Free radicals are any species capable of an independent existence that contain one or more unpaired electrons. ROS is a collective term, referring not only to oxygen-centered radicals such as superoxide (O'2) and the hydroxy radical ('OH), but also to hydrogen peroxide (H2O2), ozone (O3), and singlet oxygen (1O2). These are produced as the by-products of normal metabolism and, as such, are highly reactive in chemical terms. In order to become more stable chemically, the free radical reacts with other molecules by either donating or taking an electron, in either case leaving behind another unstable molecule, and hence this becomes a chain reaction. So, although oxygen is essential for life, in certain circumstances it may also be toxic. Damage caused by ROS to cellular target sites includes oxidative damage to proteins, membranes (lipid and proteins), and DNA. PUFA are particularly vulnerable to ROS attack because they have unstable double bonds in their structure. This process is termed 'lipid peroxidation'; because PUFA are an essential part of the phospholipid fraction of cell membranes, uncontrolled lipid peroxidation can lead to considerable cellular damage. The balance of MUFA in cell membranes is also critical to cell function, but, as already noted, MUFA are far less vulnerable to lipid peroxidation.
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