**6. Plants as source of antioxidants**

Plants have long been consumed as food which is rich in vitamins and other nutrients that are useful for the body. Also, various plants were used in folk medicine for various therapeutic purposes. Though these uses, the notion of plant as a source of antioxidant became more evident in recent time as oxidative stress was considered a major attribute to most diseases in humans and the antioxidant defense system in human was usually not sufficient to overcome the free radical level in the body. As such, plants have gained considerable interest as a source of antioxidants and so much research has been done to identify plants substances with antioxidant activities.

Like other humans, plants do have enzymatic and non-enzymatic antioxidant defense systems to protect them against free radicals. The enzymatic system includes catalase, SOD, glutathione peroxidase(GPx), and glutathione reductase (GRx) [7], while non-enzymatic systems consist of low molecular weight antioxidants (LMWA) such as ascorbic acid, proline, glutathione, carotenoids, flavonoids, phenolic acids, etc. and the high molecular weight antioxidants (HMWA) which are mostly secondary metabolites such as tannins [53]. The possible reason for the presence of these antioxidants in plants is that plants lack an immune system unlike animals thus, utilize the antioxidant defense system to protect them against microbial pathogens and animal herbivores. Also, these phytochemicals serve as a defense system against environmental stress.
