Cambridge (CIE) IGCSE Your notes
Biology
Biological Molecules
Contents
Chemicals & Life
Food Tests
DNA Structure
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Chemicals & Life
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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
Chemical Elements Table
Large Molecules are Made from Smaller
Molecules
Carbohydrates
Long chains of simple sugars
Glucose is a simple sugar ( a monosaccharide)
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)
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Glycogen, cellulose and starch are all made from glucose molecules
Fats
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) and oils (liquids at room
temperature)
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Structure of a triglyceride
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
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General amino acid structure
Amino acids join together to form proteins
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Food Tests
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Food tests
Food 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
A positive test for glucose will show a colour change from blue to orange or brick red
Use this image
Food test for starch
We can use iodine to test for the presence or absence of starch in a food sample.
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We can use iodine to test for the presence or absence of starch in a food sample
Use this image
Add drops of iodine solution to the food sample
A positive test will show a colour change from orange-brown to blue-black
A positive test for starch will show a colour change from orange-brown to blue-black
Use this image
Food 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
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A positive test for protein will show a colour change from blue to violet / purple
Use this image
Food 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
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A positive test for lipids will show a cloudy emulsion forming
Use this image
Food 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
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A positive test for vitamin C will show the blue colour of the dye disappearing
Your notes
Use this image
Examiner Tips and Tricks
When describing food tests in exam answers, make sure you give the starting colour
of the solution and the colour it changes to for a positive result.
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DNA Structure
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Structure of a DNA Molecule: Extended
Extended Tier Only
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
DNA, chromosomes and the nucleus
The individual units of DNA are called nucleotides
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A nucleotide
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)
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DNA base pairs
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
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The DNA helix is made from two strands of DNA held together by hydrogen bonds
It is this sequence of bases that holds the code for the formation of proteins Your notes
Examiner Tips and Tricks
You do not need to learn the names of the bases, just their letter. Make sure you know
which bonds with which, as this is the most commonly asked question about this
topic.
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