急求一篇有关生命 人类的跟医学有关的文章,最好是英文的.

急求一篇有关生命 人类的跟医学有关的文章,最好是英文的.
同上,最好是英文的.

Have you ever wondered why we age?What exactly is happening inside our bodies to bring on the wrinkles,gray hair,and the other changes seen in older people?Considering the universality of the process,you might be surprised to know that there remain many unanswered questions about how aging happens at the cellular level.However,theories abound,and the roles played by various suspects in the aging process are beginning to take shape.
Cell death,on the other hand,is an area in which scientists have made great leaps in understanding in recent years.Far from being strictly harmful,scientists have found that cell death,when carefully controlled,is critical to life as we know it.Without it,you wouldn't have your fingers and toes or the proper brain cell connections to be able to read the words on this page.
Aging:A World of Theories
Most scientists now agree that aging is,at least in part,the result of accumulating damage to the molecules—such as proteins,lipids,and nucleic acids (DNA and RNA)—that make up our cells.If enough molecules are damaged,our cells will function less well,our tissues and organs will begin to deteriorate,and eventually,our health will decline.So in many respects,we appear to age much like a car does:Our parts start to wear out,and we gradually lose the ability to function.
The question is,where does the damage come from?It turns out that damage can come from many different sources,both internal and external.
Thieving Oxygen
Take a deep breath.Oxygen in the air you just breathed entered your lungs,passed into the tiny blood vessels that line them,and then went on a wild ride through the creeks,rivers,and cascades of your bloodstream.Thanks to your rich network of blood vessels,oxygen gets carried to every cell in every corner of your body.Once delivered to a cell,oxygen heads for the mitochondria,where it slurps up the electrons coming off the end of the energy-production assembly line.Mitochondria need oxygen to generate cellular energy,and humans need a constant supply of that energy to survive.That's why people die within a few minutes if deprived of oxygen.
But oxygen has a darker side,and it has attracted the attention of scientists who study aging.Normally,an oxygen molecule (O 2 ) absorbs four electrons and is eventually safely converted into water.But if an oxygen molecule only takes up one or two electrons,the result is one of a group of highly unstable molecules called reactive oxygen species that can damage many kinds of biological molecules by stealing their electrons.These renegade oxygen-containing species can mutate your genes,damage the lipids that make up your cellular membranes,and break the proteins that do much of the cell's work,thereby causing cellular injury in multiple and overlapping ways.