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 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
Endoplasm refers to the inner, dense part of a cell's cytoplasm. This is opposed to the ectoplasm, the outer layer of the cytoplasm, watery and adjacent to the plasma membrane; these two terms are used to describe the cytoplasm of the amoeba, a protozoan, eukaryotic cell. The nucleus is separated from the endoplasm by the nuclear envelope; the different makeups/viscosities of the endoplasm and ectoplasm contribute to the amoeba's locomotion through the formation of a pseudopod. However, other types of cells have cytoplasm divided into endo- and ectoplasm; the endoplasm, along with its granules, contains water, nucleic acids amino acids, inorganic ions, lipids and other molecular compounds. It is the site of most cellular processes as it houses the organelles that make up the endomembrane system, as well as those that stand alone; the endoplasm is necessary for most metabolic activities, including cell division. The endoplasm, like the cytoplasm, is far from static, it is in a constant state of flux through intracellular transport, as vesicles are shuttled between organelles and to/from the plasma membrane.
Materials are both degraded and synthesized within the endoplasm based on the needs of the cell and/or organism. Some components of the cytoskeleton run throughout the endoplasm though most are concentrated in the ectoplasm - towards the cells edges, closer to the plasma membrane; the endoplasm’s granules are suspended in cytosol. The term granule refers to a small particle within the endoplasm the secretory vesicles; the granule is the defining characteristic of the endoplasm, as they are not present within the ectoplasm. These offshoots of the endomembrane system are enclosed by a phospholipid bilayer and can fuse with other organelles as well as the plasma membrane, their membrane is only semipermeable and allows them to house substances that could be harmful to the cell if they were allowed to flow within the cytosol. These granules give the cell a large amount of regulation and control over the wide variety of metabolic activities that take place within the endoplasm. There are many different types, characterized by the substance.
These granules/vesicles can contain enzymes, neurotransmitters and waste. The contents are destined for another cell/tissue; these vesicles act as a form of storage and release their contents when needed prompted by a signaling pathway. Once signaled to move, the vesicles can travel along aspects of the cytoskeleton via motor proteins to reach their final destination; the cytosol makes up the semifluid portion of the endoplasm. It is a concentrated aqueous gel with molecules so crowded and packed together within the water base that its behavior is more gel-like than liquid, it contains both small and large molecules, giving it density. It has several functions, including physical support of the cell, preventing collapse, as well as degrading nutrients, transport of small molecules, containing the ribosomes responsible for protein synthesis. Cytosol contains predominantly water, but has a complex mixture of large hydrophilic molecules, smaller molecules and proteins, dissolved ions; the contents of the cytosol change based on the needs of the cell.
Not to be confused with the cytoplasm, the cytosol is only the gel matrix of the cell which does not include many of the macromolecules essential to cellular function. Though amoeba locomotion is assisted by appendages like flagella and cilia, the main source of movement in these cells is pseudopodial locomotion; this process takes advantage of the different consistencies of the endoplasm and ectoplasm to create a pseudopod. Pseudopod, or “false foot” is the term for the extension of a cell’s plasma membrane into what appears to be an appendage that pulls the cell forward; the process behind this involves the gel of the ectoplasm, sol, more fluid, portion of the endoplasm. To create the pseudopod, the gel of the ectoplasm begins to convert to sol which, along with the endoplasm, pushes a portion of the plasma membrane into an appendage. Once the pseudopod is extended, the sol within begins to peripherally convert back to gel, converting back to the ectoplasm as the lagging cell body flows up into the pseudopod moving the cell forward.
Though research has shown aspects of the cytoskeleton assist with pseudopod formation, the exact mechanism is unknown. Research on the shelled amoeba Difflugia demonstrated that microfilaments lie both parallel and perpendicular to the axis of contraction of the plasma membrane to assist with plasma membrane extension into an appendage; the mitochondria are vital to the efficiency of eukaryotes. These organelles breakdown simple sugars like glucose to create a multitude of ATP molecules. ATP provides the energy for protein synthesis, which takes about 75% of the cell’s energy, as well as other cellular processes like signaling pathways. Present in a cell’s endoplasm, the number of mitochondria varies based on the cell’s metabolic needs. Cells that must make a large amount of proteins or breakdown a lot of material require a large amount of mitochondria. Glucose is broken down through three sequential processes: glycolysis, the citric acid cycle, the electron transport chain. Protein synthesis begins at the ribosome, both free ones and those bound to the rough endoplasmic reticulum.
Each ribosome is composed of 2 subunits and is responsible for translating genetic codes from mRNA into proteins by creating strings of amino acids called peptides. Proteins are not ready for their final target after leaving the ribosome. Ribosomes attached to e
Metabolism is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; these enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, respond to their environments.. Metabolic reactions may be categorized as catabolic - the breaking down of compounds. Catabolism releases energy, anabolism consumes energy; the chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts - they allow a reaction to proceed more - and they allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.
