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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Eukaryote | 2/3 | https://en.wikipedia.org/wiki/Eukaryote | reference | science, encyclopedia | 2026-05-05T07:15:19.756158+00:00 | kb-cron |
Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts, in this case cyanobacteria. They usually take the form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose) through photosynthesis. Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from other eukaryotes through secondary endosymbiosis or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty, occurs in many types of modern eukaryotic organisms.
=== Cytoskeletal structures ===
The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable the cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins. These include α-actinin, fimbrin, and filamin in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin, and myosin of actin filaments, make the network dynamic. Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or multiple shorter structures called cilia. These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin, and are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (mastigonemes), as in many stramenopiles. Their interior is continuous with the cell's cytoplasm. Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division.
=== Cell wall ===
The cells of plants, algae, fungi and most chromalveolates, but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major polysaccharides making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.
=== Sexual reproduction ===
Eukaryotes have a life cycle that involves sexual reproduction, alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa, to form a zygote; this may grow into a body, with its cells dividing by mitosis, and at some stage produce haploid gametes through meiosis, a division that reduces the number of chromosomes and creates genetic variability. There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases. Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have a smaller surface area to volume ratio. The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. A core set of genes that function in meiosis is present in both Trichomonas vaginalis and Giardia intestinalis, two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.
== Evolution ==
=== History of classification ===
In antiquity, the two lineages of animals and plants were recognized by Aristotle and Theophrastus. The lineages were given the taxonomic rank of kingdom by Linnaeus in the 18th century. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as ciliates, and this group was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista, in 1866. The eukaryotes thus came to be seen as four kingdoms:
Kingdom Protista Kingdom Plantae Kingdom Fungi Kingdom Animalia The protists were at that time thought to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing, leading to a system of domains rather than kingdoms as top level rank being put forward by Carl Woese, Otto Kandler, and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that "'eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create a "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)".