Misplaced Pages

Life

Article snapshot taken from Wikipedia with creative commons attribution-sharealike license. Give it a read and then ask your questions in the chat. We can research this topic together.

This is an old revision of this page, as edited by UberAetheist (talk | contribs) at 17:07, 30 April 2007. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Revision as of 17:07, 30 April 2007 by UberAetheist (talk | contribs)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Life" – news · newspapers · books · scholar · JSTOR (December 2006) (Learn how and when to remove this message)
For other uses, see Life (disambiguation). For life on Earth, see Organism.

Life
Life colonizing a rocky peak
Scientific classification
(unranked): Life
Domains and Kingdoms

Life is a condition that distinguishes organisms from inorganic objects and dead organisms, the distinguishing feature attributed to life being the maintenane of an internal environment different from its surroundings. Booyah!!!!!!


Definitions

Conventional definition: Often scientists say that life is a characteristic of organisms that exhibit the following phenomena:

  1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
  2. Organization: Being composed of one or more cells, which are the basic units of life.
  3. Metabolism: Consumption of energy by converting nonliving material into cellular components (anabolism) and decomposing organic matter (catabolism). 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.

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. Also, the Rickettsia and Chlamydia are examples of bacteria that cannot independently fulfill many vital biochemical processes, and depend on entry, growth, and replication within the cytoplasm of eukaryotic host cells. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.

Still others contest such definitions of life on philosophical grounds. They offer the following as examples of life: viruses which reproduce; storms or flames which "burn"; certain computer software programs which are programmed to mutate and evolve; future software programs which may evince (even high-order) behavior; 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.

File:Nwhi - French Frigate Shoals reef - many fish.jpg
Marine life around a coral reef

The systemic definition of life 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.

Yet other definitions of life are:

  1. 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.
  2. Life is a characteristic of self-organizing, cannibalistic systems 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. 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.
  3. Type of organization of matter producing various interacting forms of variable complexity, whose main property is to replicate almost perfectly by using matter and energy available in their environment to which they may adapt. In this definition "almost perfectly" relates to mutations happening during replication of organisms that may have adaptative benefits.
  4. Life is a potentially self-perpetuating open system of linked organic reactions, catalyzed simultaneously and almost isothermally by complex chemicals (enzymes) that are themselved produced by the open system.

Origin of life

Main article: Origin of life
Microbial mats around the Grand Prismatic Spring of Yellowstone National Park

Although it cannot be pinpointed exactly, evidence suggests that life on Earth has existed for about 3.7 billion years .

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, and in the work of Sidney Fox.
  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, Despite the length of scientist's current speculations, the origin of life remains as one of science's greatest mysteries.

Extraterrestrial life

Main articles: Extraterrestrial life, Astrobiology

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 (although strictly speaking Drake equation estimates the number of extraterrestrial civilizations in our galaxy with which we might come in contact - not probability of life elsewhere). Depending on those values, the equation may either suggest that life arises frequently or infrequently. Drake himself estimated the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time as equal to one.

Relating to the origin of life on Earth, panspermia and exogenesis are theories proposing that life originated elsewhere in the universe and was subsequently transferred to Earth perhaps via meteorites, comets or cosmic dust. However those theories do not help explain the origin of this extraterrestrial life.

See also

References

Bibliography

  • Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from
  • Lynn Margulis and Dorion Sagan - What Is Life? (1995). Simon & Schuster. ISBN 0-684-81087-5
  • Erwin Schrödinger - What is Life? (1944 to 2000). Cambridge University Press (Canto). ISBN 0-521-42708-8
  • Walker, Martin G. LIFE! Why We Exist...And What We Must Do to Survive ( Wiki Book Page) ( Web Site), Dog Ear Publishing, 2006, ISBN 1-59858-243-7

Notes

  1. http://www.ucmp.berkeley.edu/exhibits/historyoflife.php
  2. http://www.scribd.com/doc/1569/Origin-of-Life-in-Universe
  3. http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&id=doi:10.1098/rsif.2005.0045

External links

ru-sib:Жысь

Categories: