Life is a characteristic that distinguishes physical entities that do have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential artificial life as "living". Biology is the primary science concerned with the study of life, although many other sciences are involved. The definition of life is controversial. The current definition is that organisms maintain homeostasis, are composed of cells, undergo metabolism, can grow, adapt to their environment, respond to stimuli, and reproduce. However, many other biological definitions have been proposed, and there are some borderline cases of life, such as viruses. Throughout history, there have been many attempts to define what is meant by "life" and many theories on the properties and emergence of living things, such as materialism, the belief that everything is made out of matter and that life is merely a complex form of it; hylomorphism, the belief that all things are a combination of matter and form, and the form of a living thing is its soul; spontaneous generation, the belief that life repeatedly emerges from non-life; and vitalism, a now largely discredited hypothesis that living organisms possess a "life force" or "vital spark". Modern definitions are more complex, with input from a diversity of scientific disciplines. Biophysicists have proposed many definitions based on chemical systems; there are also some living systems theories, such as the Gaia hypothesis, the idea that the Earth itself is alive.
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