The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals; the basal metabolic rate of an organism is the measure of the amount of energy consumed by all of these chemical reactions. A striking feature of metabolism is the similarity of the basic metabolic pathways among vastly different species. For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms like elephants; these similarities in metabolic pathways are due to their early appearance in evolutionary history, their retention because of their efficacy. Most of the structures that make up animals and microbes are made from three basic classes of molecule: amino acids and lipids; as these molecules are vital for life, metabolic reactions either focus on making these molecules during the construction of cells and tissues, or by breaking them down and using them as a source of energy, by their digestion.
These biochemicals can be joined together to make polymers such as DNA and proteins, essential macromolecules of life. Proteins are made of amino acids arranged in a linear chain joined together by peptide bonds. Many proteins are enzymes. Other proteins have structural or mechanical functions, such as those that form the cytoskeleton, a system of scaffolding that maintains the cell shape. Proteins are important in cell signaling, immune responses, cell adhesion, active transport across membranes, the cell cycle. Amino acids contribute to cellular energy metabolism by providing a carbon source for entry into the citric acid cycle when a primary source of energy, such as glucose, is scarce, or when cells undergo metabolic stress. Lipids are the most diverse group of biochemicals, their main structural uses are as part of biological membranes both internal and external, such as the cell membrane, or as a source of energy. Lipids are defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as benzene or chloroform.
The fats are a large group of compounds that contain fatty glycerol. Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, hydrophilic groups such as phosphate as in phospholipids. Steroids such as cholesterol are another major class of lipids. Carbohydrates are aldehydes or ketones, with many hydroxyl groups attached, that can exist as straight chains or rings. Carbohydrates are the most abundant biological molecules, fill numerous roles, such as the storage and transport of energy and structural components; the basic carbohydrate units are called monosaccharides and include galactose and most glucose. Monosaccharides can be linked together to form polysaccharides in limitless ways; the two nucleic acids, DNA and RNA, are polymers of nucleotides. Each nucleotide is composed of a phosphate attached to a ribose or deoxyribose sugar group, attached to a nitrogenous base. Nucleic acids are critical for the storage and use of genetic information, its interpretation through the processes of transcription and protein biosynthesis.
This information is propagated through DNA replication. Many viruses have an RNA genome, such as HIV, which uses reverse transcription to create a DNA template from its viral RNA genome. RNA in ribozymes such as spliceosomes and ribosomes is similar to enzymes as it can catalyze chemical reactions. Individual nucleosides are made
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials, from a hard and brittle "glassy" state into a viscous or rubbery state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass; the reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification. The glass-transition temperature Tg of a material characterizes the range of temperatures over which this glass transition occurs, it is always lower than the melting temperature, Tm, of the crystalline state of the material, if one exists. Hard plastics like polystyrene and poly are used well below their glass transition temperatures, i.e. when they are in their glassy state. Their Tg values are well above room temperature, both at around 100 °C. Rubber elastomers like polyisoprene and polyisobutylene are used above their Tg, that is, in the rubbery state, where they are soft and flexible. Despite the change in the physical properties of a material through its glass transition, the transition is not considered a phase transition.
Such conventions include a constant cooling rate and a viscosity threshold of 1012 Pa·s, among others. Upon cooling or heating through this glass-transition range, the material exhibits a smooth step in the thermal-expansion coefficient and in the specific heat, with the location of these effects again being dependent on the history of the material; the question of whether some phase transition underlies the glass transition is a matter of continuing research. The glass transition of a liquid to a solid-like state may occur with either compression; the transition comprises a smooth increase in the viscosity of a material by as much as 17 orders of magnitude within a temperature range of 500 K without any pronounced change in material structure. The consequence of this dramatic increase is a glass exhibiting solid-like mechanical properties on the timescale of practical observation; this transition is in contrast to the freezing or crystallization transition, a first-order phase transition in the Ehrenfest classification and involves discontinuities in thermodynamic and dynamic properties such as volume and viscosity.
In many materials that undergo a freezing transition, rapid cooling will avoid this phase transition and instead result in a glass transition at some lower temperature. Other materials, such as many polymers, lack a well defined crystalline state and form glasses upon slow cooling or compression; the tendency for a material to form a glass while quenched is called glass forming ability. This ability can be predicted by the rigidity theory. Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used; the expansion coefficient for the glassy state is equivalent to that of the crystalline solid. If slower cooling rates are used, the increased time for structural relaxation to occur may result in a higher density glass product. By annealing the glass structure in time approaches an equilibrium density corresponding to the supercooled liquid at this same temperature. Tg is located at the intersection between the cooling curve for the glassy state and the supercooled liquid.
