## Reactive particles have important functions in cells too

So far, we have only mentioned the negative properties of reactive particles (the various types of damage they cause), but these molecules also have their physiological functions in the cells. They are used, for example, as signalling molecules, which means that they ensure communication between cellular structures, but also between individual cells. Reactive particles are ideal to act as a signal molecule for several reasons: they are small, so they can relatively easily pass through membranes and thus reliably transmit a signal, their concentration in the cell can increase sharply if necessary (thus the cell can immediately react to various stimuli), and they are very quickly eliminated as soon as the signal was recorded and the cell began to react.

The importance of reactive particles as signal transmitters can be illustrated by the example of mitochondria, which naturally generate them. If the cell has functional and undamaged mitochondria, it generates reactive particles of a certain concentration and subsequently degrades them by antioxidant mechanisms. If any component of the respiratory chain is damaged, the concentration of reactive particles starts to sharply rise. The cell also notices this change due to the increased occurrence of oxidative damage. To reverse this condition, various repair mechanisms are activated to either repairs or degrades the damaged component of the respiratory chain. In extreme conditions, when the damage is severe or extensive, the entire mitochondrion can be eliminated.

A very interesting example of reactive particles role in cellular signalling is the role of nitric oxide (NO) in the relaxation of blood vessels. Excretion of this substance is the body's natural reaction to increased blood pressure and therefore increased blood flow in the vessels. In order to ensure normal blood flow, the communication of up to three types of cells is necessary - nerve, vascular and endothelial (forms the inner lining of blood vessels) cells. The nerve cell is the main signal messenger from the brain to the blood vessels. With the help of small molecules of acetylcholine, the nerve cell sends a signal to the endothelial cell to relax the vessels. As a response to the presence of acetylcholine, the endothelial cell begins to form nitric oxide, which quickly and efficiently passes through the cell membrane and the intercellular space to the blood cell. In this case, nitric oxide is not generated by the endothelial cell randomly when some process is damaged, but generated purposefully by cell to transmit a signal. The result of such effective communication is the relaxation of the blood vessel structure, which allows better blood flow. The discovery of the physiological function of nitric oxide, a reactive particle occurring in an influent state, as a signal transmitter was so significant that the Nobel Prize was awarded in 1998 for this discovery.