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Life

2007 Schools Wikipedia Selection. Related subjects: General Biology

   Life is the characteristic state of organisms and individual cells.
   Properties common to the known organisms found on Earth (plants,
   animals, fungi, protists, archaea and bacteria) are that they are
   carbon-and- water-based, are cellular with complex organization,
   undergo metabolism, possess a capacity to grow, respond to stimuli,
   reproduce and, through natural selection, adapt in succeeding
   generations.

   An entity with the above properties is considered to be an organism.
   However, not every definition of life considers all of these properties
   to be essential. For example, the capacity for descent with
   modification is often taken as the only essential property of life.
   This definition notably includes viruses, which do not qualify under
   narrower definitions as they are acellular and do not metabolise.
   Broader definitions of life may also include theoretical
   non-carbon-based life and other alternative biology.

   Although it cannot be pinpointed exactly, evidence suggests that life
   on Earth has existed for about 3.7 billion years . Currently the entire
   Earth contains about 75 billion tons of biomass (life), which lives
   within various environments within the biosphere.

Definition by Opinion

   Life itself is a set of processes that are carried out by an organism
   causing it to survive.

A conventional definition

   Although there is no universal agreement on the definition of life,
   scientists generally accept that the biological manifestation of life
   exhibits the following phenomena:
    1. Homeostasis: Regulation of the internal environment to maintain a
       constant state; for example, sweating to cool off.
    2. Organization: Being composed of one or more cells, which are the
       basic units of life.
    3. Metabolism: Production of energy by converting nonliving material
       into cellular components ( synthesis) and decomposing organic
       matter ( catalysis). Living things require energy to maintain
       internal organization ( homeostasis) and to produce the other
       phenomena associated with life.
    4. Growth: Maintenance of a higher rate of synthesis than catalysis. A
       growing organism increases in size in all of its parts, rather than
       simply accumulating matter. The particular species begins to
       multiply and expand as the evolution continues to flourish.
    5. Adaptation: The ability to change over a period of time in response
       to the environment. This ability is fundamental to the process of
       evolution and is determined by the organism's heredity as well as
       the composition of metabolized substances, and external factors
       present.
    6. Response to stimuli: A response can take many forms, from the
       contraction of a unicellular organism when touched to complex
       reactions involving all the senses of higher animals. A response is
       often expressed by motion, for example, the leaves of a plant
       turning toward the sun or an animal chasing its prey.
    7. Reproduction: The ability to produce new organisms. Reproduction
       can be the division of one cell to form two new cells. Usually the
       term is applied to the production of a new individual (either
       asexually, from a single parent organism, or sexually, from at
       least two differing parent organisms), although strictly speaking
       it also describes the production of new cells in the process of
       growth.

Exceptions to the conventional definition

   It is important to note that life is a definition that applies
   primarily at the level of species, so even though many individuals of
   any given species do not reproduce, possibly because they belong to
   specialized sterile castes (such as ant workers), these are still
   considered forms of life. One could say that the property of life is
   inherited; hence, sterile hybrid species such as the mule are
   considered life although not themselves capable of reproduction. It is
   also worth noting that non-reproducing individuals may still help the
   spread of their genes through such mechanisms as kin selection.

   For similar reasons, viruses and aberrant prion proteins are often
   considered replicators rather than forms of life, a distinction
   warranted because they cannot reproduce without very specialized
   substrates such as host cells or proteins, respectively. However, most
   forms of life rely on foods produced by other species, or at least the
   specific chemistry of Earth's environment.

   Some individuals contest such definitions of life on philosophical
   grounds, and offer the following as examples of life: viruses which
   reproduce; storms or flames which "grow"; certain computer software
   programs which are programmed to mutate and evolve; future software
   programs which may evince (even high-order) behaviour; machines which
   can move; and some forms of proto-life consisting of metabolizing cells
   without the ability to reproduce.

   Still, most scientists would not call such phenomena expressive of
   life. Generally all seven characteristics are required for a population
   to be considered a life form.

Descent with modification

   A useful characteristic upon which to base a definition of life is that
   of descent with modification: the ability of a life form to produce
   offspring that are like its parent or parents, but with the possibility
   of some variation due to chance. Descent with modification is
   sufficient by itself to allow evolution, assuming that the variations
   in the offspring allow for differential survival. The study of this
   form of heritability is called genetics. In all known life forms
   (assuming prions are not counted as such), the genetic material is
   primarily DNA or the related molecule, RNA.

   Unlike other definitions, this definition of life includes viruses, as
   they are replicators with a genotype and phenotype, making them capable
   of natural selection and evolution. The definition may also include
   other replicating elements, including plasmids, which are otherwise
   considered part of a larger organism.

   Also difficult for this definition is organisms which cannot reproduce
   directly, such as worker bees—which may also continue their gene-line
   by helping to produce siblings, and sterilised organisms, such as
   spayed or neutered pets, which are no longer capable of descent.

   More abstract concepts may also be considered alive by this definition,
   including memes and the artificial life of computer software, such as
   self-modifying computer viruses and programs created through genetic
   programming.

Other definitions

   The systemic definition is that living things are self-organizing and
   autopoietic (self-producing). These objects are not to be confused with
   dissipative structures (e.g. fire).

   Variations of this definition include Stuart Kauffman's definition of
   life as an autonomous agent or a multi-agent system capable of
   reproducing itself or themselves, and of completing at least one
   thermodynamic work cycle.

   Another definition is: "Living things are systems that tend to respond
   to changes in their environment, and inside themselves, in such a way
   as to promote their own continuation."

   Yet another definition: "Life is a self-organizing, cannibalistic
   system consisting of a population of replicators that are capable of
   mutation, around most of which homeostatic, metabolizing organisms
   evolve." This definition does not include flames, but does include
   worker ants, viruses and mules. Without 'most of', it does not include
   viruses.

   Self reproduction and energy consumption is only one means for a system
   to promote its own continuation. This explains why bees can be alive
   and yet commit suicide in defending their hive. In this case the whole
   colony works as such a living system.

Origin of life

   There is no truly "standard" model for the origin of life, but most
   currently accepted scientific models build in one way or another on the
   following discoveries, which are listed roughly in order of postulated
   emergence:
    1. Plausible pre-biotic conditions result in the creation of the basic
       small molecules of life. This was demonstrated in the Miller-Urey
       experiment.
    2. Phospholipids spontaneously form lipid bilayers, the basic
       structure of a cell membrane.
    3. Procedures for producing random RNA molecules can produce
       ribozymes, which are able to produce more of themselves under very
       specific conditions.

   There are many different hypotheses regarding the path that might have
   been taken from simple organic molecules to protocells and metabolism.
   Many models fall into the " genes-first" category or the "
   metabolism-first" category, but a recent trend is the emergence of
   hybrid models that do not fit into either of these categories.

The possibility of extraterrestrial life

   Earth is the only planet in the universe known to harbour life. The
   Drake equation has been used to estimate the probability of life
   elsewhere, but scientists disagree on many of the values of variables
   in this equation. Depending on those values, the equation may either
   suggest that life arises frequently or infrequently.

   Retrieved from " http://en.wikipedia.org/wiki/Life"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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