A pseudopod or pseudopodium is a temporary arm-like projection of a eukaryotic cell membrane. Filled with cytoplasm, pseudopodia consist of actin filaments and may contain microtubules and intermediate filaments. Pseudopods are used for ingestion. Different types of pseudopodia can be classified by their distinct appearances. Lamellipodia are thin. Filopodia are slender, thread-like, are supported by microfilaments. Lobopodia are amoebic. Reticulopodia are complex structures bearing individual pseudopodia. Axopodia are the phagocytosis type with long, thin pseudopods supported by complex microtubule arrays enveloped with cytoplasm; however some pseudopodial cells are able to use multiple types of pseudopodia depending on the situation: Most of them use a combination of lamellipodia and filopodia to migrate. The human foreskin fibroblasts can either use lamellipodia- or lobopodia-based migration in a 3D matrix depending on the matrix elasticity. Several pseudopodia arise from the surface of the body, or, a single pseudopod may form on the surface of the body.
Cells which make pseudopods are referred to as amoeboids. To move towards a target, the cell uses chemotaxis, it senses extracellular signalling molecules, chemoattractants, to extend pseudopodia at the membrane area facing the source of these molecules. The chemoattractants bind to G protein-coupled receptors, which activate GTPases of the Rho family via G-proteins. Rho GTPases are able to activate WASp which in turn activate Arp2/3 complex which serve as nucleation sites for actin polymerization; the actin polymers push the membrane as they grow, forming the pseudopod. The pseudopodium can adhere to a surface via its adhesion proteins, pull the cell's body forward via contraction of an actin-myosin complex in the pseudopod; this type of locomotion is called Amoeboid movement. Rho GTPases can activate phosphatidylinositol 3-kinase which recruit PIP3 to the membrane at the leading edge and detach the PIP3-degrading enzyme PTEN from the same area of the membrane. PIP3 activate GTPases back via GEF stimulation.
This serves as a feedback loop to amplify and maintain the presence of local GTPase at the leading edge. Otherwise, pseudopodia can't grow on other sides of the membrane than the leading edge because myosin filaments prevent them to extend; these myosin filaments are induced by cyclic GMP in D. discoideum or Rho kinase in neutrophils for example. In the case there is no extracellular cue, all moving cells navigate in random directions, but they can keep the same direction for some time before turning; this feature allows cells to explore large areas for colonization or searching for a new extracellular cue. In Dictyostelium cells, a pseudopodium can form either de novo as normal, or from an existing pseudopod, forming a Y-shaped pseudopodium; the Y-shaped pseudopods are used by Dictyostelium to advance straight forward by alternating between retraction of the left or right branch of the pseudopod. The de novo pseudopodia form at different sides than pre-existing ones, they are used by the cells to turn.
Y-shaped pseudopods are more frequent than de novo ones, which explain the preference of the cell to keep moving to the same direction. This persistence is modulated by PLA2 and cGMP signalling pathways; the functions of pseudopodia include locomotion and ingestion: Pseudopodia are critical in sensing targets which can be engulfed. A common example of this type of amoeboid cell is the macrophage, they are essential to amoeboid-like locomotion. Human mesenchymal stem cells are a good example of this function: these migratory cells are responsible for in-utero remodeling. Pseudopods can be classified into several varieties according to the number of projections, according to their appearance: Lamellipodia are broad and flat pseudopodia used in locomotion, they are supported by microfilaments which form at the leading edge, creating a mesh-like internal network. Filopodia are slender and filiform with pointed ends, consisting of ectoplasm; these formations are supported by microfilaments which, unlike the filaments of lamellipodia with their net-like actin, form loose bundles by cross-linking.
