<P> Ionizing radiation can generate damaging intermediates through the interaction with water, a process termed radiolysis . Since water comprises 55--60% of the human body, the probability of radiolysis is quite high under the presence of ionizing radiation . In the process, water loses an electron and becomes highly reactive . Then through a three - step chain reaction, water is sequentially converted to hydroxyl radical (OH), hydrogen peroxide (H O), superoxide radical (O) and ultimately oxygen (O). </P> <P> The hydroxyl radical is extremely reactive and immediately removes electrons from any molecule in its path, turning that molecule into a free radical and thus propagating a chain reaction . However, hydrogen peroxide is actually more damaging to DNA than the hydroxyl radical, since the lower reactivity of hydrogen peroxide provides enough time for the molecule to travel into the nucleus of the cell, subsequently reacting with macromolecules such as DNA . </P> <P> ROS are produced intracellularly through multiple mechanisms and depending on the cell and tissue types, the major sources being the "professional" producers of ROS: NADPH oxidase (NOX) complexes (7 distinct isoforms) in cell membranes, mitochondria, peroxisomes, and endoplasmic reticulum . Mitochondria convert energy for the cell into a usable form, adenosine triphosphate (ATP). The process in which ATP is produced, called oxidative phosphorylation, involves the transport of protons (hydrogen ions) across the inner mitochondrial membrane by means of the electron transport chain . In the electron transport chain, electrons are passed through a series of proteins via oxidation - reduction reactions, with each acceptor protein along the chain having a greater reduction potential than the previous . The last destination for an electron along this chain is an oxygen molecule . In normal conditions, the oxygen is reduced to produce water; however, in about 0.1--2% of electrons passing through the chain (this number derives from studies in isolated mitochondria, though the exact rate in live organisms is yet to be fully agreed upon), oxygen is instead prematurely and incompletely reduced to give the superoxide radical (O), most well documented for Complex I and Complex III . Superoxide is not particularly reactive by itself, but can inactivate specific enzymes or initiate lipid peroxidation in its protonated form, hydroperoxyl HO . The pK of hydroperoxyl is 4.8 . Thus, at physiological pH, the majority will exist as superoxide anion . </P> <P> If too much damage is present in mitochondria, a cell undergoes apoptosis or programmed cell death . Bcl - 2 proteins are layered on the surface of the mitochondria, detect damage, and activate a class of proteins called Bax, which punch holes in the mitochondrial membrane, causing cytochrome C to leak out . This cytochrome C binds to Apaf - 1, or apoptotic protease activating factor - 1, which is free - floating in the cell's cytoplasm . Using energy from the ATPs in the mitochondrion, the Apaf - 1 and cytochrome C bind together to form apoptosomes . The apoptosomes bind to and activate caspase - 9, another free - floating protein . The caspase - 9 then cleaves the proteins of the mitochondrial membrane, causing it to break down and start a chain reaction of protein denaturation and eventually phagocytosis of the cell . </P>

Where are reactive oxygen species generated in the cell
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