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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 eukaryotes (/ j uː ˈ k ær i oʊ t s,-ə t s / yoo-KARR-ee-ohts, -əts) [4] constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus.All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes.
A unicellular organism is a microorganism such as a protist, bacterium, or archaean, composed of a single cell, which may contain functional structures called organelles. [22] A multicellular organism such as an animal , plant , fungus , or alga is composed of many cells, often specialised. [ 22 ]
Clockwise from top right: Amoeba proteus, Actinophrys sol, Acanthamoeba sp., Nuclearia thermophila., Euglypha acanthophora, neutrophil ingesting bacteria. An amoeba (/ ə ˈ m iː b ə /; less commonly spelled ameba or amœba; pl.: amoebas (less commonly, amebas) or amoebae (amebae) / ə ˈ m iː b i /), [1] often called an amoeboid, is a type of cell or unicellular organism with the ability ...
The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology". [16] John Hogg's illustration of the Four Kingdoms of Nature, showing "Primigenal" as a greenish haze at the base of the Animals and Plants, 1860
Bacteria and archaea are unicellular organisms that lack organelles–specifically, a nucleus. [1] They lack nuclei but contain other organelles that assist with the replication processes. Viruses are sometimes considered akaryocytes but the suffix "cyte" means cells.
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia. In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill ...
Unicellular eukaryotes consist of a single cell throughout their life cycle. This qualification is significant since most multicellular eukaryotes consist of a single cell called a zygote only at the beginning of their life cycles. Microbial eukaryotes can be either haploid or diploid, and some organisms have multiple cell nuclei. [60]