Eukaryotic domain
Eukaryota (scientific name: Eukaryota) is a general term for single-celled and multicellular organisms whose cells have nuclei, and it includes all animals, plants, fungi, and other organisms with complex subcellular structures wrapped by membranes, not bacteria and archaea, because their organelles are membrane-free.
The fundamental difference between eukaryotes and prokaryotes is that the cells of the former contain nuclei, hence the eukaryotic name for this type of cell. Many eukaryotic cells also contain other organelles, such as mitochondria, chloroplasts, Golgi, etc.
Because of their nucleus, the cell division process of eukaryotic cells is also very different from that of prokaryotes without nuclei.
Eukaryotes are evolutionarily singular, belonging to the eukaryotic domains of the tridenologic system of taxonomy, and the other two domains are bacteria and archaea that also belong to prokaryotes. However, due to the similarity between eukaryotes and archaea in some biochemical properties and genes, the two are sometimes attributed to the new wall total domain evolutionary branch.
Scientists believe that from the genetic evidence, eukaryotes are the genetic fusion of bacteria and archaea, which is the product of a certain symbiosis and heterogeneity of some archaea and bacteria.
<h1 class="pgc-h-arrow-right" data-track="8" >1</h1>
Eukaryotic cells can develop nuclei and organelles surrounded by plasma membranes, which are thought to have two processes: intramembrane folding and endosymbiosis. Intramembrane fold refers to the intraplasmic membrane folding of archaeopteryx paleontological cells into the membrane system of eukaryotic cells. After a second inversion, the endoplasmic omentum forms organelles such as the Golgi apparatus. Endosymbiosis refers to the fact that paleontological organisms swallow aerobic, smaller, archaeopteryx prokaryotes (aerobic eubacteriums, such as deformed bacteria) and transition from parasitism to symbiosis and become mitochondria. Through a similar process, paleontological cells symbiosis with smaller, archaeopterytic photosynthetic prokaryotes (such as cyanobacteria) produce green and red algae, which are swallowed by paleontology for the second time to become symbiotics and evolve into chloroplasts. The original endoplasmic omentum system produced by the in-membrane fold limits the activity of swallowing bacteria and avoids the "eating" of cellular components by them.
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<h1 class="pgc-h-arrow-right" data-track="11" >(1) traditional classification</h1>
In ancient times, it was recognized that animals and plants should be divided into two categories. When Linnaeus established the taxonomic order, although there were some doubts, the fungus was classified as the plant kingdom in the two-boundary theory. Later scholars divided the fungi into one realm, that is, divided eukaryotes into four realms: the protists, the plant kingdom, the animal kingdom, and the fungal kingdom, but the composition of the fungal kingdom was still not fully understood by the academic community until the 1980s. After the invention of the microscope, many of the protists found were divided into native plants and protozoa according to whether they were autotrophic or heterotrophic, and were classified into the plant kingdom and the animal kingdom respectively. Haeckel added the protists in 1866, i.e., divided eukaryotes into four realms.
Before the 1980s, the classification of single-celled protists was still in a very chaotic stage, until the maturity of DNA sequencing technology led to the emergence of phylogenetics, which changed this situation and also led to the beginning of rewriting the classification of eukaryotes.
<h1 class="pgc-h-arrow-right" data-track="14" >(2) 2005 International Society of Native Biologists Classification</h1>
Based on phylogenetic studies, based on homology on evolutionary branches, the International Society of Protistologists in 2005 proposed a then-generally accepted classification of eukaryotes, divided into six categories. This taxonomic approach does not specify which taxonomic hierarchy these classes are, but other literature treats them as "boundaries.".
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Ancient insectichea (Excavata)
Native organisms such as flagellars
Amoeba community (Amoebozoa)
Amoebic and slime molds, etc
Posterior flagellar (Opisthokonta)
Animals, fungi, choanoflagellates, etc
Foraminifera (Rhizaria)
There are foraminifera , radiolaria , and some amoeba protists
Vesicle algae (Chromalveolata)
Heterokonta, Haptophyta, Cryptophyta, Alveolata
Archaeplastida
Terrestrial plants, green algae, red algae, gray cell algae
However, this "six-realm theory" was soon challenged by new research, especially questions about whether the vesicle algae world had a common origin. The classification of eukaryotes is still highly uncertain. For example, eukaryotes are divided into unikonta, which includes the amoeba and post-flagellar, and Bikonta, which is the other four. However, many studies have shown that monoflagents and biflagents are not homologous evolutionary clades. In addition, the foraminifera boundary should be incorporated into the vesicle algae boundary. The pan-plant kingdom should also not be homologous and need to be subdivided.
