Magnesium in biology
Magnesium is an essential element in biological systems. Magnesium occurs as the Mg2+ ion, it is present in every cell type in every organism. For example, ATP, the main source of energy in cells, must bind to a magnesium ion in order to be biologically active. What is called ATP is actually Mg-ATP; as such, magnesium plays a role in the stability of all polyphosphate compounds in the cells, including those associated with the synthesis of DNA and RNA. Over 300 enzymes require the presence of magnesium ions for their catalytic action, including all enzymes utilizing or synthesizing ATP, or those that use other nucleotides to synthesize DNA and RNA. In plants, magnesium is necessary for synthesis of photosynthesis. A balance of magnesium is vital to the well-being of all organisms. Magnesium is a abundant ion in Earth's crust and mantle and is bioavailable in the hydrosphere; this availability, in combination with a useful and unusual chemistry, may have led to its utilization in evolution as an ion for signaling, enzyme activation, catalysis.
However, the unusual nature of ionic magnesium has led to a major challenge in the use of the ion in biological systems. Biological membranes are impermeable to magnesium, so transport proteins must facilitate the flow of magnesium, both into and out of cells and intracellular compartments. Chlorophyll in plants converts water to oxygen as O2. Hemoglobin in vertebrate animals transports oxygen as O2 in the blood. Chlorophyll is similar to hemoglobin, except magnesium is at the center of the chlorophyll molecule and iron is at the center of the hemoglobin molecule, with other variations; this process keeps living cells on earth alive and maintains baseline levels of CO2 and O2 in the atmosphere. Inadequate magnesium intake causes muscle spasms, has been associated with cardiovascular disease, high blood pressure, anxiety disorders, migraines and cerebral infarction. Acute deficiency is rare, is more common as a drug side-effect than from low food intake per se, but it can occur in people fed intravenously for extended periods of time.
The most common symptom of excess oral magnesium intake is diarrhea. Supplements based on amino acid chelates are much better-tolerated by the digestive system and do not have the side-effects of the older compounds used, while sustained-release dietary supplements prevent the occurrence of diarrhea. Since the kidneys of adult humans excrete excess magnesium efficiently, oral magnesium poisoning in adults with normal renal function is rare. Infants, which have less ability to excrete excess magnesium when healthy, should not be given magnesium supplements, except under a physician's care. Pharmaceutical preparations with magnesium are used to treat conditions including magnesium deficiency and hypomagnesemia, as well as eclampsia; such preparations are in the form of magnesium sulfate or chloride when given parenterally. Magnesium is absorbed with reasonable efficiency by the body from any soluble magnesium salt, such as the chloride or citrate. Magnesium is absorbed from Epsom salts, although the sulfate in these salts adds to their laxative effect at higher doses.
Magnesium absorption from the insoluble oxide and hydroxide salts is erratic and of poorer efficiency, since it depends on the neutralization and solution of the salt by the acid of the stomach, which may not be complete. Magnesium orotate may be used as adjuvant therapy in patients on optimal treatment for severe congestive heart failure, increasing survival rate and improving clinical symptoms and patient's quality of life. Magnesium can affect muscle relaxation through direct action on cell membranes. Mg2+ ions close certain types of calcium channels, which conduct positively charged calcium ions into neurons. With an excess of magnesium, more channels will be blocked and nerve cells activity will decrease. Intravenous magnesium sulphate is used in treating pre-eclampsia. For other than pregnancy-related hypertension, a meta-analysis of 22 clinical trials with dose ranges of 120 to 973 mg/day and a mean dose of 410 mg, concluded that magnesium supplementation had a small but statistically significant effect, lowering systolic blood pressure by 3–4 mm Hg and diastolic blood pressure by 2–3 mm Hg.
The effect was larger. Higher dietary intakes of magnesium correspond to lower diabetes incidence. For people with diabetes or at high risk of diabetes, magnesium supplementation lowers fasting glucose; the U. S. Institute of Medicine updated Estimated Average Requirements and Recommended Dietary Allowances for magnesium in 1997. If there is not sufficient information to establish EARs and RDAs, an estimate designated Adequate Intake is used instead; the current EARs for magnesium for women and men ages 31 and up are 265 mg/day and 350 mg/day, respectively. The RDAs are 420 mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy is 350 to 400 mg/day depending on age of the woman. RDA for lactation ranges 310 to 360 mg/day for same reason. For children ages 1–13 years the RDA increases with age from 65 to 200 mg/day; as for safety, the IOM sets Tolerable upper intake levels for vitamins and minerals when evidence is sufficient.
