A vacuole is a membrane-bound organelle, present in all plant and fungal cells and some protist and bacterial cells. Vacuoles are enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are just larger forms of these; the organelle has size. The function and significance of vacuoles varies according to the type of cell in which they are present, having much greater prominence in the cells of plants and certain protists than those of animals and bacteria. In general, the functions of the vacuole include: Isolating materials that might be harmful or a threat to the cell Containing waste products Containing water in plant cells Maintaining internal hydrostatic pressure or turgor within the cell Maintaining an acidic internal pH Containing small molecules Exporting unwanted substances from the cell Allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole By increasing in size, allows the germinating plant or its organs to grow quickly and using up just water.
In seeds, stored proteins needed for germination are kept in'protein bodies', which are modified vacuoles. Vacuoles play a major role in autophagy, maintaining a balance between biogenesis and degradation, of many substances and cell structures in certain organisms, they aid in the lysis and recycling of misfolded proteins that have begun to build up within the cell. Thomas Boller and others proposed that the vacuole participates in the destruction of invading bacteria and Robert B. Mellor proposed. In protists, vacuoles have the additional function of storing food, absorbed by the organism and assisting in the digestive and waste management process for the cell; the vacuole evolved several times independently within the Viridiplantae. Contractile vacuoles were first observed by Spallanzani in protozoa, although mistaken for respiratory organs. Dujardin named these "stars" as vacuoles. In 1842, Schleiden applied the term for plant cells, to distinguish the structure with cell sap from the rest of the protoplasm.
In 1885, de Vries named the vacuoule membrane as tonoplast. Large vacuoles are found in three genera of filamentous sulfur bacteria, the Thioploca and Thiomargarita; the cytosol is reduced in these genera and the vacuole can occupy between 40–98% of the cell. The vacuole contains high concentrations of nitrate ions and is therefore thought to be a storage organelle. Gas vesicles known as gas vacuoles, are nanocompartments which are permeable to gas, are present in some species of Cyanobacteria, they allow the bacteria to control their buoyancy. Most mature plant cells have one large vacuole that occupies more than 30% of the cell's volume, that can occupy as much as 80% of the volume for certain cell types and conditions. Strands of cytoplasm run through the vacuole. A vacuole is surrounded by a membrane filled with cell sap. Called the vacuolar membrane, the tonoplast is the cytoplasmic membrane surrounding a vacuole, separating the vacuolar contents from the cell's cytoplasm; as a membrane, it is involved in regulating the movements of ions around the cell, isolating materials that might be harmful or a threat to the cell.
Transport of protons from the cytosol to the vacuole stabilizes cytoplasmic pH, while making the vacuolar interior more acidic creating a proton motive force which the cell can use to transport nutrients into or out of the vacuole. The low pH of the vacuole allows degradative enzymes to act. Although single large vacuoles are most common, the size and number of vacuoles may vary in different tissues and stages of development. For example, developing cells in the meristems contain small provacuoles and cells of the vascular cambium have many small vacuoles in the winter and one large one in the summer. Aside from storage, the main role of the central vacuole is to maintain turgor pressure against the cell wall. Proteins found in the tonoplast control the flow of water into and out of the vacuole through active transport, pumping potassium ions into and out of the vacuolar interior. Due to osmosis, water will diffuse into the vacuole. If water loss leads to a significant decline in turgor pressure, the cell will plasmolyze.
Turgor pressure exerted by vacuoles is required for cellular elongation: as the cell wall is degraded by the action of expansins, the less rigid wall is expanded by the pressure coming from within the vacuole. Turgor pressure exerted by the vacuole is essential in supporting plants in an upright position. Another function of a central vacuole is that it pushes all contents of the cell's cytoplasm against the cellular membrane, thus keeps the chloroplasts closer to light. Most plants store chemicals in the vacuole. If the cell is broken, for example by a herbivore the two chemicals can react forming toxic chemicals. In garlic and the enzyme alliinase are separated but form allicin if the vacuole is broken. A similar reaction is responsible for the production of syn-propanethial-S-oxide when onions are cut. Vacuoles in fungal cells perform similar functions to those in
The Golgi apparatus known as the Golgi complex, Golgi body, or the Golgi, is an organelle found in most eukaryotic cells. It was identified in 1897 by the Italian scientist Camillo Golgi and named after him in 1898. Part of the endomembrane system in the cytoplasm, the Golgi apparatus packages proteins into membrane-bound vesicles inside the cell before the vesicles are sent to their destination; the Golgi apparatus resides at the intersection of the secretory and endocytic pathways. It is of particular importance in processing proteins for secretion, containing a set of glycosylation enzymes that attach various sugar monomers to proteins as the proteins move through the apparatus. Owing to its large size and distinctive structure, the Golgi apparatus was one of the first organelles to be discovered and observed in detail, it was discovered in 1898 by Italian physician Camillo Golgi during an investigation of the nervous system. After first observing it under his microscope, he termed the structure as apparato reticolare interno.