This formation is due to bundling proteins such as fimbrins and fascins. Filopodia are observed in some animal cells: in part of Filosa, in "Testaceafilosia", in Vampyrellidae and Pseudosporida and in Nucleariida. Lobopodia are bulbous and blunt in form; these finger-like, tubular pseudopodia contain both endoplasm. They can be found in different kind of cells, notably in Lobosa and other Amoebozoa and in some Heterolobosea. High-pressure lobopodia can be found in human fibroblasts travelling through a complex network of 3D matrix. Contrarily to other pseudopodia using the pressure exerted by actin polymerization on the membrane to extend, fibroblast lobopods use the nuclear piston mechanism consisting in pulling the nucleus via actomyosin contractility to push the cytoplasm that in turn push the membrane, leading to pseudopod formation. To occur, this lobopodia-based fibroblast migration needs nesprin 3, RhoA
A sporangium is an enclosure in which spores are formed. It can be multicellular. All plants and many other lineages form sporangia at some point in their life cycle. Sporangia can produce spores by mitosis, but in nearly all land plants and many fungi, sporangia are the site of meiosis and produce genetically distinct haploid spores. In some phyla of fungi, the sporangium plays a role in asexual reproduction, may play an indirect role in sexual reproduction; the sporangium contains haploid nuclei and cytoplasm. Spores are formed in the sporangiophore by encasing each haploid nucleus and cytoplasm in a tough outer membrane. During asexual reproduction, these spores are dispersed via germinate into haploid hyphae. Although sexual reproduction in fungi varies between phyla, for some fungi the sporangium plays an indirect role in sexual reproduction. For Zygomycota, sexual reproduction occurs when the haploid hyphae from two individuals join to form a zygosporangium in response to unfavorable conditions.
The haploid nuclei within the zygosporangium fuse into diploid nuclei. When conditions improve the zygosporangium germinates, undergoes meiosis and produces a sporangium, which releases spores. In mosses and hornworts, an unbranched sporophyte produces a single sporangium, which may be quite complex morphologically. Most non-vascular plants, as well as many lycophytes and most ferns, are homosporous; some bryophytes, most lycophytes, some ferns are heterosporous. These plants produce microspores and megaspores, which give rise to gametophytes that are functionally male or female, respectively. In some cases, both kinds of spores are produced in the same sporangium, may develop together as part of a spore tetrad. However, in most heterosporous plants there are two kinds of sporangia, termed microsporangia and megasporangia. A few ferns and some lycophytes are heterosporous with two kinds of sporangia, as are all the seed plants. Sporangia can associated with leaves. In ferns, sporangia are found on the abaxial surface of the leaf and are densely aggregated into clusters called sori.
Sori may be covered by a structure called an indusium. Some ferns have their sporangia scattered along reduced leaf segments or along the margin of the leaf. Lycophytes, in contrast, bear their sporangia on the adaxial surface of leaves or laterally on stems. Leaves that bear sporangia are called sporophylls. If the plant is heterosporous, the sporangia-bearing leaves are distinguished as either microsporophylls or megasporophylls. In seed plants, sporangia are located within strobili or flowers. Cycads form their microsporangia on microsporophylls. Megasporangia are formed within ovules, which are borne on megasporophylls, which are aggregated into strobili on separate plants. Conifers bear their microsporangia on microsporophylls aggregated into papery pollen strobili, the ovules, are located on modified stem axes forming compound ovuliferous cone scales. Flowering plants contain microsporangia in the anthers of stamens and megasporangia inside ovules inside ovaries. In all seed plants, spores are produced by meiosis and develop into gametophytes while still inside the sporangium.
The microspores become microgametophytes. The megaspores become megagametophytes. Categorized based on developmental sequence and leptosporangia are differentiated in the vascular plants. In a leptosporangium, found only in ferns, development involves a single initial cell that becomes the stalk and spores within the sporangium. There are around 64 spores in a leptosporangium. In a eusporangium, characteristic of all other vascular plants and some primitive ferns, the initials are in a layer. A eusporangium is larger, its wall is multi-layered. Although the wall may be stretched and damaged, resulting in only one cell-layer remaining. A cluster of sporangia that have become fused in development is called a synangium; this structure is most prominent in Psilotum and Marattiaceae such as Christensenia and Marattia. A columella is a sterile structure that supports the sporangium. In fungi, the columella, which may be branched or unbranched, may be of host origin. Secotium species have a simple, unbranched columella, while in Gymnoglossum species, the columella is branched.