In the case of magnesium the UL is set at 350 mg/day. The UL is specific to magnesium consumed as a dietary supplement, the reason being that too much magnesium consumed at one time can caus
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
Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue and can serve as a fuel source; as a fuel, proteins provide as much energy density as carbohydrates: 4 kcal per gram. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. Proteins are polymer chains made of amino acids linked together by peptide bonds. During human digestion, proteins are broken down in the stomach to smaller polypeptide chains via hydrochloric acid and protease actions; this is crucial for the absorption of the essential amino acids that cannot be biosynthesized by the body. There are nine essential amino acids which humans must obtain from their diet in order to prevent protein-energy malnutrition and resulting death, they are phenylalanine, threonine, methionine, isoleucine and histidine. There has been debate as to whether there are 9 essential amino acids; the consensus seems to lean towards 9. There are five amino acids.
These five are alanine, aspartic acid, glutamic acid and serine. There are six conditionally essential amino acids whose synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress; these six are arginine, glycine, glutamine and tyrosine. Dietary sources of protein include both animals and plants: meats, dairy products and eggs, as well as grains and nuts. Vegans can get enough essential amino acids by eating plant proteins. Protein is a nutrient needed by the human body for maintenance. Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body muscle; this includes body organs and skin. Proteins are used in membranes, such as glycoproteins; when broken down into amino acids, they are used as precursors to nucleic acid, co-enzymes, immune response, cellular repair, other molecules essential for life.
Additionally, protein is needed to form blood cells. Protein can be found in a wide range of food; the best combination of protein sources depends on the region of the world, cost, amino acid types and nutrition balance, as well as acquired tastes. Some foods are high in certain amino acids, but their digestibility and the anti-nutritional factors present in these foods make them of limited value in human nutrition. Therefore, one must consider digestibility and secondary nutrition profile such as calories, cholesterol and essential mineral density of the protein source. On a worldwide basis, plant protein foods contribute over 60 percent of the per capita supply of protein, on average. In North America, animal-derived foods contribute about 70 percent of protein sources. Meat, products from milk, eggs and fish are sources of complete protein. Whole grains and cereals are another source of proteins. However, these tend to be limiting in the amino acid lysine or threonine, which are available in other vegetarian sources and meats.
Examples of food staples and cereal sources of protein, each with a concentration greater than 7.0%, are buckwheat, rye, maize, wheat, sorghum and quinoa. Vegetarian sources of proteins include legumes, nuts and fruits. Legumes, some of which are called pulses in certain parts of the world, have higher concentrations of amino acids and are more complete sources of protein than whole grains and cereals. Examples of vegetarian foods with protein concentrations greater than 7 percent include soybeans, kidney beans, white beans, mung beans, cowpeas, lima beans, pigeon peas, wing beans, Brazil nuts, pecans, cotton seeds, pumpkin seeds, hemp seeds, sesame seeds, sunflower seeds. Food staples that are poor sources of protein include roots and tubers such as yams and sweet potato. Plantains, another major staple, are a poor source of essential amino acids. Fruits, while rich in other essential nutrients, are another poor source of amino acids; the protein content in roots and fruits is between 0 and 2 percent.
Food staples with low protein content must be complemented with foods with complete, quality protein content for a healthy life in children for proper development. A good source of protein is a combination of various foods, because different foods are rich in different amino acids. A good source of dietary protein meets two requirements: The requirement for the nutritionally indispensable amino acids under all conditions and for conditionally indispensable amino acids under specific physiological and pathological conditions The requirement for nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids and other physiologically important nitrogen-containing compounds such as nucleic acids and porphyrins. Healthy people eating a balanced diet need protein supplements; the table below presents the most important food groups as protein sources, from a worldwide perspective. It lists their respective performance as source of the limiting amino acids, in milligrams of limiting amino acid per gram of total protein in the food source.
The table reiterates the need for a balanced mix of
The rosids are members of a large clade of flowering plants, containing about 70,000 species, more than a quarter of all angiosperms. The clade is divided into 16 to 20 orders, depending upon circumscription and classification; these orders, in turn, together comprise about 140 families. Fossil rosids are known from the Cretaceous period. Molecular clock estimates indicate that the rosids originated in the Aptian or Albian stages of the Cretaceous, between 125 and 99.6 million years ago. The name is based upon the name "Rosidae", understood to be a subclass. In 1967, Armen Takhtajan showed that the correct basis for the name "Rosidae" is a description of a group of plants published in 1830 by Friedrich Gottlieb Bartling; the clade was renamed "Rosidae" and has been variously delimited by different authors. The name "rosids" is informal and not assumed to have any particular taxonomic rank like the names authorized by the ICBN; the rosids are monophyletic based upon evidence found by molecular phylogenetic analysis.
