Oxygen is what it is all about. Ironically, the stuff that gives us life eventually kills it. The ultimate life force lies in tiny cellular factories of energy, called mitochondria, ① that burn nearly all the oxygen we breathe in. But breathing has a price. The combustion of oxygen that keeps us alive and active ② sending out by-products called oxygen free radicals. They have Dr. Jekyll and Mr. Hyde characteristics. On the one hand, they help guarantee our survival. For example, when the body mobilizes ③ to fight off infectious agents, it generates a burst of free radicals to destroy the invaders very efficiently. On the other hand, free radicals move ④ uncontrollably through the body, attacking cells, rusting their proteins, piercing their membranes and corrupting their genetic code until the cells become dysfunctional and sometimes give up and die. These fierce radicals, ⑤ built into life as both protectors and avengers, are potent agents of aging.

* oxygen free radical: 활성 산소 ** membrane: (해부학) 얇은 막