**Section 1**

**Introduction** 

**1** 

*Ukraine* 

 **Introductory Chapter** 

*Vassyl Stefanyk Precarpathian National University,* 

Oxidative stress, which will be defined and described in details below, is inevitable attribute of most strong stresses. In this book, the induction of oxidative stress by environmental challenges like physical, chemical as well as biological factors is described. These factors can induce oxidative stress in direct and non-direct ways, which will be covered by several chapters. Substantial bulk of chapters will describe the defensive mechanisms against deleterious effects of reactive species in different organisms. The book gives a broad description of the processes related to production of reactive species and their elimination.

Particular attention will be given to natural and chemically synthesised antioxidants.

Free radicals are relatively unstable particles with one or more unpaired electrons on outer atomic or molecular orbitals. Many of them have as short life time and they can exist for only microseconds or even less. That is why most scientists for long time believed that free radicals were too unstable to exist in biological systems. The presence of free radicals in biological systems was discovered about 60 years ago and was virtually immediately implicated by Rebecca Gerschman and colleagues (1954) in human diseases. Two years later Denham Harman (1956) suggested that free radicals could be involved in pathologies as well as animal and human aging, and he first proposed free radical hypothesis of aging. Since 1950th critically important discoveries on roles of free radicals in living organisms promoted deep understanding that they are involved in many pathologies of animal and human organisms. D. Harman also specified later mitochondria as a place in the cell principally determining lifespan and proposed that mitochondria could be the "biological clock" and in this manner govern longevity, and further the hypothesis proposed was developed in mitochondrial theory of aging with key role of free radicals (Harman, 1972). Investigations on ROS roles in living organisms, particularly, in organisms' aging culminated by the formulation of free radical theory of aging (Harman, 1983), which in different formulations has been applied to all organisms – bacteria, fungi, plants and animals (Lushchak, 2011a). In 1995, D. Harman was nominated for the Nobel Prize in medicine for his works on the role of free radicals in diseases and aging. It seems that among all theories of aging, the Harman's one has the most consistent experimental support to date. The development of the theory extended it to age-related pathologies and also

**2. Introduction in oxidative stress theory** 

disturbances not directly related to aging.

**1. Introduction** 

Volodymyr I. Lushchak