Three different definitions of the rosids were used. Some authors included the orders Vitales in the rosids. Others excluded both of these orders; the circumscription used in this article is that of the APG IV classification, which includes Vitales, but excludes Saxifragales. The rosids and Saxifragales form the superrosids clade; this is one of three groups that compose the Pentapetalae, the others being Dilleniales and the superasterids. The rosids consist of two groups: the eurosids; the eurosids, in turn, are divided into two groups: malvids. The rosids consist of 17 orders. In addition to Vitales, there are 8 orders in malvids; some of the orders have only been recognized. These are Vitales, Crossosomatales and Huerteales; the phylogeny of Rosids shown below is adapted from the Angiosperm Phylogeny Group website. The nitrogen-fixing clade contains a high number of actinorhizal plants. Not all plants in this clade are actinorhizal, however. Media related to Rosids at Wikimedia Commons
Manganese is a chemical element with symbol Mn and atomic number 25. It is not found as a free element in nature. Manganese is a metal with important industrial metal alloy uses in stainless steels. Manganese is named for pyrolusite and other black minerals from the region of Magnesia in Greece, which gave its name to magnesium and the iron ore magnetite. By the mid-18th century, Swedish-German chemist Carl Wilhelm Scheele had used pyrolusite to produce chlorine. Scheele and others were aware that pyrolusite contained a new element, but they were unable to isolate it. Johan Gottlieb Gahn was the first to isolate an impure sample of manganese metal in 1774, which he did by reducing the dioxide with carbon. Manganese phosphating is used for corrosion prevention on steel. Ionized manganese is used industrially as pigments of various colors, which depend on the oxidation state of the ions; the permanganates of alkali and alkaline earth metals are powerful oxidizers. Manganese dioxide is used as the cathode material in alkaline batteries.
In biology, manganese ions function as cofactors for a large variety of enzymes with many functions. Manganese enzymes are essential in detoxification of superoxide free radicals in organisms that must deal with elemental oxygen. Manganese functions in the oxygen-evolving complex of photosynthetic plants. While the element is a required trace mineral for all known living organisms, it acts as a neurotoxin in larger amounts. Through inhalation, it can cause manganism, a condition in mammals leading to neurological damage, sometimes irreversible. Manganese is a silvery-gray metal, it is hard and brittle, difficult to fuse, but easy to oxidize. Manganese metal and its common ions are paramagnetic. Manganese tarnishes in air and oxidizes like iron in water containing dissolved oxygen. Occurring manganese is composed of one stable isotope, 55Mn. Eighteen radioisotopes have been isolated and described, ranging in atomic weight from 46 u to 65 u; the most stable are 53Mn with a half-life of 3.7 million years, 54Mn with a half-life of 312.3 days, 52Mn with a half-life of 5.591 days.
All of the remaining radioactive isotopes have half-lives of less than three hours, the majority of less than one minute. The primary decay mode before the most abundant stable isotope, 55Mn, is electron capture and the primary mode after is beta decay. Manganese has three meta states. Manganese is part of the iron group of elements, which are thought to be synthesized in large stars shortly before the supernova explosion. 53Mn decays to 53Cr with a half-life of 3.7 million years. Because of its short half-life, 53Mn is rare, produced by cosmic rays impact on iron. Manganese isotopic contents are combined with chromium isotopic contents and have found application in isotope geology and radiometric dating. Mn–Cr isotopic ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites suggest an initial 53Mn/55Mn ratio, which indicates that Mn–Cr isotopic composition must result from in situ decay of 53Mn in differentiated planetary bodies.
Hence, 53Mn provides additional evidence for nucleosynthetic processes before coalescence of the solar system. The most common oxidation states of manganese are +2, +3, +4, +6, +7, though all oxidation states from −3 to +7 have been observed. Mn2+ competes with Mg2+ in biological systems. Manganese compounds where manganese is in oxidation state +7, which are restricted to the unstable oxide Mn2O7, compounds of the intensely purple permanganate anion MnO4−, a few oxyhalides, are powerful oxidizing agents. Compounds with oxidation states +5 and +6 are strong oxidizing agents and are vulnerable to disproportionation; the most stable oxidation state for manganese is +2, which has a pale pink color, many manganese compounds are known, such as manganese sulfate and manganese chloride. This oxidation state is seen in the mineral rhodochrosite. Manganese most exists with a high spin, S = 5/2 ground state because of the high pairing energy for manganese. However, there are a few examples of S = 1/2 manganese.
