What is Unicellular organism?
[[File:Ventricaria ventricosa.JPG|thumb|''[[Valonia ventricosa]]'', a kind of [[alga]], is among the largest unicellular species. Its diameter can reach {{convert|5|cm}}.]] '''Unicellular organisms''' are [[organism]]s that have one [[Cell (biology)|cell]]. They are divided into two quite different types, from different [[Scientific classification|classification]] [[Kingdom (biology)|kingdoms]]. *The [[prokaryote]]s, [[bacteria]] and [[archaea]], have cells with no [[cell nucleus|nucleus]] and a simple cell structure. *[[Eukaryote]]s have a [[Cell nucleus|nucleus]], and a more complex cell structure. The differences between the prokaryota and eukaryota are significant. Eukaryotes possess a nucleus and various subcellular organs called [[organelle]]s, and prokaryotes do not. == Observations == Almost all unicellular organisms are [[microorganism]]s but you can observe eukaryotes and many prokaryotes by using a compound [[microscope]]. Bacteria just appear as dark dots. To gather unicellular organisms for observation, one can place a cover slip on the surface of pond water, and leave it overnight. By the next morning, numerous unicellular organisms will have grown entire colonies on the bottom of the slip. They multiply fast: colonies can double their size in between 30 minutes and a few hours. Unicellular organisms can be found everywhere. The oldest forms of life existed 3.8 [[billion]] years ago, if not longer. They use a variety of strategies for survival: [[photosynthesis]] ([[cyanobacteria]]), [[chemotrophy]] (many [[archaea]]), and [[heterotrophy]] ([[amoeba]]). Some unicellular organisms have [[flagella]], little tails they use for locomotion, or lobopods, extensions of the cellular skeleton (cytoskeleton), which appear as bloblike arms. The flagella of our unicellular ancestors occurs all the way up into the animals, where it makes an appearance as flagellated [[sperm]]. Of all the six eukaryote supergroups, four are exclusively composed of unicellular organisms. Unicellular organisms vary in size,