In some Geastrum species, the columella appears as an extension of the stalk into the spore mass. Microsporangium Archegonium Antheridium Spore formation
Orders of magnitude (length)
The following are examples of orders of magnitude for different lengths. To help compare different orders of magnitude, the following list describes various lengths between 1.6 × 10 − 35 metres and 10 10 10 122 metres. To help compare different orders of magnitude, this section lists lengths shorter than 10−23 m. 1.6 × 10−11 yoctometres – the Planck length. 1 ym – 1 yoctometre, the smallest named subdivision of the metre in the SI base unit of length, one septillionth of a metre 1 ym – length of a neutrino. 2 ym – the effective cross-section radius of 1 MeV neutrinos as measured by Clyde Cowan and Frederick Reines To help compare different orders of magnitude, this section lists lengths between 10−23 metres and 10−22 metres. To help compare different orders of magnitude, this section lists lengths between 10−22 m and 10−21 m. 100 ym – length of a top quark, one of the smallest known quarks To help compare different orders of magnitude, this section lists lengths between 10−21 m and 10−20 m. 2 zm – length of a preon, hypothetical particles proposed as subcomponents of quarks and leptons.
2 zm – radius of effective cross section for a 20 GeV neutrino scattering off a nucleon 7 zm – radius of effective cross section for a 250 GeV neutrino scattering off a nucleon To help compare different orders of magnitude, this section lists lengths between 10−20 m and 10−19 m. 15 zm – length of a high energy neutrino 30 zm – length of a bottom quark To help compare different orders of magnitude, this section lists lengths between 10−19 m and 10−18 m. 177 zm – de Broglie wavelength of protons at the Large Hadron Collider To help compare different orders of magnitude, this section lists lengths between 10−18 m and 10−17 m. 1 am – sensitivity of the LIGO detector for gravitational waves 1 am – upper limit for the size of quarks and electrons 1 am – upper bound of the typical size range for "fundamental strings" 1 am – length of an electron 1 am – length of an up quark 1 am – length of a down quark To help compare different orders of magnitude, this section lists lengths between 10−17 m and 10−16 m. 10 am – range of the weak force To help compare different orders of magnitude, this section lists lengths between 10−16 m and 10−15 m. 100 am – all lengths shorter than this distance are not confirmed in terms of size 850 am – approximate proton radius The femtometre is a unit of length in the metric system, equal to 10−15 metres.
In particle physics, this unit is more called a fermi with abbreviation "fm". To help compare different orders of magnitude, this section lists lengths between 10−15 metres and 10−14 metres. 1 fm – length of a neutron 1.5 fm – diameter of the scattering cross section of an 11 MeV proton with a target proton 1.75 fm – the effective charge diameter of a proton 2.81794 fm – classical electron radius 7 fm – the radius of the effective scattering cross section for a gold nucleus scattering a 6 MeV alpha particle over 140 degrees To help compare different orders of magnitude, this section lists lengths between 10−14 m and 10−13 m. 1.75 to 15 fm – Diameter range of the atomic nucleus To help compare different orders of magnitude, this section lists lengths between 10−13 m and 10−12 m. 570 fm – typical distance from the atomic nucleus of the two innermost electrons in the uranium atom, the heaviest naturally-occurring atom To help compare different orders of magnitude this section lists lengths between 10−12 and 10−11 m. 1 pm – distance between atomic nuclei in a white dwarf 2.4 pm – The Compton wavelength of the electron 5 pm – shorter X-ray wavelengths To help compare different orders of magnitude this section lists lengths between 10−11 and 10−10 m. 25 pm – approximate radius of a helium atom, the smallest neutral atom 50 pm – radius of a hydrogen atom 50 pm – bohr radius: approximate radius of a hydrogen atom ~50 pm – best resolution of a high-resolution transmission electron microscope 60 pm – radius of a carbon atom 93 pm – length of a diatomic carbon molecule To help compare different orders of magnitude this section lists lengths between 10−10 and 10−9 m. 100 pm – 1 ångström 100 pm – covalent radius of sulfur atom 120 pm – van der Waals radius of a neutral hydrogen atom 120 pm – radius of a gold atom 126 pm – covalent radius of ruthenium atom 135 pm – covalent radius of technetium atom 150 pm – Length of a typical covalent bond 153 pm – covalent radius of silver atom 155 pm – covalent radius of zirconium atom 175 pm – covalent radius of thulium atom 200 pm – highest resolution of a typical electron microscope 225 pm – covalent radius of caesium atom 280 pm – Average size of the water molecule 298 pm – radius of a caesium atom, calculated to be the largest atomic radius 340 pm – thickness of single layer graphene 356.68 pm – width of diamond unit cell 403 pm – width of lithium fluoride unit cell 500 pm – Width of protein α helix 543 pm – silicon lattice spacing 560 pm – width of sodium chloride unit cell 700 pm – width of glucose molecule 780 pm – mean width of quartz unit cell 820 pm – mean width of ice unit cell 900 pm – mean width of coesite unit cell To help compare different orders
