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| Throughout history man has had to accept the fact that all living things must die. But people now live longer than they__1__.Yet, all living things still show the__2_of aging, which will eventually__3__death. Aging is not a disease, but as a person passes maturity (成熟期), the cells of the body and the__4__ they form do not function as well as they__5__in childhood and teenage years. The body provides less __6__against disease and is more__7__to have an accident. A number of related causes may__8__to aging. Some cells of the body have a fairly long life, but they are not__9__when they die. As a person ages, 10 of brain cells and muscle cells decreases. 11 body cells die and are replaced by new cells. In an aging person the 12 cells may not be as workable or as capable 13 growth as those of a young person. Another 14 in aging may be changes within the cells 15 . Some of the protein chemicals in cells are known 16 with age and become less elastic (有弹性). This is why the skin of old people wrinkles and 17 . This is also the reason why old people 18 in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and 19 information that the cells need. Aging may affect this 20 and change the informationcarrying molecules so that they do not transmit the information as well. | ||||
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