Precursor

Plants make phytochemicals by starting with one chemical, a <i>parent compound</i> and changing it into another, called a <i>derivative</i> -- meaning 'derived from the parent compound.' There is a sometimes lengthy series of chemical transformations wherein the original molecule changes shape, splits up, or joins with other molecules or atoms on the way to becoming the derivative. Every series of chemical changes is organized by a specific group of enzymes that help change each form of the molecule into the next form 'downstream.' A molecule that is 'upstream' in this process is called a 'precursor' of any molecule that is 'downstream.' This terminology also applies to transformations which occur as an ingested compound is metabolized in the body. For example, salicin (a salicylate in occurring in Willow bark) is a precursor of salicylic acid, which forms as salicin breaks down in the stomach. -- Ganora