Chemicals & Life (Cambridge O Level Biology)

• 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

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

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

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

Amino acid chemical structure diagram

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

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

DNA, chromosomes and the nucleus

• The individual units of DNA are called nucleotides

Nucleotide structure diagram

The structure of a nucleotide