What are small molecules?
A small molecule (or metabolite) is a low molecular weight organic compound, typically involved in a biological process as a substrate or product. Metabolomics usually studies small molecules within a mass range of 50 – 1500 daltons (Da).
Some examples of small molecules include: sugars, lipids, amino acids, fatty acids, phenolic compounds, alkaloids and many more (Figure 2).
Figure 2 Examples of small molecules.
To give you an idea of the variation of metabolites between species, it is estimated there are around 200,000 metabolites across the plant kingdom, and somewhere between 7,000 and 15,000 within an individual plant species (1,2). By contrast, in humans, there are thought to be around 3,000 endogenous or common metabolites (3). These estimates are approximations that are likely to be underestimates because it is difficult to detect low-abundance molecules. Nonetheless, it can be concluded that plants are particularly biochemically rich by comparison with many other species. They also typically contain larger numbers of genes.