Gamma rays create free radicals in tissue primarily because they are a form of ionizing radiation that carries enough energy to eject electrons from atoms and molecules within the tissue. When gamma rays penetrate living tissue, they interact with the atoms and molecules, especially water molecules which are abundant in biological tissues. This interaction causes ionization — the removal of electrons — resulting in the formation of charged ions and highly reactive free radicals.
Here’s how this process unfolds in more detail:
Gamma rays have very high energy and can pass through cells, transferring energy to electrons in atoms along their path. When a gamma photon hits an atom, it can knock out an electron from its orbit. This ejected electron is free and highly reactive because it has unpaired electrons. The atom that lost the electron becomes a positively charged ion. Together, these form an ion pair: a free electron and an ionized atom.
The free electron can then collide with other molecules, especially water molecules (H₂O), which make up about 70% of the tissue. When water molecules are hit by these energetic electrons, they break apart into fragments called free radicals. The most common free radicals formed in this way are hydroxyl radicals (•OH) and hydrogen atoms (•H). These radicals are extremely reactive because they have unpaired electrons and seek to stabilize themselves by reacting with nearby molecules.
These free radicals can then attack important biological molecules such as DNA, proteins, and lipids. For example, hydroxyl radicals can cause breaks in DNA strands or modify bases, leading to mutations or cell death. Lipids in cell membranes can undergo peroxidation, damaging the membrane’s integrity and function. This cascade of damage is a key reason why gamma radiation is harmful to living tissue.
The creation of free radicals by gamma rays is often called the *indirect action* of radiation because the damage is not caused directly by the gamma photons hitting DNA or other critical molecules, but rather by the reactive species generated from water and other molecules. This indirect action is especially important for low linear energy transfer (LET) radiation like gamma rays, where the energy is spread out and the radicals diffuse to cause damage at some distance from the initial ionization event.
In addition to causing direct DNA damage, gamma ray-induced free radicals can also disrupt mitochondrial function. Mitochondria, the energy-producing organelles in cells, are sensitive to oxidative stress caused by free radicals. When mitochondria are damaged, they produce even more reactive oxygen species (ROS), amplifying the oxidative damage and triggering inflammatory responses and cell death pathways.
The overall effect of gamma ray exposure in tissue is a complex interplay of direct ionization damage and indirect damage mediated by free radicals. The free radicals generated can initiate chain reactions, damaging multiple cellular components and leading to cell dysfunction or death. This is why gamma radiation is both a powerful tool in medical treatments like radiotherapy and a significant hazard when tissues are exposed unintentionally.
To summarize the key points:
– Gamma rays have enough energy to eject electrons from atoms in tissue.
– Ejected electrons interact with water molecules, producing free radicals like hydroxyl radicals.
– These free radicals are highly reactive and damage DNA, proteins, and lipids.
– Damage to mitochondria by free radicals increases oxidative stress and inflammation.
– The indirect action of gamma rays via free radicals is a major cause of radiation-induced tissue injury.
This mechanism explains why gamma rays are effective at killing cells but also why they can cause harmful side effects in healthy tissues exposed to radiation.