In cell biology a centriole is a cylindrical organelle composed of a protein called tubulin. Centrioles are found in most eukaryotic cells. A bound pair of centrioles, surrounded by a shapeless mass of dense material, called the pericentriolar material, makes up a structure called a centrosome. Centrioles are present in the cells of most eukaryotes, for example those of animals. However, they are absent from conifers, flowering plants and most fungi, are only present in the male gametes of charophytes, seedless vascular plants and ginkgo. Centrioles are made up of nine sets of short microtubule triplets, arranged in a cylinder. Deviations from this structure include crabs and Drosophila melanogaster embryos, with nine doublets, Caenorhabditis elegans sperm cells and early embryos, with nine singlets; the main function of centrioles is to produce cilia during interphase and the aster and the spindle during cell division. Edouard van Beneden made the first observation of centrioles in 1883. In 1895, Theodor Boveri named the organelle a "centriole".
The pattern of centriole duplication was first worked out independently by Etienne de Harven and Joseph G. Gall c. 1950. Centrioles are involved in the organization of the mitotic spindle and in the completion of cytokinesis. Centrioles were thought to be required for the formation of a mitotic spindle in animal cells. However, more recent experiments have demonstrated that cells whose centrioles have been removed via laser ablation can still progress through the G1 stage of interphase before centrioles can be synthesized in a de novo fashion. Additionally, mutant flies lacking centrioles develop although the adult flies' cells lack flagella and cilia and as a result, they die shortly after birth; the centrioles can self replicate during cell division. Centrioles are a important part of centrosomes, which are involved in organizing microtubules in the cytoplasm; the position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of the cell.
Sperm centrioles are important for 2 functions: to form the sperm flagellum and sperm movement and for the development of the embryo after fertilization. In flagellates and ciliates, the position of the flagellum or cilium is determined by the mother centriole, which becomes the basal body. An inability of cells to use centrioles to make functional flagella and cilia has been linked to a number of genetic and developmental diseases. In particular, the inability of centrioles to properly migrate prior to ciliary assembly has been linked to Meckel-Gruber syndrome. Proper orientation of cilia via centriole positioning toward the posterior of embryonic node cells is critical for establishing left–right asymmetry during mammalian development. Before DNA replication, cells contain two centrioles; the older of the two centrioles is termed the other the daughter. During the cell division cycle, a new centriole grows at the proximal end of both mother and daughter centrioles. After duplication, the two centriole pairs will remain attached to each other orthogonally until mitosis.
At that point the mother and daughter centrioles separate dependently on an enzyme called separase. The two centrioles in the centrosome are tied to one another; the mother centriole has radiating appendages at the distal end of its long axis and is attached to its daughter at the proximal end. Each daughter cell formed. Centrioles start duplicating; the last common ancestor of all eukaryotes was a ciliated cell with centrioles. Some lineages of eukaryotes, such as land plants, do not have centrioles except in their motile male gametes. Centrioles are absent from all cells of conifers and flowering plants, which do not have ciliate or flagellate gametes, it is unclear if the last common ancestor had two cilia. Important genes required for centriole growth, like centrins, are only found in eukaryotes and not in bacteria or archaeans; the word centriole uses combining forms of centri- and -ole, yielding "little central part", which describes a centriole's typical location near the center of the cell.
Typical centrioles are made of 9 triplets of microtubules organized with radial symmetry. Centrioles can vary the number of microtubules and can be made of 9 doublets of microtubules or 9 singlets of microtubules as in C. elegans. Atypical centrioles are centrioles that do not have microtubules, such as the Proximal Centriole-Like found in D. melanogaster sperm, or that have microtubules with no radial symmetry, such as in the distal centriole of human spermatozoon
A microchromosome is a type of small chromosome, a typical component of the karyotype of birds, some reptiles and amphibians. They are less than 20 Mb in size. Microchromosomes are characteristically small and cytogenetically indistinguishable in a karyotype. While thought to be insignificant fragments of chromosomes, in species where they have been studied they have been found to be rich in genes. In chickens, microchromosomes have been estimated to contain between 75 % of all genes; the presence of microchromosomes makes ordering and identifying chromosomes into a coherent karyotype difficult. During metaphase, they appear as 0.5-1.5 μm long specks. Their small size and poor condensation into heterochromatin means they lack the diagnostic banding patterns and distinct centromere locations used for chromosome identification. Birds have karyotypes of 80 chromosomes, with only a few being distinguishable macrochromosomes and an average of 60 being microchromosomes, they are more abundant in birds than any other group of animals.