Some doubted the discovery at first, arguing that the appearance of the structure was an optical illusion created by the observation technique used by Golgi. With the development of modern microscopes in the 20th century, the discovery was confirmed. Early references to the Golgi apparatus referred to it by various names including the "Golgi–Holmgren apparatus", "Golgi–Holmgren ducts", "Golgi–Kopsch apparatus"; the term "Golgi apparatus" was used in 1910 and first appeared in the scientific literature in 1913, while "Golgi complex" was introduced in 1956. The subcellular localization of the Golgi apparatus varies among eukaryotes. In mammals, a single Golgi apparatus is located near the cell nucleus, close to the centrosome. Tubular connections are responsible for linking the stacks together. Localization and tubular connections of the Golgi apparatus are dependent on microtubules. In experiments it is seen that as microtubules are depolymerized the Golgi apparatuses lose mutual connections and become individual stacks throughout the cytoplasm.
In yeast, multiple Golgi apparatuses are scattered throughout the cytoplasm. In plants, Golgi stacks are not concentrated at the centrosomal region and do not form Golgi ribbons. Organization of the plant Golgi depends on actin cables and not microtubules; the common feature among Golgi is. In most eukaryotes, the Golgi apparatus is made up of a series of compartments and is a collection of fused, flattened membrane-enclosed disks known as cisternae, originating from vesicular clusters that bud off the endoplasmic reticulum. A mammalian cell contains 40 to 100 stacks of cisternae. Between four and eight cisternae are present in a stack; this collection of cisternae is broken down into cis and trans compartments, making up two main networks: the cis Golgi network and the trans Golgi network. The CGN is the first cisternal structure, the TGN is the final, from which proteins are packaged into vesicles destined to lysosomes, secretory vesicles, or the cell surface; the TGN is positioned adjacent to the stack, but can be separate from it.
The TGN may act as an early endosome in yeast and plants. There are organizational differences in the Golgi apparatus among eukaryotes. In some yeasts, Golgi stacking is not observed. Pichia pastoris does have stacked Golgi. In plants, the individual stacks of the Golgi apparatus seem to operate independently; the Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large amounts of substances. In all eukaryotes, each cisternal stack has a trans exit face; these faces are characterized by unique biochemistry. Within individual stacks are assortments of enzymes responsible for selectively modifying protein cargo; these modifications influence the fate of the protein. The compartmentalization of the Golgi apparatus is advantageous for separating enzymes, thereby maintaining consecutive and selective processing steps: enzymes catalyzing early modifications are gathered in the cis face cisternae, enzymes catalyzing modifications are found in trans face cisternae of the Golgi stacks.
The Golgi apparatus is a major collection and dispatch station of protein products received from the endoplasmic reticulum. Proteins synthesized in the ER are packaged into vesicles, which fuse with the Golgi apparatus; these cargo proteins are destined for secretion via exocytosis or for use in the cell. In this respect, the Golgi can be thought of as similar to a post office: it packages and labels items which it sends to different parts of the cell or to the extracellular space; the Golgi apparatus is involved in lipid transport and lysosome formation. The structure and function of the Golgi apparatus are intimately linked. Individual stacks have different assortments of enzymes, allowing for progressive processing of cargo proteins as they travel from the cisternae to the trans Golgi face. Enzymatic reactions within the Golgi stacks occur near its membrane surfaces, where enzymes are anchored; this feature is in contrast to the ER, which has soluble enzymes in its lumen. Much of the enzymatic processing is post-translational modification of proteins.
