4. Biological Molecules
4.1 Biological Molecules
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Cambridge (CIE) O Level
Biology
Biology
- 4.1 Biological Molecules
- Contents
- Chemicals & Life
Food Tests
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Chemicals & Life
- Chemical Elements
- Most of the molecules in living organisms fall into four categories: carbohydrates,
proteins, lipids and DNA
These all
- These all contain carbon and so are described as organic molecules
- Chemical Elements Table
- Molecule Chemical Elements
- Carbohydrate Carbon, oxygen, and hydrogen
Protein All
- 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
- Large Molecules are Made from Smaller
Molecules
- Carbohydrates
- Long chains of simple sugars
Glucose
- 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
- 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
- Their basic unit is one glycerol molecule chemically bonded to three fatty acid chains
- The fatty acids vary in size and structure
Lipids
- 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
- 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
- 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
- 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
- Amino acids join together to form proteins
Amino acids
- Amino acids are the monomers that join to form a peptide chain that folds into a functional
protein
- DNA
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DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for the
growth and development of all organisms
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
- DNA, chromosomes and the nucleus
The individual units of DNA
- The individual units of DNA are called nucleotides
- Nucleotide structure diagram
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- The structure of a nucleotide
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Food Tests
- Food Tests
- Test for starch using iodine
We can
- 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
- A positive test will show a colour change from orange-brown to blue-black
- Placing iodine on a potato diagram
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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
- A positive test will show a colour change from blue to orange or brick red
- The Benedict's test for glucose diagram
- 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
- A positive test will show a colour change from blue to violet / purple
- The Biuret test for protein diagram
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- Use this image
- Biuret solution turns purple in the presence of protein
- Test for lipids
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Food sample is mixed with 2cm of ethanol and shaken
The ethanol
- The ethanol is added to an equal volume of cold water
A positive test
- 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.