Oxygen toxicity: chemistry and biology of reactive oxygen species.

Oxygen has a central role in the evolution of complex life on Earth mainly because of the biochemical symmetry of oxygenic photosynthesis and aerobic respiration that can maintain homeostasis within our planet biosphere. Oxygen can also produce toxic molecules, reactive oxygen species (ROS). ROS is a collective term that includes both oxygen radicals and certain oxidizing agents that are easily converted into radicals. They can be produced from both endogenous and exogenous substances. ROS play a dual role in biological systems, since they can be either harmful or beneficial to living systems. They can be considered a double-edged sword because on the one hand oxygen-dependent reactions and aerobic respiration have significant advantages but, on the other, overproduction of ROS has the potential to cause damage. Copyright 2010 Elsevier Ltd. All rights reserved.