For example, phosphorylation of oligosaccharides on lysosomal proteins occurs in the early CGN. Cis cisterna are associ
The endoplasmic reticulum is a type of organelle found in eukaryotic cells that forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known as cisternae. The membranes of the ER are continuous with the outer nuclear membrane; the endoplasmic reticulum occurs in most types of eukaryotic cells, but is absent from red blood cells and spermatozoa. There are two types of ER: smooth endoplasmic reticulum; the outer face of the rough endoplasmic reticulum is studded with ribosomes that are the sites of protein synthesis. The rough endoplasmic reticulum is prominent in cells such as hepatocytes; the smooth endoplasmic reticulum lacks ribosomes and functions in lipid synthesis but not metabolism, the production of steroid hormones, detoxification. The smooth ER is abundant in mammalian liver and gonad cells; the ER was observed with light microscope by Garnier in 1897, who coined the term "ergastoplasm". With electron microscopy, the lacy membranes of the endoplasmic reticulum were first seen in 1945 by Keith R. Porter, Albert Claude, Ernest F. Fullam.
The word "reticulum", which means "network", was applied by Porter in 1953 to describe this fabric of membranes. The general structure of the endoplasmic reticulum is a network of membranes called cisternae; these sac-like structures are held together by the cytoskeleton. The phospholipid membrane encloses the cisternal space, continuous with the perinuclear space but separate from the cytosol; the functions of the endoplasmic reticulum can be summarized as the synthesis and export of proteins and membrane lipids, but varies between ER and cell type and cell function. The quantity of both rough and smooth endoplasmic reticulum in a cell can interchange from one type to the other, depending on the changing metabolic activities of the cell. Transformation can include embedding of new proteins in membrane as well as structural changes. Changes in protein content may occur without noticeable structural changes; the surface of the rough endoplasmic reticulum is studded with protein-manufacturing ribosomes giving it a "rough" appearance.
The binding site of the ribosome on the rough endoplasmic reticulum is the translocon. However, the ribosomes are not a stable part of this organelle's structure as they are being bound and released from the membrane. A ribosome only binds to the RER; this special complex forms when a free ribosome begins translating the mRNA of a protein destined for the secretory pathway. The first 5–30 amino acids polymerized encode a signal peptide, a molecular message, recognized and bound by a signal recognition particle. Translation pauses and the ribosome complex binds to the RER translocon where translation continues with the nascent protein forming into the RER lumen and/or membrane; the protein is processed in the ER lumen by an enzyme. Ribosomes at this point may be released back into the cytosol; the membrane of the rough endoplasmic reticulum forms large double membrane sheets that are located near, continuous with, the outer layer of the nuclear envelope. The double membrane sheets are stacked and connected through several right or left-handed helical ramps, the so-called Terasaki ramps, giving rise to a structure resembling a multi-storey car park.
Although there is no continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membrane-bound transport vesicles shuttle proteins between these two compartments. Vesicles are surrounded by coating proteins called COPI and COPII. COPII targets vesicles to the Golgi apparatus and COPI marks them to be brought back to the rough endoplasmic reticulum; the rough endoplasmic reticulum works in concert with the Golgi complex to target new proteins to their proper destinations. A second method of transport out of the endoplasmic reticulum involves areas called membrane contact sites, where the membranes of the endoplasmic reticulum and other organelles are held together, allowing the transfer of lipids and other small molecules; the rough endoplasmic reticulum is key in multiple functions: Manufacture of lysosomal enzymes with a mannose-6-phosphate marker added in the cis-Golgi network. Manufacture of secreted proteins, either secreted constitutively with no tag or secreted in a regulatory manner involving clathrin and paired basic amino acids in the signal peptide.