There are no spin-allowed d–d transitions in manganese, explaining why manganese compounds are pale to colorless. The +3 oxidation state is known in compounds like manganese acetate, but these are quite powerful oxidizing agents and prone to disproportionation in solution, forming manganese and manganese. Solid compounds of manganese are characterized by its strong purple-red color and a preference for distorted octahedral coordination resulting from the Jahn-Teller effect; the oxidation state +5 can be produced by dissolving manganese dioxide in molten sodium nitrite. Manganate salts can be produced by dissolving Mn compounds, such as manganese dioxide, in molten alkali while exposed to air. Permanganate compounds are purple, can give glass a violet color. Potassium permanganate, sodium permanganate, barium permanganate are all potent oxidizers. Potassium permanganate called Condy's crystals, is a used laboratory reagent because of its oxidizing properties. Solutions of potassium permanganate were among the first stains and fixatives to be used in the preparation of biological cells and tissues for electron microscopy
Glycine is an amino acid that has a single hydrogen atom as its side chain. It is the simplest amino acid, with the chemical formula NH2‐CH2‐COOH. Glycine is one of the proteinogenic amino acids, it is encoded by all the codons starting with GG. Glycine is known as a "helix breaker", due to its ability to act as a hinge in the secondary structure of proteins. Glycine is a sweet-tasting crystalline solid, it is the only achiral proteinogenic amino acid. It can fit into hydrophilic or hydrophobic environments, due to its minimal side chain of only one hydrogen atom; the acyl radical is glycyl. Glycine was discovered in 1820 by the French chemist Henri Braconnot when he hydrolyzed gelatin by boiling it with sulfuric acid, he called it "sugar of gelatin", but the French chemist Jean-Baptiste Boussingault showed that it contained nitrogen. The American scientist Eben Norton Horsford a student of the German chemist Justus von Liebig, proposed the name "glycocoll"; the name comes from the Greek word γλυκύς "sweet tasting".
In 1858, the French chemist Auguste Cahours determined. Although glycine can be isolated from hydrolyzed protein, this is not used for industrial production, as it can be manufactured more conveniently by chemical synthesis; the two main processes are amination of chloroacetic acid with ammonia, giving glycine and ammonium chloride, the Strecker amino acid synthesis, the main synthetic method in the United States and Japan. About 15 thousand tonnes are produced annually in this way. Glycine is cogenerated as an impurity in the synthesis of EDTA, arising from reactions of the ammonia coproduct. In aqueous solution, glycine itself is amphoteric: at low pH the molecule can be protonated with a pKa of about 2.4 and at high pH it loses a proton with a pKa of about 9.6. Glycine is not essential to the human diet, as it is biosynthesized in the body from the amino acid serine, in turn derived from 3-phosphoglycerate, but the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis.
In most organisms, the enzyme serine hydroxymethyltransferase catalyses this transformation via the cofactor pyridoxal phosphate: serine + tetrahydrofolate → glycine + N5,N10-Methylene tetrahydrofolate + H2OIn the liver of vertebrates, glycine synthesis is catalyzed by glycine synthase. This conversion is reversible: CO2 + NH+4 + N5,N10-Methylene tetrahydrofolate + NADH + H+ ⇌ Glycine + tetrahydrofolate + NAD+ Glycine is degraded via three pathways; the predominant pathway in animals and plants is the reverse of the glycine synthase pathway mentioned above. In this context, the enzyme system involved is called the glycine cleavage system: Glycine + tetrahydrofolate + NAD+ ⇌ CO2 + NH+4 + N5,N10-Methylene tetrahydrofolate + NADH + H+In the second pathway, glycine is degraded in two steps; the first step is the reverse of glycine biosynthesis from serine with serine hydroxymethyl transferase. Serine is converted to pyruvate by serine dehydratase. In the third pathway of glycine degradation, glycine is converted to glyoxylate by D-amino acid oxidase.
