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A precursor is something that existed before and was incorporated into something that came later. Also known as a forerunner or predecessor.
For example, cacao is a precursor of chocolate, but chocolate is not a precursor of ice cream. There, chocolate is an ingredient.
In biological processes especially metabolism, a precursor is a substance from which another, usually more active or mature substance is formed. eg protein precursor.
For instance, certain liver enzymes are precursors to insulin. Or, this is the opposite of the body breaking down a more complex substance, like protein] into individual chemicals for use within the body.
Chemically, a precursor is a substance that, following a reaction, becomes an intrinsic part of a product chemical.
Medically, symptoms that, following a certain progression, indicate the development of a different problem are also called precursors.
For example, chest pain can be a precursor to a heart attack. A heart attack involves chest pains, but not all chest pains indicate a heart attack.