Integral membrane proteins that stay embedded in the membrane as vesicles exit and bind to new membranes. Rab proteins are key in targeting the membrane. Initial glycosylation as assembly continues; this is N-linked. N-linked glycosylation: If the protein is properly folded, Oligosaccharyltransferase recognizes the AA sequence NXS or NXT and adds a 14-sugar backbone to the side-chain nitrogen of Asn. In most cells the smooth endoplasmic reticulum is scarce. Instead there are areas where the ER is smooth and rough, this area is called the transitional ER; the transitional ER gets its name. These are areas where the transport vesicles that contain lipids and proteins made in the ER, detach from the ER and start moving to the Golgi apparatus. Specialized cells can have a lot of smooth endoplasmic reticulum and in these cells the smooth ER has many functions
Photosystem I is the second photosystem in the photosynthetic light reactions of algae and some bacteria. Photosystem I is an integral membrane protein complex that uses light energy to produce the high energy carriers ATP and NADPH. PSI comprises more than 110 cofactors more than photosystem II; this photosystem is known as PSI because it was discovered before photosystem II. Aspects of PSI were discovered in the 1950s, but the significances of these discoveries was not yet known. Louis Duysens first proposed the concepts of photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a cohesive theory of serial photosynthetic reactions. Hill and Bendall's hypothesis was justified in experiments conducted in 1961 by Duysens and Witt groups. Two main subunits of PSI, PsaA and PsaB, are related proteins involved in the binding of P700, A0, A1, Fx. PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that seem to contain 11 transmembrane segments.
The Fx 4Fe-4S iron-sulfur center is coordinated by four cysteines. The two cysteines in both proteins are proximal and located in a loop between the ninth and tenth transmembrane segments. A leucine zipper motif seems to be present downstream of the cysteines and could contribute to dimerisation of psaA/psaB; the terminal electron acceptors, FA and FB, are located in a 9 kDa protein called PsaC. Photoexcitation of the pigment molecules in the antenna complex induces electron transfer; the antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins. These pigment molecules transmit the resonance energy from photons. Antenna molecules can absorb all wavelengths of light within the visible spectrum; the number of these pigment molecules varies from organism to organism. For instance, the cyanobacterium Synechococcus elongatus has about 100 chlorophylls and 20 carotenoids, whereas spinach chloroplasts have around 200 chlorophylls and 50 carotenoids. Located within the antenna complex of PSI are molecules of chlorophyll called P700 reaction centers.
The energy passed around by antenna molecules is directed to the reaction center. There may be as many as 120 or as few as 25 chlorophyll molecules per P700; the P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm, with higher wavelengths causing bleaching. P700 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level; these electrons are moved in pairs in an oxidation/reduction process from P700 to electron acceptors. P700 has an electric potential of about −1.2 volts. The reaction center is therefore referred to as a dimer; the dimer is thought to be composed of one chlorophyll a ′ molecule. However, if P700 forms a complex with other antenna molecules, it can no longer be a dimer. Modified chlorophyll a0 is an early electron acceptor in PSI. Chlorophyll a0 accepts electrons from P700 before passing them along to another early electron acceptor. Phylloquinone A1 is the next early electron acceptor in PSI.
Phylloquinone is sometimes called vitamin K1. Phylloquinone A1 oxidizes A0 in order to receive the electron and in turn reduces Fx in order to pass the electron to Fb and Fa. Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled Fx, Fa, Fb, they serve as electron relays. Fa and Fb are bound to protein subunits of the PSI complex and Fx is tied to the PSI complex. Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order. Ferredoxin is a soluble protein that facilitates reduction of NADP+ to NADPH. Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces NADP+. Thylakoid membranes have one binding site for each function of Fd; the main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme ferredoxin–NADP+ reductase. This enzyme transfers the electron from reduced ferredoxin to NADP+ to complete the reduction to NADPH. FNR may accept an electron from NADPH by binding to it.
Plastocyanin is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI. The Ycf4 protein domain is found on the thylakoid membrane and is vital to photosystem I; this thylakoid transmembrane protein helps assemble the components of photosystem I, without it, photosynthesis would be inefficient. Molecular data show that PSI evolved from the photosystems of green sulfur bacteria; the photosystems of green sulfur bacteria and those of cyanobacteria and higher plants are not the same, however there are many analogous functions and similar structures. Three main features are similar between the different photosystems. First, redox potential is negative enough to reduce ferredoxin. Next, the electron-accepting reaction centers include iron–sulfur proteins. Last, redox centres in complexes of both photosystems are constructed upon a protein subunit dimer; the photosystem of green sulfur bacteria contains all of the same cofactors of the electron transport chain in PSI.
