Most reactive oxygen species (ROS) are generated in cells by the mitochondrial respiratory chain. Mitochondrial ROS production is modulated largely by the rate of electron flow through respiratory chain complexes. Recently, it has become clear that under hypoxic conditions, the mitochondrial respiratory chain also produces nitric oxide (NO), which can generate other reactive nitrogen species (RNS). Although excess ROS and RNS can lead to oxidative and nitrosative stress, moderate to low levels of both function in cellular signaling pathways. Especially important are the roles of these mitochondrially generated free radicals in hypoxic signaling pathways, which have important implications for cancer, inflammation and a variety of other diseases.
Well the cell membrane accepts nonpolar molecules, and O2 is nonpolar.
The Krebs cycle<span> occurs right after glycolysis. The substance that begins the </span>Krebs cycle<span> is a 3-carbon molecule called pyruvic </span>acid<span>. The process breaks down the pyruvic </span>acid<span> into acetyl coenzyme A, releasing one of the carbon atoms into carbon dioxide.
hope it help</span>