Chickens are an important model organism for studying microchromosomes. Examination of microchromosomes in birds has led to the hypotheses that they may have originated as conserved fragments of ancestral macrochromosomes, conversely that macrochromosomes could have arisen as aggregates of microchromosomes. Comparative genomic analysis shows that microchromosomes contain genetic information, conserved across multiple classes of chromosomes; this indicates that at least ten chicken microchromosomes arose from fission of larger chromosomes and that the typical bird karyotype arose 100–250 mya. Chickens have a diploid number of 78 chromosomes, as is usual in birds, the majority are microchromosomes. Classification of chicken chromosomes varies by author; some classify them as 6 pairs of macrochromosomes, one pair of sex chromosomes, with the remaining 32 pairs being intermediate or microchromosomes. Other arrangements such as that used by the International Chicken Genome Sequencing Consortium include five pairs of macrochromosomes, five pairs of intermediate chromosomes, twenty-eight pairs of microchromosomes.
Microchromosomes represent one third of the total genome size, have been found to have a much higher gene density than macrochromosomes. Because of this, it is estimated that the majority of genes are located on microchromosomes, though due to the difficulty in physically identifying microchromosomes and the lack of microsatellite markers, it has been difficult to place genes on specific microchromosomes. Replication timing and recombination rates have been found to differ between microchromosomes and macrochromosomes in chickens. Microchromosomes replicate earlier in the S phase of interphase than macrochromosomes. Recombination rates have been found to be higher on microchromosomes. Due to the high recombination rates, chicken chromosome 16 has been found to contain the most genetic diversity of any chromosome in certain chicken breeds; this is due to the presence on this chromosome of the major histocompatibility complex. For the many small linkage groups in the chicken genome which have not been placed on chromosomes, the assumption has been made that they are located on the microchromosomes.
Groups of these correspond exactly with large sections of certain human chromosomes. For example, linkage groups E29C09W09, E21E31C25W12, E48C28W13W27, E41W17, E54 and E49C20W21 correspond with chromosome 7; the turkey has a diploid number of 80 chromosomes. The karyotype contains an additional chromosomal pair relative to the chicken due to the presence of at least two fission/fusion differences. Given these differences involving the macrochromosomes, an additional fission/fusion must exist between the species involving the microchromosomes if the diploid numbers are valid. Other rearrangements have been identified through comparative genetic maps, physical maps and whole genome sequencing. Microchromosomes are absent from the karyotypes of mammals and frogs. In rare cases, microchromosomes have been observed in the karotypes of individual humans. A link has been suggested between microchromosome presence and certain genetic disorders like Down syndrome and fragile X syndrome; the smallest chromosome in humans is chromosome 21, 47 Mb.
The mitochondrion is a double-membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms may, lack them. A number of unicellular organisms, such as microsporidia and diplomonads, have reduced or transformed their mitochondria into other structures. To date, only one eukaryote, Monocercomonoides, is known to have lost its mitochondria; the word mitochondrion comes from the Greek μίτος, mitos, "thread", χονδρίον, chondrion, "granule" or "grain-like". Mitochondria generate most of the cell's supply of adenosine triphosphate, used as a source of chemical energy. A mitochondrion was thus termed the powerhouse of the cell. Mitochondria are between 0.75 and 3 μm in diameter but vary in size and structure. Unless stained, they are not visible. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, cell death, as well as maintaining control of the cell cycle and cell growth. Mitochondrial biogenesis is in turn temporally coordinated with these cellular processes.
Mitochondria have been implicated in several human diseases, including mitochondrial disorders, cardiac dysfunction, heart failure and autism. The number of mitochondria in a cell can vary by organism and cell type. For instance, red blood cells have no mitochondria, whereas liver cells can have more than 2000; the organelle is composed of compartments. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, the cristae and matrix. Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome that shows substantial similarity to bacterial genomes. Mitochondrial proteins vary depending on the species. In humans, 615 distinct types of protein have been identified from cardiac mitochondria, whereas in rats, 940 proteins have been reported; the mitochondrial proteome is thought to be dynamically regulated. The first observations of intracellular structures that represented mitochondria were published in the 1840s.
