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Animation: Signs of Life

by Pearson
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While there is a tremendous diversity of life on Earth, all living things-bacteria, archaea, protists, fungi, plants, and animals- share common traits that are associated with life. No single definition applies to all forms of life, so instead of defining life, we can only characterize it. Most biologists agree that, in general, the following statements characterize life. First, living things contain nucleic acids, proteins, carbohydrates, and lipids. The nucleic acid DNA (deoxyribonucleic acid) is especially important because DNA molecules can be replicated, an ability that enables organisms to reproduce themselves. Second, living things are composed of cells. Cells are the smallest units of life. Some organisms have only a single cell; others are multicellular. Humans, for example, are composed of trillions of cells. Third, living things reproduce. Living things have ways of generating new individuals that carry some of the genetic material of the parents. Some organisms, such as bacteria, reproduce simplyby making new and virtually exact copies of themselves. Fourth, living things use energy and raw materials. Through metabolic activities, organisms extract energy from various nutrients and transform it to do many kinds of work, allowing the organism to maintain life and grow. Fifth, living things respond. A snail, for example, detects the pressure of touch and responds by retracting into its shell. Sixth, living things maintain homeostasis- the relatively constant and self-correcting internal environment of a living organism. A snake remains cool by finding shade- a behavioral means of maintaining homeostasis. Animals also have physiological means to maintain homeostasis. For example, humans sweat to remain cool. Seventh, populations of living things evolve and have adaptive traits. In a process called natural selection, members of a population possessing beneficial genetic traits will survive and reproduce better than members of the population that lack these traits. In the case of the giraffe, a classic explanation for the long neck is that it is an adaptive trait for eating leaves that other animals may not be able to reach. Another explanation, however, is that male giraffes with longer necks can win fights with males having shorter necks and thereby gain greater access to females.
While there is a tremendous diversity of life on Earth, all living things-bacteria, archaea, protists, fungi, plants, and animals- share common traits that are associated with life. No single definition applies to all forms of life, so instead of defining life, we can only characterize it. Most biologists agree that, in general, the following statements characterize life. First, living things contain nucleic acids, proteins, carbohydrates, and lipids. The nucleic acid DNA (deoxyribonucleic acid) is especially important because DNA molecules can be replicated, an ability that enables organisms to reproduce themselves. Second, living things are composed of cells. Cells are the smallest units of life. Some organisms have only a single cell; others are multicellular. Humans, for example, are composed of trillions of cells. Third, living things reproduce. Living things have ways of generating new individuals that carry some of the genetic material of the parents. Some organisms, such as bacteria, reproduce simplyby making new and virtually exact copies of themselves. Fourth, living things use energy and raw materials. Through metabolic activities, organisms extract energy from various nutrients and transform it to do many kinds of work, allowing the organism to maintain life and grow. Fifth, living things respond. A snail, for example, detects the pressure of touch and responds by retracting into its shell. Sixth, living things maintain homeostasis- the relatively constant and self-correcting internal environment of a living organism. A snake remains cool by finding shade- a behavioral means of maintaining homeostasis. Animals also have physiological means to maintain homeostasis. For example, humans sweat to remain cool. Seventh, populations of living things evolve and have adaptive traits. In a process called natural selection, members of a population possessing beneficial genetic traits will survive and reproduce better than members of the population that lack these traits. In the case of the giraffe, a classic explanation for the long neck is that it is an adaptive trait for eating leaves that other animals may not be able to reach. Another explanation, however, is that male giraffes with longer necks can win fights with males having shorter necks and thereby gain greater access to females.