The number and degree of similarities between the two photosystems indicates that PSI is derived from the analogous photosystem of green sulfur bacteria. Biohybrid solar cell Photosystem I: Molecule of the Month in the Protein Data Bank Photosystem I in A Companion to Plant Physiology James Barber FRS Photosystems I & II
The cytosol known as intracellular fluid or cytoplasmic matrix, is the liquid found inside cells. It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into many compartments. In the eukaryotic cell, the cytosol is surrounded by the cell membrane and is part of the cytoplasm, which comprises the mitochondria and other organelles; the cytosol is thus a liquid matrix around the organelles. In prokaryotes, most of the chemical reactions of metabolism take place in the cytosol, while a few take place in membranes or in the periplasmic space. In eukaryotes, while many metabolic pathways still occur in the cytosol, others take place within organelles; the cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, its structure and properties within cells is not well understood; the concentrations of ions such as sodium and potassium are different in the cytosol than in the extracellular fluid.
The cytosol contains large amounts of macromolecules, which can alter how molecules behave, through macromolecular crowding. Although it was once thought to be a simple solution of molecules, the cytosol has multiple levels of organization; these include concentration gradients of small molecules such as calcium, large complexes of enzymes that act together and take part in metabolic pathways, protein complexes such as proteasomes and carboxysomes that enclose and separate parts of the cytosol. The term "cytosol" was first introduced in 1965 by H. A. Lardy, referred to the liquid, produced by breaking cells apart and pelleting all the insoluble components by ultracentrifugation; such a soluble cell extract is not identical to the soluble part of the cell cytoplasm and is called a cytoplasmic fraction. The term cytosol is now used to refer to the liquid phase of the cytoplasm in an intact cell; this excludes any part of the cytoplasm, contained within organelles. Due to the possibility of confusion between the use of the word "cytosol" to refer to both extracts of cells and the soluble part of the cytoplasm in intact cells, the phrase "aqueous cytoplasm" has been used to describe the liquid contents of the cytoplasm of living cells.
Prior to this, other terms, including "hyaloplasm", were used for the cell fluid, not always synonymously, as its nature was not clear. The proportion of cell volume, cytosol varies: for example while this compartment forms the bulk of cell structure in bacteria, in plant cells the main compartment is the large central vacuole; the cytosol consists of water, dissolved ions, small molecules, large water-soluble molecules. The majority of these non-protein molecules have a molecular mass of less than 300 Da; this mixture of small molecules is extraordinarily complex, as the variety of molecules that are involved in metabolism is immense. For example, up to 200,000 different small molecules might be made in plants, although not all these will be present in the same species, or in a single cell. Estimates of the number of metabolites in single cells such as E. coli and baker's yeast predict that under 1,000 are made. Most of the cytosol is water; the pH of the intracellular fluid is 7.4. While human cytosolic pH ranges between 7.0 - 7.4, is higher if a cell is growing.
The viscosity of cytoplasm is the same as pure water, although diffusion of small molecules through this liquid is about fourfold slower than in pure water, due to collisions with the large numbers of macromolecules in the cytosol. Studies in the brine shrimp have examined. Although water is vital for life, the structure of this water in the cytosol is not well understood because methods such as nuclear magnetic resonance spectroscopy only give information on the average structure of water, cannot measure local variations at the microscopic scale; the structure of pure water is poorly understood, due to the ability of water to form structures such as water clusters through hydrogen bonds. The classic view of water in cells is that about 5% of this water is bound in by solutes or macromolecules as water of solvation, while the majority has the same structure as pure water; this water of solvation is not active in osmosis and may have different solvent properties, so that some dissolved molecules are excluded, while others become concentrated.
However, others argue that the effects of the high concentrations of macromolecules in cells extend throughout the cytosol and that water in cells behaves differently from the water in dilute solutions. These ideas include the proposal that cells contain zones of low and high-density water, which could have widespread effects on the structures and functions of the other parts of the cell. However, the use of advanced nuclear magnetic resonance methods to directly measure the mobility of water in living cells contradicts this idea, as it suggests that 85% of cell water acts like that pure water, while the remainder is less mobile and bound to macromolecules; the concentrations of the other ions in cytosol are quite different from those in extracellular flui
An endosymbiont or endobiont is any organism that lives within the body or cells of another organism in a mutualistic relationship with the host body or cell but not always to mutual benefit. The term endosymbiosis is from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living"). Examples are nitrogen-fixing bacteria, which live in root nodules on legume roots, single-cell algae inside reef-building corals, bacterial endosymbionts that provide essential nutrients to about 10–15% of insects. Many instances of endosymbiosis are obligate; the most common examples of obligate endosymbioses are chloroplasts. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp, they can both be eliminated from said hosts by treatments. However, not all endosymbioses are obligate and some endosymbioses can be harmful to either of the organisms involved. Two major types of organelle in eukaryotic cells and plastids such as chloroplasts, originated by symbiogenesis as bacterial endosymbionts.