Richard Altmann, in 1890, established them as cell organelles and called them "bioblasts". The term "mitochondria" was coined by Carl Benda in 1898. Leonor Michaelis discovered that Janus green can be used as a supravital stain for mitochondria in 1900. In 1904, Friedrich Meves, made the first recorded observation of mitochondria in plants in cells of the white waterlily, Nymphaea alba and in 1908, along with Claudius Regaud, suggested that they contain proteins and lipids. Benjamin F. Kingsbury, in 1912, first related them with cell respiration, but exclusively based on morphological observations. In 1913, particles from extracts of guinea-pig liver were linked to respiration by Otto Heinrich Warburg, which he called "grana". Warburg and Heinrich Otto Wieland, who had postulated a similar particle mechanism, disagreed on the chemical nature of the respiration, it was not until 1925, when David Keilin discovered cytochromes, that the respiratory chain was described. In 1939, experiments using minced muscle cells demonstrated that cellular respiration using one oxygen atom can form two adenosine triphosphate molecules, and, in 1941, the concept of the phosphate bonds of ATP being a form of energy in cellular metabolism was developed by Fritz Albert Lipmann.
In the following years, the mechanism behind cellular respiration was further elaborated, although its link to the mitochondria was not known. The introduction of tissue fractionation by Albert Claude allowed mitochondria to be isolated from other cell fractions and biochemical analysis to be conducted on them alone. In 1946, he concluded that cytochrome oxidase and other enzymes responsible for the respiratory chain were isolated to the mitochondria. Eugene Kennedy and Albert Lehninger discovered in 1948 that mitochondria are the site of oxidative phosphorylation in eukaryotes. Over time, the fractionation method was further developed, improving the quality of the mitochondria isolated, other elements of cell respiration were determined to occur in the mitochondria; the first high-resolution electron micrographs appeared in 1952, replacing the Janus Green stains as the preferred way of visualising the mitochondria. This led to a more detailed analysis of the structure of the mitochondria, including confirmation that they were surrounded by a membrane.
It showed a second membrane inside the mitochondria that folded up in ridges dividing up the inner chamber and that the size and shape of the mitochondria varied from cell to cell. The popular term "powerhouse of the cell" was coined by Philip Siekevitz in 1957. In 1967, it was discovered. In 1968, methods were developed for mapping the mitochondrial genes, with the genetic and physical map of yeast mitochondrial DNA being completed in 1976. There are two hypotheses about the origin of mitochondria: autogenous; the endosymbiotic hypothesis suggests that mitochondria were prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells. In the autogenous hypothesis, mitochondria were born by splitting off a portion of DNA from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes. Since mitochondria have many features in common with bacteria, the endosymbiotic hypothesis is more accepted. A mitochondrion contains DNA, which i
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
A basal body is a protein structure found at the base of a eukaryotic undulipodium. It is formed from a centriole and several additional protein structures, is a modified centriole; the basal body serves as a nucleation site for the growth of the axoneme microtubules. Centrioles, from which basal bodies are derived, act as anchoring sites for proteins that in turn anchor microtubules, are known as the microtubule organizing center; these microtubules provide structure and facilitate movement of vesicles and organelles within many eukaryotic cells. Cilia and basal bodies form during the G1 phase of the cell cycle. Before the cell enters G1 phase, i.e. before the formation of the cilium, the mother centriole serves as a component of the centrosome. In cells that are destined to have only one primary cilium, the mother centriole differentiates into the basal body upon entry into G1 or quiescence. Thus, the basal body in such a cell is derived from the centriole; the basal body differs from the mother centriole in at least 2 aspects.
First, basal bodies have basal feet, which are anchored to cytoplasmic microtubules and are necessary for polarized alignment of the cilium. Second, basal bodies have pinwheel-shaped transition fibers that originate from the appendages of mother centriole. In multiciliated cells, however, in many cases basal bodies are not made from centrioles but are generated de novo from a special protein structure called the deuterosome. During cell cycle quiescence, basal bodies organize primary cilia and reside at the cell cortex in proximity to plasma membrane. On cell cycle entry, cilia resorb and the basal body migrates to the nucleus where it functions to organize centrosomes. Centrioles, basal bodies, cilia are important for mitosis, cell division, protein trafficking, signaling and sensation. Mutations in proteins that localize to basal bodies are associated with several human ciliary diseases, including Bardet–Biedl syndrome, orofaciodigital syndrome, Joubert syndrome, cone-rod dystrophy, Meckel syndrome, nephronophthisis.
