The most abundant substance in living things. It is the main component of cells and body fluids like blood.

Substances that can be found in living things in the form of dissolved ions, such as a sodium ion (Na +) and potassium ion (K +), or precipitated as crystals, such as phosphate and calcium carbonate.

Bioelements are chemical elements, such as carbon (C), that make up living things. 

Bioelements join together to form biomolecules. These can be classified into two types: inorganic and organic.

They are water and mineral salts.

• Water is the most abundant substance found in living things. It makes up about 65% of the human body. 
• Mineral salts can be found in living things in the form of dissolved ions or as crystals.

Organic biomolecules are unique to living things and contain significant amounts of the chemical element carbon.
 
The major types of organic biomolecules are carbohydrates, lipids, proteins and nucleic acids.

• Carbohydrates can be composed of one molecule (glucose); disaccharides (sucrose or maltose), formed when two monosaccharides are joined; or polysaccharides (cellulose or glycogen), formed when many monosaccharides are joined.
• Lipids include fats, phospholipids and cholesterol. Fats are the simplest lipids and can be broken down into fatty acids and glycerol (a sugar alcohol).
• Proteins are macromolecules composed of many smaller molecules called amino acids.  Some important human proteins are collagen, haemoglobin and antibodies.
• Nucleic acids are macromolecules composed of smaller ones called nucleotides. There are two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).