**1. Introduction**

Free radicals are formed during the reactions required for the maintenance of normal metabolism and energy formation in biological systems. Under normal conditions, the most significant source of free radicals in cells is the leakage of electrons into molecular oxygen from electron flow in the mitochondria and endoplasmic reticulum during oxidative respiration. The superoxide anion formed in this way is converted into hydrogen peroxide, a reactive oxygen type. Hydrogen peroxide forms the peroxyl radical, which is the most reactive radical type in the organism in the presence of transition metal ions. When free radicals cannot be removed from the environment, they cause damage at the cell, tissue, and organ level by disrupting the structure of biomolecules such as lipids, proteins, and nucleic acids due to their high reactivity. Lipid peroxidation begins when polyunsaturated fatty acids (PUFAs), which are in the structure of membrane lipids in cells, are affected by free radicals. If peroxyl radicals are not cleaned, a chain reaction starts affecting the intact PUFA. Lipid peroxidation is damaging as it is a self-sustaining chain reaction. When lipid peroxides (LOOH) are broken down, aldehydes are formed, many of which are biologically active. These compounds are either metabolized at the cellular level or

diffuse from their initial domains and spread damage to other parts of the cell. When peroxidation of fatty acids containing three or more double bonds. Malondialdehyde (MDA) arises following the peroxidation of fatty acids comprised of three or more double bonds and appears in blood and urine. Due to its ability to corelate well with the degree of which lipid peroxidation occurs despite not being a specific or quantitative indicator of fatty acid oxidation, the measurement of MDA in biological material is used as an indicator of lipid peroxide levels. Nonenzymatic lipid peroxidation is a very harmful chain reaction. It both damages the membrane structure directly and also damages other cell components indirectly with the reactive aldehydes it produces. Thus, it causes tissue damage and many subsequent diseases.

#### **2. Free radicals**

According to quantum chemistry, only two electrons can enter the structure of a bond together. Electron pairs exist in a very stable state. Electrons in the human body exist almost entirely in electron pairs. When a bond breaks, the two electrons are either separated but remain in the same atom or both remain in the atom separately. If they remain together, the atom formed becomes an ion, and when they leave, the atom formed becomes a free radical [1]. Atoms or compounds that contain the unpaired electron in their final orbital are defined as free radicals. In other words, they are atoms or molecules that have an open electron shell configuration and contain an odd number of electrons in their structure [2]. The term Reactive Oxygen Species (ROS) is more commonly used in place of the term free oxygen radical as it includes molecules that are radical and are not actually radical, but that cause the formation of oxygen radicals with their reactions [1].

Despite oxygen being crucial for life, in some cases it can also damage cells. This damage is caused by increased oxygen-induced ROS production. The amounts of ROS produced under normal physiological conditions do not exceed the capacity of the natural antioxidant defense systems in the body. ROSs are chemical derivatives with unpaired high energy electrons in their outer orbits. In order to stabilize, ROS interact with any molecule they can find in their vicinity and exchange electrons. Molecules that react with free radicals turn into free radicals and initiate the damage chain reaction. These radicals react with organic and inorganic chemicals such as protein, lipid, and carbohydrate. When radicals occur in cells, they react with nucleic acids and various membrane molecules and break them down. While radicals affect intracellular organelles, they also create distant effects by passing to the extracellular compartment [1]. Although oxygen is crucial for human life, some ROS that occur during normal metabolism have the potential to cause great harm to the body. Compared to normal oxygen molecules, ROS, which are mostly composed of free radicals, appear as oxygen forms with higher chemical reactivity [3].

Free radicals are any atom or molecule with one or more unpaired electrons produced in many physiological or pathological conditions. These molecules, also known as oxidant molecules or reactive oxygen particles, easily exchange electrons with other molecules [4]. A compound can return to a free radical by losing an electron (reduction) or gaining an additional electron (oxidation). Free radicals can be part of a larger structure, as well as in immobile or small and freely spreading species [5–8]. Free radicals are frequently produced by the mitochondria during the body's normal use of oxygen. These free radicals, which are formed as a result of energy production, can change the structure of lipids, proteins, and nucleic acids. Free radicals are produced from many endogenous and exogenous sources as well as mitochondria and cause a variety of damage alongside their benefits. The benefits of free radicals only occur when they are of low concentration. Low concentration

#### *Lipid Peroxidation DOI: http://dx.doi.org/10.5772/intechopen.95802*

free radicals are involved in the activation of cellular signals such as calcium release from intracellular stores, and the activation of tyrosine phosphating and growth factor signals, along with defense functions such as defense against infections, the killing cancer cells and detoxification of xenobiotics [9].