Regulation of basal body production and spatial orientation is a function of the nucleotide-binding domain of γ-tubulin. Plants lack centrioles and only lower plants with motile sperm have basal bodies. Histology image: 21804loa – Histology Learning System at Boston University - "Ultrastructure of the Cell: ciliated epithelium and basal bodies"
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
The plastid is a membrane-bound organelle found in the cells of plants and some other eukaryotic organisms. Plastids were discovered and named by Ernst Haeckel, but A. F. W. Schimper was the first to provide a clear definition. Plastids are the site of manufacture and storage of important chemical compounds used by the cells of autotrophic eukaryotes, they contain pigments used in photosynthesis, the types of pigments in a plastid determine the cell's color. They have a common evolutionary origin and possess a double-stranded DNA molecule, circular, like that of prokaryotic cells. Plastids that contain chlorophyll are called chloroplasts. Plastids can store products like starch and can synthesize fatty acids and terpenes, which can be used for producing energy and as raw material for the synthesis of other molecules. For example, the components of the plant cuticle and its epicuticular wax are synthesized by the epidermal cells from palmitic acid, synthesized in the chloroplasts of the mesophyll tissue.
All plastids are derived from proplastids, which are present in the meristematic regions of the plant. Proplastids and young chloroplasts divide by binary fission, but more mature chloroplasts have this capacity. In plants, plastids may differentiate into several forms, depending upon which function they play in the cell. Undifferentiated plastids may develop into any of the following variants: Chloroplasts: green plastids for photosynthesis; each plastid creates multiple copies of a circular 75–250 kilobase plastome. The number of genome copies per plastid is variable, ranging from more than 1000 in dividing cells, which, in general, contain few plastids, to 100 or fewer in mature cells, where plastid divisions have given rise to a large number of plastids; the plastome contains about 100 genes encoding ribosomal and transfer ribonucleic acids as well as proteins involved in photosynthesis and plastid gene transcription and translation. However, these proteins only represent a small fraction of the total protein set-up necessary to build and maintain the structure and function of a particular type of plastid.
Plant nuclear genes encode the vast majority of plastid proteins, the expression of plastid genes and nuclear genes is co-regulated to coordinate proper development of plastids in relation to cell differentiation. Plastid DNA exists as large protein-DNA complexes associated with the inner envelope membrane and called'plastid nucleoids'; each nucleoid particle may contain more than 10 copies of the plastid DNA. The proplastid contains a single nucleoid located in the centre of the plastid; the developing plastid has many nucleoids, localized at the periphery of the plastid, bound to the inner envelope membrane. During the development of proplastids to chloroplasts, when plastids convert from one type to another, nucleoids change in morphology and location within the organelle; the remodelling of nucleoids is believed to occur by modifications to the composition and abundance of nucleoid proteins. Many plastids those responsible for photosynthesis, possess numerous internal membrane layers. In plant cells, long thin protuberances called stromules sometimes form and extend from the main plastid body into the cytosol and interconnect several plastids.
Proteins, smaller molecules, can move within stromules. Most cultured cells that are large compared to other plant cells have long and abundant stromules that extend to the cell periphery. In 2014, evidence of possible plastid genome loss was found in Rafflesia lagascae, a non-photosynthetic parasitic flowering plant, in Polytomella, a genus of non-photosynthetic green algae. Extensive searches for plastid genes in both Rafflesia and Polytomella yielded no results, however the conclusion that their plastomes are missing is still controversial; some scientists argue that plastid genome loss is unlikely since non-photosynthetic plastids contain genes necessary to complete various biosynthetic pathways, such as heme biosynthesis. In algae, the term leucoplast is used for all unpigmented plastids, their function differs from the leucoplasts of plants. Etioplasts and chromoplasts are plant-specific and do not occur in algae. Plastids in algae and hornworts may differ from plant plastids in that they contain pyrenoids.
Glaucophyte algae contain muroplasts, which are similar to chloroplasts except that they have a peptidoglycan cell wall, similar to that of prokaryotes. Red algae contain rhodoplasts, which are red chloroplasts that allow them to photosynthesise to a depth of up to 268 m; the chloroplasts of plants differ from the rhodoplasts of red algae in their ability to synthesize starch, stored in the form of granules within the plastids. In red algae, floridean starch is stored outside the plastids in the cytosol. Most plants inherit the plastids from only one parent. In general, angiosperms inherit plastids from the female gamete, whereas