But if oxygen only gains one or two electrons this can lead to the production of molecules such as superoxide ions or peroxide ions. And if these are released by complex four, that can cause damage to our cells. And because of that, we call these reactive oxygen species. They're highly reactive and they can cause damage to the cells of our body. So Ros stands for reactive oxygen species. Now, of course, the electron transport chain isn't a perfect process and these molecules will, in fact, form.