Glyoxylate is oxidized by hepatic lactate dehydrogenase to oxalate in an NAD+-dependent reaction. The half-life of glycine and its elimination from the body varies based on dose. In one study, the half-life varied between 4.0 hours. The principal function of glycine is as a precursor to proteins. Most proteins incorporate only small quantities of glycine, a notable exception being collagen, which contains about 35% glycine due to its periodically repeated role in the formation of collagen's helix structure in conjunction with hydroxyproline. In the genetic code, glycine is coded by all codons starting with GG, namely GGU, GGC, GGA and GGG. In higher eukaryotes, δ-aminolevulinic acid, the key precursor to porphyrins, is biosynthesized from glycine and succinyl-CoA by the enzyme ALA synthase. Glycine provides the central C2N subunit of all purines. Glycine is an inhibitory neurotransmitter in the central nervous system in the spinal cord and retina; when glycine receptors are activated, chloride enters the neuron via ionotropic receptors, causing an Inhibitory postsynaptic potential.
Strychnine is a strong antagonist at ionotropic glycine receptors, whereas bicuculline is a weak one. Glycine is a required co-agonist along with glutamate for NMDA receptors. In contrast to the inhibitory role of glycine in the spinal cord, this behaviour is facilitated at the glutamatergic receptors which are excitatory; the LD50 of glycine is 7930 mg/kg in rats, it causes death by hyperexcitability. In the US, glycine is sold in two grades: United States Pharmacopeia, technical grade. USP grade sales account for 80 to 85 percent of the U. S. market for glycine. If purity greater than the USP standard is needed, for example for intravenous injections, a more expensive pharmaceutical grade glycine can be used. Technical grade glycine, which may or may not meet USP grade standards, is sold at a lower price for use in industrial applications, e.g. as an agent in metal complexing and finishing. USP glycine has a wide variety of uses, including as an additive in pet food and animal feed, in foods and pharmaceuticals as a sweetener/taste enhancer, or as a component of food supplements and protein drinks.
Two glycine molecules in a dipeptide form are referred to as a diglycinate. Because they use a different s
A carbohydrate is a biomolecule consisting of carbon and oxygen atoms with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cmn. This formula holds true for monosaccharides; some exceptions exist. The carbohydrates are technically hydrates of carbon; the term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides and polysaccharides. Monosaccharides and disaccharides, the smallest carbohydrates, are referred to as sugars; the word saccharide comes from the Greek word σάκχαρον, meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides often end in the suffix -ose, as in the monosaccharides fructose and glucose and the disaccharides sucrose and lactose. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as structural components; the 5-carbon monosaccharide ribose is an important component of coenzymes and the backbone of the genetic molecule known as RNA.
The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, preventing pathogenesis, blood clotting, development, they are found in a wide variety of processed foods. Starch is a polysaccharide, it is abundant in cereals and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet as table sugar, lactose and fructose, both of which occur in honey, many fruits, some vegetables. Table sugar, milk, or honey are added to drinks and many prepared foods such as jam and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, are metabolized by these bacteria to yield short-chain fatty acids.
In scientific literature, the term "carbohydrate" has many synonyms, like "sugar", "saccharide", "ose", "glucide", "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are used with other meanings. In food science and in many informal contexts, the term "carbohydrate" means any food, rich in the complex carbohydrate starch or simple carbohydrates, such as sugar. In lists of nutritional information, such as the USDA National Nutrient Database, the term "carbohydrate" is used for everything other than water, fat and ethanol; this includes chemical compounds such as acetic or lactic acid, which are not considered carbohydrates. It includes dietary fiber, a carbohydrate but which does not contribute much in the way of food energy though it is included in the calculation of total food energy just as though it were a sugar. In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.
The name "carbohydrate" was used in chemistry for any compound with the formula Cm n. Following this definition, some chemists considered formaldehyde to be the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today, the term is understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the known carbohydrates and abundant carbohydrates deviate from this. For example, carbohydrates display chemical groups such as: N-acetyl, carboxylic acid and deoxy modifications. Natural saccharides are built of simple carbohydrates called monosaccharides with general formula n where n is three or more. A typical monosaccharide has the structure H–x–y–H, that is, an aldehyde or ketone with many hydroxyl groups added one on each carbon atom, not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose and glyceraldehydes.
However, some biological substances called "monosaccharides" do not conform to this formula and there are many chemicals that do conform to this formula but are not considered to be monosaccharides. The open-chain form of a monosaccharide coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon and hydroxyl group react forming a hemiacetal with a new C–O–C bridge. Monosaccharides can be linked togeth