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Molecules For Life

1.2 Molecules for life (ESG44)

Revise the concept of the atom, elements, molecules and compounds.

Although life at the macro level is diverse, the chemistry making up that life is remarkably similar. All living things are made up of basic building blocks called elements. An element is a substance that cannot be broken down into simpler substances using chemical means. Carbon, oxygen, hydrogen, nitrogen, sulfur, calcium, sodium and iron are examples of elements you will come across in Life Sciences.

Each element is distinguished by the composition of its atom. An atom is the basic unit of matter. Molecules are formed when one or more atoms are covalently bonded together. The atoms of a molecule can be identical, such as \(0_{2}\) or \(\text{H}_{2}\) or differ such as \(\text{H}_{2}\text{O}\). A compound is formed when atoms of different elements join together.

Because all compounds contain more than one atom, all compounds are molecules. However, not all molecules are compounds.

Simulation on building a molecule

Video: 2CMH

Compounds are divided into organic and inorganic compounds. Organic compounds always contain carbon, but not all compounds that contain carbon are organic. A general rule of thumb is that organic compounds contain carbon, with at least one of these carbons bonded to hydrogen atoms. Carbon dioxide is therefore an inorganic compound even though it contains carbon. The major organic compounds found in living organisms include: carbohydrates, fats, proteins and nucleic acids. These will be discussed in detail later in this chapter.


Percentage (\(\%\))




Mineral salts







Other organic macromolecules


Table 1.1: The composition of macromolecules in humans by percentage.