Chemicals & Life (Cambridge International IGCSE Biology)

Chemical elements

Spec point — 4.1.1 · List the chemical elements that make up: carbohydrates, fats and proteins

• 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

Spec point — 4.1.1 · State that large molecules are made from smaller molecules, limited to: (a) starch, glycogen and cellulose from glucose (b) proteins from amino acids (c) fats and oils from fatty acids and glycerol

Large molecules are made from smaller molecules

Carbohydrates

• Long chains of simple sugars(monosaccharides)
• Glucose is a simple sugar
• 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)

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 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)

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

General amino acid structure

Amino acids join together to form proteins