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An organism is any living thing that functions as an individual. [1] Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what an organism is.
A microorganism, or microbe, [a] is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.. The possible existence of unseen microbial life was suspected from ancient times, such as in Jain scriptures from sixth century BC India.
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea
The following outline is provided as an overview of and topical guide to life forms: . A life form (also spelled life-form or lifeform) is an entity that is living, [1] [2] such as plants (), animals (), and fungi ().
The notion of system (or apparatus) relies upon the concept of vital or organic function: [2] a system is a set of organs with a definite function. This idea was already present in Antiquity (Galen, Aristotle), but the application of the term "system" is more recent.
In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics , which are effective at demonstrating ...
The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by a set of characters shared by all its living representatives.
In the first organisms, the gradient could have been provided by the difference in chemical composition between the flow from a hydrothermal vent and the surrounding seawater, [156] or perhaps meteoric quinones that were conducive to the development of chemiosmotic energy across lipid membranes if at a terrestrial origin.