schematically
Part of a DNA molecule
