Plant secondary metabolites are those compounds which are not directly involved in the growth and development of plants. Some of these metabolites help to prevent some plants from being consumed. Some may be poisonous to human but may have other importance. These metabolites can be in the leaves, stem and root of a plant, and prevent the growth of some organisms that infect the plant. They are divided into three based on their biosynthesis. They include;
 1) Flavonoids 
2) Terpenes
3) Alkaloids

 Flavonoids                                                                                                                                 Flavonoids are phenolic water soluble compounds with variable colors. They can be crimson, purple or yellow and remain present in vacuoles and chloroplasts. The basic skeleton of flavonoids consists of two aromatic rings A and B, joined by a 3 carbon residue. They are commonly found in higher plants. Examples of flavonoid include Anthocyanidins, Catechin, Leukocyanidin, and Sulpahuretin.

                                            Functions of flavonoids

1)      Flavonoids attract insects and animals and help in pollination.

2)      They help in the dispersal of fruits.

3)      They repel invading pathogenic organisms.

4)      They provide every color of spectrum except green to the plant parts and flowers.

5)      They have pleasant smells and can be used in the production of perfumes.


Terpenes                                                                                                                                              Terpenes are lipids synthesized from acetyl-CoA through marvelonic acid pathway. They are large and diverse class of organic compounds produced by a variety of plants, particularly conifers. They are often strong smelling and thus protect the plant that produce their structures contain a basic 5-carbon unit, isopentane unit. Examples are Geraniol, Farnesiol, squalene, carotenoids, and abscisic acid.

                                         Functions of terpenes

1)      They have penetrating odor which helps in attracting and repelling insects during pollination.

2)      They help in the regulation of seed dormancy and leaf abscission.

3)      Carotenoids are accessory pigments and help in light reaction of photosynthesis.

4)      They show antifungal action against pathogens.

5)       Carotenoids acts as photoreceptors for response during phototropism                                                                        

Alkaloids are nitrogen containing compounds.  They are alkaline in nature and can be synthesized from amino acids such as aspartic acid, lysine, and tyrosine. Examples of alkaloids are cocaine, atropine, codeine, morphine, and hyoscyamine.

                  Functions of alkaloids

                 1)      They are used in the production of drugs.
                 2)      They act as growth regulators.
                 3)      They help in maintaining ionic balance. 
                 4)      They are used as stimulants.