The configuration of the glass in this temperature range changes with time towards the equilibrium structure. The principle of the minimization of the Gibbs free energy provides the thermodynamic driving force necessary for the eventual change, it should be noted here that at somewhat higher temperatures than Tg, the structure corresponding to equilibrium at any temperature is achieved quite rapidly. In contrast, at lower temperatures, the configuration of the glass remains sensibly stable over extended periods of time. Thus, the liquid-glass transition is not a transition between states of thermodynamic equilibrium, it is believed that the true equilibrium state is always crystalline. Glass is believed to exist in a kinetically locked state, its entropy, so on, depend on the thermal history. Therefore, the glass transition is a dynamic phenomenon. Time and temperature are interchangeable quantities when dealing with glasses, a fact expressed in the time–temperature superposition principle. On cooling a liquid, internal degrees of freedom successively fall out of equilibrium.
However, there is a longstanding debate whether there is an underlying second-order phase transition in the hypothetical limit of infinitely long relaxation times. Refer to the figure on the upper right plotting the heat capacity as a function of temperature. In this context, Tg is the temperature corresponding to point A on the curve; the linear sections below and above Tg are colored green. Tg is the temperature at the intersection of the red regression lines. Different operational definitions of the glass transition temperature Tg are in use, several of them are endorsed as accepted scientific standards. All definitions are arbitrary, all yield different numeric results: at best, values of Tg for a given substance agree within a few kelvins. One definition refers to the viscosity; as evidenced experimentally, this value is close to the annealing point of many glasses. In contrast to viscosity, the thermal expansion, heat capaci
Calcium is a chemical element with symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air, its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium; the most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life. The name derives from Latin calx "lime", obtained from heating limestone; some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via electrolysis of its oxide by Humphry Davy, who named the element. Calcium compounds are used in many industries: in foods and pharmaceuticals for calcium supplementation, in the paper industry as bleaches, as components in cement and electrical insulators, in the manufacture of soaps.
On the other hand, the metal in pure form has few applications due to its high reactivity. Calcium is the fifth-most abundant element in the human body; as electrolytes, calcium ions play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger. Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes as well as proper bone formation. Calcium is a ductile silvery metal whose properties are similar to the heavier elements in its group, strontium and radium. A calcium atom has twenty electrons, arranged in the electron configuration 4s2. Like the other elements placed in group 2 of the periodic table, calcium has two valence electrons in the outermost s-orbital, which are easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a noble gas, in this case argon. Hence, calcium is always divalent in its compounds, which are ionic.
Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to disproportionation to the divalent salts and calcium metal, because the enthalpy of formation of MX2 is much higher than those of the hypothetical MX. This occurs because of the much greater lattice energy afforded by the more charged Ca2+ cation compared to the hypothetical Ca+ cation. Calcium, strontium and radium are always considered to be alkaline earth metals. Beryllium and magnesium are different from the other members of the group in their physical and chemical behaviour: they behave more like aluminium and zinc and have some of the weaker metallic character of the post-transition metals, why the traditional definition of the term "alkaline earth metal" excludes them; this classification is obsolete in English-language sources, but is still used in other countries such as Japan. As a result, comparisons with strontium and barium are more germane to calcium chemistry than comparisons with magnesium.
Calcium metal melts at 842 °C and boils at 1494 °C. It crystallises in the face-centered cubic arrangement like strontium, its density of 1.55 g/cm3 is the lowest in its group. Calcium can be cut with a knife with effort. While calcium is a poorer conductor of electricity than copper or aluminium by volume, it is a better conductor by mass than both due to its low density. While calcium is infeasible as a conductor for most terrestrial applications as it reacts with atmospheric oxygen, its use as such in space has been considered; the chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more than magnesium and less than strontium to produce calcium hydroxide and hydrogen gas, it reacts with the oxygen and nitrogen in the air to form a mixture of calcium oxide and calcium nitride. When finely divided, it spontaneously burns in air to produce the nitride. In bulk, calcium is less reactive: it forms a hydration coating in moist air, but below 30% relative humidity it may be stored indefinitely at room temperature.
Besides the simple oxide CaO, the peroxide CaO2 can be made by direct oxidation of calcium metal under a high pressure of oxygen, there is some evidence for a yellow superoxide Ca2. Calcium hydroxide, Ca2, is a strong base, though it is not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known. Calcium carbonate and calcium sulfate are abundant minerals. Like strontium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, calcium metal dissolves directly in liquid ammonia to give a dark blue solution. Due to the large size of the Ca2+ ion, high coordination numbers are common, up to 24 in some intermetallic compounds such as CaZn13. Calcium is complexed by oxygen chelates such as EDTA and polyphosphates, which are useful in an
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts and the green algae, but excludes the red and brown algae. Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria, their chloroplasts contain b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs; the scientific study of plants is known as a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships; these are not yet settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, they are no longer classified as plants as defined here; the Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common, they undergo closed mitosis without centrioles, have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. Two additional groups, the Rhodophyta and Glaucophyta have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour.
These groups differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event; this is the broadest modern definition of the term'plant'. In contrast, most other algae not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in the broadest modern definition of the plant kingdom, although they were in the past; the green plants or Viridiplantae were traditionally divided into the green algae (including