although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction. Protozoa perform, in a single cell, the same life functions as those carried on by the complex tissues and organs of humans and other highly developed organisms. The attributes of life are inherent in such minute structures as viruses, bacteria, and genes, just as they are in the whale and the giant sequoia. In seeking an understanding of life, scientists have broken down many barriers that once separated the physical sciences from the biological sciences; a result of the growth of biochemistry, biophysics, and other interrelated fields of study has been a better understanding of the composition and functioning of living tissues of all kinds.
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