Foraminifera and unequal flagella and vesicles are now widely considered to be sister taxa, belonging to the same supergroup (SAR supergroup), so foraminifera are no longer considered one of the main taxa of eukaryotes, while monoflages and their subgroups Amoeba and posterior flagellates are regarded as monophyletic groups, that is, animals and fungi belong to the same monophyletic group - posterior flagellar. Beyond that, there is no other consensus.
<h1 class="pgc-h-arrow-right" data-track="30" >(3) the current classification</h1>
The protists
Fungal boundaries
Plant kingdom
animal kingdom
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Eukaryotic cells are usually much larger in size than prokaryotic cells, about 10,000 times the latter. It has an intimal system composed of various intimal structures. Organelles and microfilaments, microtubules, intermediate filaments and other cytoskeles play an important role in the function and structure of cells. Eukaryotic DNA is linearly divided on several chromosomes, usually in the form of chromatin in the nucleus, separated by spindle filaments composed of microtubules in the case of mitosis.
<h1 class="pgc-h-arrow-right" data-track="37" >(1) inner membrane system</h1>
Eukaryotic cells contain a variety of membrane structures, collectively referred to as the intimal system. The membrane can be partially detached, producing a separate chamber called a vesicle or vacuole. Many cells ingest food and other substances through endocytosis, and the outer membrane is first sheathed and then tightened to form independent vesicles that pop out. Most other organelles surrounded by membranes may form from this vacuole.
The nucleus is surrounded by two membranes, often called the nuclear membrane, which contains a nuclear pore that allows matter to move in and out. Tubular and flaky protrusions of the nuclear membrane form an endoplasmic reticulum responsible for protein production and transport. There are ribosomes on the coarse endoplasmic reticulum for protein synthesis, which then enter the intracellular cavity, enter the vesicles, and then eject from the glossy endoplasmic reticulum. For most eukaryotic cells, these protein-carrying vesicles are released into a pile of flat vesicles, the Golgi apparatus, and are further modified.
Small bubbles can be specialized and used for a variety of different purposes. For example, lysosomals are loaded with enzymes that can break down food [22], and peroxidase is used to break down toxic peroxides. Many protists have shrinkable vacuoles that are used to collect and drain excess water, and extrusomes can spew out substances that can be used to deflect predators or catch prey. In higher plants, the central vacuole occupies most of the volume of the cell, and the vacuole also maintains its osmotic pressure.
< h1 class="pgc-h-arrow-right" data-track="41" >(2) inner symbiotic body</h1>
The mitochondrials are organelles common to almost all eukaryotic cells, with two membranes, that is, two layers of phospholipid bilayers, which are folded inward into crests, where aerobic respiration is performed. Mitochondria have their own DNA, which today is generally thought to have developed from prokaryotes of endosynthetic theory, most likely proteophyllum bacteria close to Rickettsia. A small number of protists do not have mitochondria, but have found organelles of mitochondrial origin, such as hydrogenated enzymes and spindlesomes, so it is likely that these protists lost their mitochondria later.
Plants and many algae have organelle pigment bodies. Similarly, these pigment bodies have their own DNA, originating from endosymbiosis, this time the endophytes being cyanobacteria. Pigment bodies, also known as plastoids, are most commonly chloroplasts, and chlorophyll, like cyanobacteria bacteria, synthesize organic matter (such as glucose) through photosynthesis. Other pigments are involved in storing food. Although the pigment body is likely to be of single origin, not all cells containing the pigment body are also of single origin, and in fact some eukaryotic cells obtain the pigment body through multiple endosymbiosis or ingestion.
Endosynthetic origin may be applicable to the nucleus, after flagella has also been thought to have originated in spirochetes. However, due to the lack of evidence of cell biology and the difficulty of reconciling with the cell reproductive process, the theory of spirochetes is not widely accepted.
