Complete revision notes for Topic 4 of the CIE IGCSE Biology course: Biological molecules. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
All the molecules in this topic contain carbon and so are described as organic molecules.
List the chemical elements that make up:
Carbohydrates
C, O and H
Fats
C, O and H
Proteins
C, O, H and N
Some contain small amounts of other elements like sulphur
State that large molecules are made from smaller molecules, limited to:
Starch and glycogen from glucose
Cellulose from glucose
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)
Polysaccharides:
Starch (energy store in plants)
Glycogen (energy store in animals)
Cellulose (makes up cell walls)
Proteins from amino acids
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
Fats and oils from fatty acids and glycerol
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)
Explain that different sequences of amino acids give different shapes to protein molecules
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