Unicellular organism

The origin of life is largely still a mystery. Primitive protocells are thought to be the precursors to today's unicellular organisms. In one theory, known as the RNA world hypothesis, early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication. Compartmentalization was necessary for chemical reactions to be more likely as well as to differentiate reactions with the external environment. For example, an early RNA replicator ribozyme may have replicated other replicator ribozymes of different RNA sequences if not kept separate. Such hypothetic cells with an RNA genome instead of the usual DNA genome are called 'ribocells' or 'ribocytes'. Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment. Because of their simplicity and ability to self-assemble in water, it is likely that these simple membranes predated other forms of early biological molecules. == Prokaryotes ==

Prokaryotes lack membrane-bound organelles, such as mitochondria or a nucleus. Instead, most prokaryotes have an irregular region that contains DNA, known as the nucleoid. Most prokaryotes have a single, circular chromosome, which is in contrast to eukaryotes, which typically have linear chromosomes. Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, cellulose, ammonia, or nitrite. Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments. Bacteria s in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm. Bacteria are one of the world's oldest forms of life, and are found virtually everywhere on Earth. Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance. Bacteria predominantly reproduce asexually through a process called binary fission. However, about 80 different species can undergo a sexual process referred to as natural genetic transformation. Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation for repairing DNA damage in the recipient cell. In addition, plasmids can be exchanged through the use of a pilus in a process known as conjugation. Stromatolites, structures made up of layers of calcium carbonate and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records. The existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the Archaean (4 billion to 2.5 billion years ago), Proterozoic (2.5 billion to 540 million years ago), and Phanerozoic (540 million years ago to present day) eons. Archaea Hydrothermal vents release heat and hydrogen sulfide, allowing extremophiles to survive using chemolithotrophic growth. Archaea are generally similar in appearance to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA. By sequencing the ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually more phylogenetically related to eukaryotes. Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic the early, harsh conditions that life was likely exposed to. Examples of these Archaean extremophiles are as follows: • Thermophiles, optimum growth temperature of 50 °C-110 °C, including the genera Pyrobaculum, Pyrodictium, Pyrococcus, Thermus aquaticus and Melanopyrus. • Psychrophiles, optimum growth temperature of less than 15 °C, including the genera Methanogenium and Halorubrum. • Acidophiles, optimum growth pH of less than 3, including the genera Sulfolobus and Picrophilus. • Piezophiles, (also known as barophiles), prefer high pressure up to 130 MPa, such as deep ocean environments, including the genera Methanococcus and Pyrococcus. Methanogens are a significant subset of archaea and include many extremophiles, but are also ubiquitous in wetland environments as well as the ruminant and hindgut of animals. This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of adenosine triphosphate. Under stressful environmental conditions that cause DNA damage, some species of archaea aggregate and transfer DNA between cells. The function of this transfer appears to be to replace damaged DNA sequence information in the recipient cell by undamaged sequence information from the donor cell. == Eukaryotes ==

Eukaryotic cells contain membrane bound organelles. Some examples include mitochondria, a nucleus, or the Golgi apparatus. Prokaryotic cells probably transitioned into eukaryotic cells between 2.0 and 1.4 billion years ago. This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by using mitosis and meiosis. Sex appears to be a ubiquitous and ancient, and inherent attribute of eukaryotic life. Meiosis, a true sexual process, allows for efficient recombinational repair of DNA damage However, if the bacteria were capable of respiration, it would have been beneficial for the larger cell to allow the parasite to live in return for energy and detoxification of oxygen. While not all eukaryotes have mitochondria or chloroplasts, mitochondria are found in most eukaryotes, and chloroplasts are found in all plants and algae. Photosynthesis and respiration are essentially the reverse of one another, and the advent of respiration coupled with photosynthesis enabled much greater access to energy than fermentation alone. Protozoa Protozoa are largely defined by their method of locomotion, including flagella, cilia, and pseudopodia. While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa: Euglenozoa, Amoebozoa, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. Protozoa, like plants and animals, can be considered heterotrophs or autotrophs. Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism. Many ciliates have trichocysts, which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense. Ciliates are also capable of sexual reproduction, and utilize two nuclei unique to ciliates: a macronucleus for normal metabolic control and a separate micronucleus that undergoes meiosis. Entamoeba histolytica appears to be capable of meiosis. Unicellular algae Unicellular algae are plant-like autotrophs and contain chlorophyll. They include groups that have both multicellular and unicellular species: • Euglenophyta, flagellated, mostly unicellular algae that occur often in fresh water. • Diatoms, unicellular algae that have siliceous cell walls. They are the most abundant form of algae in the ocean, although they can be found in fresh water as well. Diatoms are very diverse, and comprise about 100,000 species. Dinoflagellates can be mixotrophic, and are the algae responsible for red tide. Unicellular fungi '' Unicellular fungi include the yeasts. Fungi are found in most habitats, although most are found on land. Yeasts reproduce through mitosis, and many use a process called budding, where most of the cytoplasm is held by the mother cell. S. cerevisiae is also an important model organism, since it is a eukaryotic organism that is easy to grow. It has been used to research cancer and neurodegenerative diseases as well as to understand the cell cycle. Furthermore, research using S. cerevisiae has played a central role in understanding the mechanism of meiotic recombination and the adaptive function of meiosis. Candida spp. are responsible for candidiasis, causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection). == Macroscopic unicellular organisms ==

Most unicellular organisms are of microscopic size and are thus classified as microorganisms. However, some unicellular protists and bacteria are macroscopic and visible to the naked eye. Examples include: • Brefeldia maxima, a slime mold, examples have been reported up to a centimetre thick with a surface area of over a square metre and weighed up to around 20 kg • Xenophyophores, protozoans of the phylum Foraminifera, are the largest examples known, with Syringammina fragilissima achieving a diameter of up to • Nummulite, foraminiferans • Valonia ventricosa, an alga of the class Chlorophyceae, can reach a diameter of • Acetabularia, algae • Caulerpa, algae, may grow to 3 metres long • Gromia sphaerica, amoeba, • Thiomargarita magnifica is the largest bacterium, reaching a length of up to 20 mm • Epulopiscium fishelsoni, a bacterium • Stentor, ciliates nicknamed trumpet animalcules • Bursaria, largest colpodean ciliates. ==See also==