<h1 class="pgc-h-arrow-right" data-track="45" >(3) cytoskeleton</h1>
Many eukaryotes have elongated, motile protrusions called flagella, and shorter but similar structures— cilia, collectively known as undulipodia, are closely related to eukaryotes' movement, sensation, and feeding, and the main component is tubulin. These organelles are distinctly different from the protist's flagella, with the matrix protruding from a cluster of microtubules that support them. The matrix, also known as the hair matrix or centriole, is the main component of the centridome. The flagella itself can also have fur, as well as scales, connecting membranes and inner rods, which in turn are connected to the cytoplasm.
Microfilament structures consist of actin and actin-binding proteins, such as α-actin, fimbrin, and filamin, which are also present in the submembrane cortex and cortical bundles. Motor proteins in microtubules, such as dyskinesins, or driver proteins and actins, such as dynamin, provide dynamic properties for the system. Even cells without flagella often have centrioles, but flowering plants and coniferous plants do not. Centrioles usually appear in groups called actuators (kinetids) to form a variety of different microtubules that form the main components of the cytoskeleton structure, often synthesized during multiple cell divisions, with one flagella retained from the parent and the other derived from the parent. In addition, centrioles may also be associated with the formation of spindles when nuclei divide.
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The division of the nucleus is usually coordinated with the division of the cell in the form of mitosis, a process that allows each daughter nucleus to receive a copy of the chromosome of the parent. In most eukaryotic cells, there is another sexual reproduction process, meiosis, in which diploid parent cells split twice to become haploid, halving the amount of DNA. However, meiosis itself is also divided into many types.
Eukaryotic cells have a smaller surface area-to-volume ratio than prokaryotic cells, so metabolism rates are slower and cell cycles are longer. In some multicellular organisms, some cells dedicated to metabolism expand the surface area, such as small intestinal villocytes.
Sexual reproduction is widely used in eukaryotes today, and there is evidence that this is a primitive, fundamental property of eukaryotes. Based on phylogenetic analysis, biologists Dax and Roger proposed that eukaryotic cells shared ancestors for random sexual behavior. A core set of genes in meiosis appears in Trichomonas vaginalis vaginalis and intestinal giardia, which were previously considered asexual. On the eukaryotic evolutionary tree, the two species were isolated very early, so it can be inferred that the core gene of meiosis existed in the common ancestor of all eukaryotes, and therefore this ancestor was sexual. Other studies of eukaryotic species have also uncovered evidence of the reproductive cycle. For example, the parasitic protigen protozoa Leishmania has recently been shown to have a reproductive cycle. In addition, there is evidence that amoebas, which were previously thought to be asexual, were also sexual in ancient times, and most asexual organisms today have only recently evolved independently into asexual.
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<h1 class="pgc-h-arrow-right" data-track="53" >(1) plant cells</h1>
Plant cells are also many different from other eukaryotic cells. Its characteristics are:
A large central vacuole surrounded by a monolayer membrane maintains the cellular osmotic pressure, the liquid of which is also the main source of plant sap.
Cell walls of cellulose, hemicellulose and pectin protect protoplasts. This is different from fungal, slime mold and water mold cell walls containing chitin components.
Intercellular filaments pass through the cell wall through the veined pores to connect each plant cell to its neighbor and communicate with each other, which is different from the functionally similar animal intercellular junction system.
Pigment bodies that contain many two-layer membranes (i.e., primitive pigment bodies, plants are also a class of primitive pigment organisms), especially chlorophyll-rich chloroplasts, which photosynthesize and make plants green.
Higher plants, including conifers and flowering plants (angiosperms), lack flagella and centrioles.
< h1 class="pgc-h-arrow-right" data-track="60" >(2) fungal cells</h1>
Fungal cells are most similar to animal cells, but differ from the following:
It has a cell wall composed of chitin.
It is difficult to distinguish between individual cells: the hyphae of higher fungi have a diaphragm, but the diaphragm can allow organelles in the cytoplasm and even the nucleus to pass through; the hyphae of the primitive fungus have little or no septum at all, so each organism is essentially a giant multinucleated superuniform.
Only the most primitive fungal chytrids have flagella.
<h1 class="pgc-h-arrow-right" data-track="65" >(3) animal cells</h1>
Animal cells are the basic units that make up animal tissue. Animal cells are very different from other eukaryotic cells, mainly in the absence of cell walls, large vacuoles, etc., but have small vascularity that can be swallowed and spit. It is precisely because of the lack of cell walls that animal cells can be converted into different forms, and it is precisely because of this that phagocytes can phagocytosis.
Animal cells can differentiate into many types, such as about 210 types of cells in adults.
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