In marine environments, bacterial endosymbionts have more been discovered. These endosymbiotic relationships are prevalent in oligotrophic or nutrient-poor regions of the ocean like that of the North Atlantic. In these oligotrophic waters, cell growth of larger phytoplankton like that of diatoms is limited by low nitrate concentrations. Endosymbiotic bacteria fix nitrogen for their diatom hosts and in turn receive organic carbon from photosynthesis; these symbioses play an important role in global carbon cycling in oligotrophic regions. One known symbiosis between the diatom Hemialus spp. and the cyanobacterium Richelia intracellularis has been found in the North Atlantic and Pacific Ocean. The Richelia endosymbiont is found within the diatom frustule of Hemiaulus spp. and has a reduced genome losing genes related to pathways the host now provides. Research by Foster et al. measured nitrogen fixation by the cyanobacterial host Richelia intracellularis well above intracellular requirements, found the cyanobacterium was fixing excess nitrogen for Hemiaulus host cells.
Additionally, both host and symbiont cell growth were much greater than free-living Richelia intracellularis or symbiont-free Hemiaulus spp. The Hemaiulus-Richelia symbiosis is not obligatory in areas with excess nitrogen. Richelia intracellularis is found in Rhizosolenia spp. A diatom found in oligotrophic oceans. Compared to the Hemaiulus host, the endosymbiosis with Rhizosolenia is much more consistent, Richelia intracellularis is found in Rhizosolenia. There are some asymbiotic Rhizosolenia, however there appears to be mechanisms limiting growth of these organisms in low nutrient conditions. Cell division for both the diatom host and cyanobacterial symbiont can be uncoupled and mechanisms for passing bacterial symbionts to daughter cells during cell division are still unknown. Other endosymbiosis with nitrogen fixers in open oceans include Calothrix in Chaetocerous spp. and UNCY-A in prymnesiophyte microalga. The Chaetocerous-Calothrix endosymbiosis is hypothesized to be more recent, as the Calothrix genome is intact.
While other species like that of the UNCY-A symbiont and Richelia have reduced genomes. This reduction in genome size occurs within nitrogen metabolism pathways indicating endosymbiont species are generating nitrogen for their hosts and losing the ability to use this nitrogen independently; this endosymbiont reduction in genome size, might be a step that occurred in the evolution of organelles. Extracellular endosymbionts are represented in all four extant classes of Echinodermata. Little is known of the nature of the association but phylogenetic analysis indicates that these symbionts belong to the alpha group of the class Proteobacteria, relating them to Rhizobium and Thiobacillus. Other studies indicate that these subcuticular bacteria may be both abundant within their hosts and distributed among the Echinoderms in general; some marine oligochaeta have obligate extracellular endosymbionts that fill the entire body of their host. These marine worms are nutritionally dependent on their symbiotic chemoautotrophic bacteria lacking any digestive or excretory system.
Mixotricha paradoxa is a protozoan. However, spherical bacteria serve the function of the mitochondria. Mixotricha has three other species of symbionts that live on the surface of the cell. Paramecium bursaria, a species of ciliate, has a mutualistic symbiotic relationship with green alga called Zoochlorella; the algae live in the cytoplasm. Paulinella chromatophora is a freshwater amoeboid which has taken on a cyanobacterium as an endosymbiont. Scientists classify insect endosymbionts in two broad categories,'Primary' and'Secondary'. Primary endosymbionts have been associated with their insect hosts for many millions of years, they form obligate associations, display cospeciation with their insect hosts. Secondary endosymbionts exhibit a more developed association, are sometimes hor
A lysosome is a membrane-bound organelle found in many animal cells and most plant cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane proteins, its lumenal proteins; the lumen's pH is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in various cell processes, including secretion, plasma membrane repair, cell signaling, energy metabolism; the lysosomes act as the waste disposal system of the cell by digesting unwanted materials in the cytoplasm, both from outside the cell and obsolete components inside the cell. Material from outside the cell is taken-up through endocytosis, while material from the inside of the cell is digested through autophagy, their sizes can be different—the largest ones can be more than 10 times the size of the smallest ones. They were discovered and named by Belgian biologist Christian de Duve, who received the Nobel Prize in Physiology or Medicine in 1974.
