- Atomic level. The atom is the smallest part of a chemical element that can take part in a chemical reaction and has the characteristics of that element. The atoms that are part of living matter are called bioelements, and the most important are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P).
- Molecular level. Molecules are formed by the union, through chemical bonds, of two or more atoms. The molecules that are part of living matter are called biomolecules, which can be:
- Inorganic biomolecules: water (H2O), calcium carbonate (CaCO3), gases such as oxygen (O2), etc.
- Organic biomolecules: they are mainly made up of carbon atoms joined by covalent bond. The carbohydrates such as glucose (C6H12O6), the lipids, the protein and nucleic acids would organic biomolecules.
There are also various levels of complexity within molecules. For example, macromolecules are formed by the union of simpler molecules. A polymer is formed by the union of monomers. Thus, starch (polysaccharide) is a polymer formed by many glucose units (monomer), proteins are formed by the union of amino acids, and nucleic acids, by the union of nucleotides.
Cell organelles, made up of supramolecular complexes, should also be included at this level, although they have a certain entity of their own, they are not living beings, since they do not fulfill the three vital functions of nutrition, relationship and reproduction. Examples of organelles are mitochondria, ribosomes, chloroplasts, etc.
The virus would also be at this level, they are macromolecular complexes formed by proteins and nucleic acids nor fulfill vital functions.