Vahlkampfia is a genus of amoeboids in Heterolobosea. Brown, Susan. "A reevaluation of the amoeba genus Vahlkampfia based on SSUrDNA sequences". European Journal of Protistology 35:49-54.doi:10.1016/S0932-473980021-2 Gonzalez-Robles, Arturo. "Vahlkampfia sp: Structural observations of cultured trophozoites". Experimental Parasitology. 130: 86–90. Doi:10.1016/j.exppara.2011.10.009. PMID 22067209. Maeda, Yasuo. "Folic Acid is a Potent Chemoattractant of Free-living Amoebae in a New and Amazing Species of Protist, Vahlkampfia sp". Zoological Science. 26: 179–186. Doi:10.2108/zsj.26.179. PMID 19341337
Discicristata is a proposed eukaryotic clade. It consists of Percolozoa, it was proposed that Cercozoa yielded Cabozoa. Another proposal is to group Discicristata with Jakobida into Discoba superphylum. Excavate
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. Eukaryotes belong to Eukarya, their name comes from the Greek εὖ and κάρυον. Eukaryotic cells contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells; these act as sex cells. Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis.
The domain Eukaryota appears to be monophyletic, makes up one of the domains of life in the three-domain system. The two other domains and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things. However, due to their much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved 1.6–2.1 billion years ago, during the Proterozoic eon. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton; the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1937 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes; however he mentioned this in only one paragraph, the idea was ignored until Chatton's statement was rediscovered by Stanier and van Niel.
In 1905 and 1910, the Russian biologist Konstantin Mereschkowski argued that plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, itself formed by symbiosis between an amoeba-like host and a bacterium-like cell that formed the nucleus. Plants had thus inherited photosynthesis from cyanobacteria. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, On the origin of mitosing cells. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA; this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles and chloroplasts. In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria. In 1979, G. W. Gould and G. J. Dring suggested that the eukaryotic cell's nucleus came from the ability of Gram-positive bacteria to form endospores. In 1987 and papers, Thomas Cavalier-Smith proposed instead that the membranes of the nucleus and endoplasmic reticulum first formed by infolding a prokaryote's plasma membrane.
In the 1990s, several other biologists proposed endosymbiotic origins for the nucleus reviving Mereschkowski's theory. Eukaryotic cells are much larger than those of prokaryotes having a volume of around 10,000 times greater than the prokaryotic cell, they have a variety of internal membrane-bound structures, called organelles, a cytoskeleton composed of microtubules and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division. Eukaryote cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and pinches off to form a vesicle, it is probable that most other membrane-bound organelles are derived from such vesicles.
Alternatively some products produced by the cell can leave in a vesicle through exocytosis. The nucleus is surrounded with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, involved in protein transport and maturation, it includes the rough endoplasmic reticulum where ribosomes are attached to synthesize proteins, which enter the interior space or lumen. Subsequently, they enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, the Golgi apparatus. Vesicles may be specialized for various purposes. For instance, lysosomes contain digestive enzymes that break down most biomolecules in the cytoplasm. Peroxisomes are used to break down peroxide, otherwise toxic. Many protozoans have contractile vacuoles, which collect and expel excess water, extrusomes, which expel material used to deflect predators or capture prey.
In higher plants, most of a cell's volume is taken up by a central vacuole, whi