Lysosomes are known to contain more than 60 different enzymes, have more than 50 membrane proteins. Enzymes of the lysosomes are synthesised in the rough endoplasmic reticulum; the enzymes are imported from the Golgi apparatus in small vesicles, which fuse with larger acidic vesicles. Enzymes destined for a lysosome are tagged with the molecule mannose 6-phosphate, so that they are properly sorted into acidified vesicles. Synthesis of lysosomal enzymes is controlled by nuclear genes. Mutations in the genes for these enzymes are responsible for more than 30 different human genetic disorders, which are collectively known as lysosomal storage diseases; these diseases result from an accumulation of specific substrates, due to the inability to break them down. These genetic defects are related to several neurodegenerative disorders, cardiovascular diseases, ageing-related diseases. Lysosomes should not be confused with micelles. Christian de Duve, the chairman of the Laboratory of Physiological Chemistry at the Catholic University of Louvain in Belgium, had been studying the mechanism of action of a pancreatic hormone insulin in liver cells.
By 1949, he and his team had focused on the enzyme called glucose 6-phosphatase, the first crucial enzyme in sugar metabolism and the target of insulin. They suspected that this enzyme played a key role in regulating blood sugar levels; however after a series of experiments, they failed to purify and isolate the enzyme from the cellular extracts. Therefore, they tried a more arduous procedure of cell fractionation, by which cellular components are separated based on their sizes using centrifugation, they succeeded in detecting the enzyme activity from the microsomal fraction. This was the crucial step in the serendipitous discovery of lysosomes. To estimate this enzyme activity, they used that of standardised enzyme acid phosphatase, found that the activity was only 10% of the expected value. One day, the enzyme activity of purified cell fractions, refrigerated for five days was measured; the enzyme activity was increased to normal of that of the fresh sample. The result was the same no matter how many times they repeated the estimation, led to the conclusion that a membrane-like barrier limited the accessibility of the enzyme to its substrate, that the enzymes were able to diffuse after a few days.
They described this membrane-like barrier as a "saclike structure surrounded by a membrane and containing acid phosphatase."It became clear that this enzyme from the cell fraction came from membranous fractions, which were cell organelles, in 1955 De Duve named them "lysosomes" to reflect their digestive properties. The same year, Alex B. Novikoff from the University of Vermont visited de Duve's laboratory, obtained the first electron micrographs of the new organelle. Using a staining method for acid phosphatase, de Duve and Novikoff confirmed the location of the hydrolytic enzymes of lysosomes using light and electron microscopic studies. De Duve won the Nobel Prize in Medicine in 1974 for this discovery. De Duve had termed the organelles the "suicide bags" or "suicide sacs" of the cells, for their hypothesized role in apoptosis. However, it has since been concluded. Lysosomes contain a variety of enzymes, enabling the cell to break down various biomolecules it engulfs, including peptides, nucleic acids and lipids.
The enzymes responsible for this hydrolysis require an acidic environment for optimal activity. In addition to being able to break down polymers, lysosomes are capable of fusing with other organelles & digesting large structures or cellular debris, they are able to break-down virus particles or bacteria in phagocytosis of macrophages. The size of lysosomes varies from 0.1 μm to 1.2 μm. With a pH ranging from ~4.5–5.0, the interior of the lysosomes is acidic compared to the basic cytosol. The lysosomal membrane protects the cytosol, therefore the rest of the cell, from the degradative enzymes within the lysosome; the cell is additionally protected from any lysosomal acid hydrolases that drain into the cytosol, as these enzymes are pH-sensitive and do not function well or at all in the alkaline environment of the cytosol. This ensures that cytosolic molecules and organelles are not destroyed in case there is leakage of the hydrolytic enzymes from the lysosome; the lysosome maintains its pH differen