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Cambridge (CIE) O Level Your notes
Biology
4.1 Biological Molecules
Contents
Chemicals & Life
Food Tests
Page 1 of 13
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Chemicals & Life
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Chemical Elements
Most of the molecules in living organisms fall into four categories: carbohydrates, proteins, lipids and
DNA
These all contain carbon and so are described as organic molecules
Chemical Elements Table
Molecule Chemical Elements
Carbohydrate Carbon, oxygen, and hydrogen
Protein All contain carbon, oxygen, hydrogen, and nitrogen (and some contain small amounts of
other elements such as sulfur)
Lipid Carbon, oxygen, and hydrogen
DNA Carbon, oxygen, hydrogen, nitrogen, and phosphorous
Large Molecules are Made from Smaller Molecules
Carbohydrates
Long chains of simple sugars
Glucose is a simple sugar (a monosaccharide)
When two glucose molecules join together maltose is formed (a disaccharide)
When lots of glucose molecules join together starch, glycogen or cellulose can form (a
polysaccharide)
Carbohydrates in animals and plants diagram
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Glycogen, cellulose and starch are all made from glucose molecules
Lipids
Most lipids (fats and oils) in the body are made up of triglycerides
Their basic unit is one glycerol molecule chemically bonded to three 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)
Triglyceride diagram
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Structure of a triglyceride with a glycerol and three fatty acids
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
Page 4 of 13
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Amino acid chemical structure diagram
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General amino acid structure. The part that is different for each amino acid is represented by the letter
R.
Amino acids join together to form proteins
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Amino acids are the monomers that join to form a peptide chain that folds into a functional protein
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
Structure of DNA in the cell diagram
Page 6 of 13
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DNA, chromosomes and the nucleus
The individual units of DNA are called nucleotides
Nucleotide structure diagram
Page 7 of 13
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The structure of a nucleotide
Page 8 of 13
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Food Tests
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Food Tests
Test for starch using iodine
We can use iodine to test for the presence or absence of starch in a food sample
Iodine test for starch diagram
Use this image
Iodine turns blue-black in the presence of starch
Add drops of iodine solution to the food sample
A positive test will show a colour change from orange-brown to blue-black
Placing iodine on a potato diagram
Page 9 of 13
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Use this image
Testing a potato to prove the presence of starch
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
The Benedict's test for glucose diagram
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Use this image
The Benedict's solution turns brick red when heated in the presence of glucose
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
The Biuret test for protein diagram
Page 11 of 13
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Use this image
Biuret solution turns purple in the presence of protein
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
The ethanol emulsion test for lipids
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Use this image
The ethanol emulsion test shows a cloudy white positive result
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.
Page 13 of 13
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