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Jun 16, 2022 · Life Definition. Life is a characteristic of a living organism that distinguishes the latter from a dead organism or a non-living thing, as specifically distinguished by the capacity to grow, metabolize, respond (to stimuli), adapt, and reproduce. Life may also pertain to the biota of a particular region. Word origin: Old English līf (life, body).
- Overview
- Definitions of life
- Metabolic
- Physiological
- Biochemical
life, living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction. Although a noun, as with other defined entities, the word life might be better cast as a verb to reflect its essential status as a process. Life comprises individuals, living beings, assignable to groups (taxa). Each individual is composed of one or more minimal living units, called cells, and is capable of transformation of carbon-based and other compounds (metabolism), growth, and participation in reproductive acts. Life-forms present on Earth today have evolved from ancient common ancestors through the generation of hereditary variation and natural selection. Although some studies state that life may have begun as early as 4.1 billion years ago, it can be traced to fossils dated to 3.5–3.7 billion years ago, which is still only slightly younger than Earth, which gravitationally accreted into a planet about 4.5 billion years ago. But this is life as a whole. More than 99.9 percent of species that have ever lived are extinct. The several branches of science that reveal the common historical, functional, and chemical basis of the evolution of all life include electron microscopy, genetics, paleobiology (including paleontology), and molecular biology.
The phenomenon of life can be approached in several ways: life as it is known and studied on planet Earth; life imaginable in principle; and life, by hypothesis, that might exist elsewhere in the universe (see extraterrestrial life). As far as is known, life exists only on Earth. Most life-forms reside in a thin sphere that extends about 23 km (14 miles) from 3 km (2 miles) beneath the bottom of the ocean to the top of the troposphere (lower atmosphere); the relative thickness is comparable to a coat of paint on a rubber ball. An estimated 10–30 million distinguishable species currently inhabit this sphere of life, or biosphere.
Much is known about life from points of view reflected in the various biological, or “life,” sciences. These include anatomy (the study of form at the visible level), ultrastructure (the study of form at the microscopic level), physiology (the study of function), molecular biology and biochemistry (the study of form and function at chemical levels)...
Metabolic definitions are popular with biochemists and some biologists. Living systems are objects with definite boundaries, continually exchanging some materials with their surroundings but without altering their general properties, at least over some period of time. However, there are exceptions. There are frozen seeds and spores that remain, so far as is known, perfectly dormant. At low temperatures they lack metabolic activity for hundreds, perhaps thousands, of years but revive perfectly well upon being subjected to more clement conditions. A candle flame has a well-defined shape with a fixed boundary and is maintained by “metabolizing” its organic waxes and the surrounding molecular oxygen to produce carbon dioxide and water. Similar reactions, incidentally, occur in animals and plants. Flames also have a well-known capacity for growth. These facts underscore the inadequacy of this metabolic definition, even as they suggest the indispensable role of energy transformation to living systems. (See metabolism.)
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Physiological definitions of life are popular. Life is defined as any system capable of performing functions such as eating, metabolizing, excreting, breathing, moving, growing, reproducing, and responding to external stimuli. But many such properties are either present in machines that nobody is willing to call alive or absent from organisms, such...
A biochemical or molecular biological definition sees living organisms as systems that contain reproducible hereditary information coded in nucleic acid molecules and that metabolize by controlling the rate of chemical reactions using the proteinaceous catalysts known as enzymes. In many respects, this is more satisfying than the physiological or metabolic definitions of life. However, even here there are counterexamples. Viruslike agents called prions lack nucleic acids, although the nucleic acids of the animal cells in which they reside may be involved in their reproduction. Ribonucleic acid (RNA) molecules may replicate, mutate, and then replicate their mutations in test tubes, although by themselves they are not alive. Furthermore, a definition strictly in chemical terms seems peculiarly vulnerable. It implies that, were a person able to construct a system that had all the functional properties of life, it would still not be alive if it lacked the molecules that earthly biologists are fond of—and made of. (See biochemistry.)
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So, let us start again in trying to produce a good definition, or given the fact that we know only of one version of life, a useful description of what we mean by life. First, the core units of life are organisms, which are individual living objects. From a structural and thermodynamic perspective, each organism is a bounded, non-equilibrium ...
Properties of Life. All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.
1 day ago · The dictionary’s chronologies chart developments in major fields including cell biology, genetics, microscopy, and vitamins. A new feature is the inclusion of entry-level web links.With over 5,500 clear and concise entries, over 400 of them new to this edition, this market-leading dictionary is the perfect guide for anyone studying biology ...
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms.It is intended as introductory material for novices; for more specific and technical definitions from sub-disciplines and related fields, see Glossary of cell biology, Glossary of genetics, Glossary of evolutionary biology, Glossary of ecology ...
Life definition: the condition that distinguishes organisms from inorganic objects and dead organisms, being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally..
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