**1. Introduction**

Man as a living creature has always indulged himself into several activities to ensure his survival and well-being. In so doing, he has induced the production or release of various reactive substances or free radicals which are either consumed or inhaled. Also, certain physiological processes in the body generate free radicals or proxidants. These free radicals or reactive species, because of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

their deficiency in electron and instability, attack electron rich centers such as lipid membranes, proteins and nucleic acids thereby damaging cells and tissues in the body. Eventual, the human body is adapted to remove these unstable molecules by a myriad of molecules including certain enzymes collectively known as antioxidants. This antioxidant defense system reduces the level of these free radicals in the body and maintains the homeostatic balance for proper functioning of the body. However, when these reactive species are overwhelming high in the body, it surpasses the capacity of the antioxidant defense system leading to a condition known as oxidative stress. This imbalance between antioxidant and proxidants is characteristic of certain disease conditions such as diabetes, atherosclerosis, cardiovascular diseases, cancer etc. One of the possible remedy for this condition is to supplement the endogenous antioxidant defense system with exogenous antioxidants. Plants have gained considerable interest in recent time in managing oxidative stress related diseases; firstly, because of their ethnopharmacological uses in managing diseases and secondly, due to their richness in phytochemicals which possess antioxidant properties. Hence, this chapter is aimed to give an overview of free radicals, their sources of origin and processes of generation in the environment and body. Also, it will highlight on the various mechanisms of free radical induced cellular damage and the associated diseases due to oxidative stress. The various mechanisms of the antioxidant defense system; both enzymatic and non-enzymatic antioxidants will be described as well as the contribution of plant phytochemicals as antioxidants. Emphasis will be laid on some plants and phytochemicals with antioxidant activities stating their mode of scavenging free radicals and prevention of oxidative stress-related diseases.

radical (OH•), superoxide ion radical (O•

O2

), and O2

physiological processes or endogenous sources.

cides, anesthetic, certain drugs, industrial solvents etc.

**Table 1.** Free radicals and non-reactive radicals of oxygen species.

form of singlet oxygen (1

aldehydes, ozone (O3

Oxygen radicals

Non-reactive oxygen radical

**2.2. Sources of free radicals**

2

substances include hypochlorous acid (HClO), hydrogen peroxide (H2

as shown in **Table 1**.

cals since they contain at least one unpaired electron in the shells around the atomic nucleus which makes them unstable and therefore can easily donate or obtain another electron to attain stability. As such, they are highly reactive and capable of independent existence [6, 7]. On the other hand, the non-reactive radicals are a group of compounds which are not radicals but are extremely reactive or can easily be converted to reactive species. Examples of these

Free Radicals and the Role of Plant Phytochemicals as Antioxidants Against Oxidative Stress-Related Diseases

As reviewed from Sultan [8], free radicals can originate either from the environment,

**External sources:** Certain organic compounds in the atmosphere can react non-enzymatically with oxygen to generate free radicals. Also, reactions initiated by ionizing radiations in the environment can generate free radicals. Thus, some external sources of free radicals include environmental pollutant, cigarette smoke, alcohol, radiations, ozone, ultraviolet light, pesti-

**Endogenous sources:** This includes processes in living organisms that necessitates enzymatic reactions to generate free radicals. These include reactions involved in the respiratory chain,

**Free radicals Name Symbol**

), peroxyl (ROO•), alkoxyl radicals (RO•), and one

http://dx.doi.org/10.5772/intechopen.76719

O2

), organic peroxides,

51

2

2

O2

O2

) as shown in **Table 1** [5]. These species are considered as free radi-

Oxygen (bi-radical) O•

Superoxide ion O•

Hydroxyl OH• Peroxyl ROO• Alkoxyl RO• Nitric oxide NO•

Hydrogen peroxide H2

Organic peroxide ROOH Hypochlorous acid HOCL Ozone O3 Aldehydes HCOR Singlet oxygen <sup>1</sup>

Peroxynitrite ONOOH
