100-487: Cariamiformes (or Cariamae ) is an order of primarily flightless birds that has existed for over 50 million years. The group includes the family Cariamidae (seriemas) and the extinct families such as Phorusrhacidae , Bathornithidae , Idiornithidae and Ameghinornithidae . Extant members (seriemas) are only known from South America , but fossils of many extinct taxa are also found in other continents including Europe and North America . Though traditionally considered
200-466: A futile cycle . Although fat is a common way of storing energy, in vertebrates such as humans the fatty acids in these stores cannot be converted to glucose through gluconeogenesis as these organisms cannot convert acetyl-CoA into pyruvate ; plants do, but animals do not, have the necessary enzymatic machinery. As a result, after long-term starvation, vertebrates need to produce ketone bodies from fatty acids to replace glucose in tissues such as
300-440: 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 by attaching a nucleobase to a ribose sugar. These bases are heterocyclic rings containing nitrogen, classified as purines or pyrimidines . Nucleotides also act as coenzymes in metabolic-group-transfer reactions. Metabolism involves
400-789: A better sense of smell. A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula , and the loss of grasping hands. † Anchiornis † Archaeopteryx † Xiaotingia † Rahonavis † Jeholornis † Jixiangornis † Balaur † Zhongjianornis † Sapeornis † Confuciusornithiformes † Protopteryx † Pengornis Ornithothoraces † Enantiornithes Metabolism Metabolism ( / m ə ˈ t æ b ə l ɪ z ə m / , from Greek : μεταβολή metabolē , "change")
500-423: A constant set of conditions within cells, a condition called homeostasis . Metabolic regulation also allows organisms to respond to signals and interact actively with their environments. Two closely linked concepts are important for understanding how metabolic pathways are controlled. Firstly, the regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals. Secondly,
600-429: A cycle of reactions that add the acyl group, reduce it to an alcohol, dehydrate it to an alkene group and then reduce it again to an alkane group. The enzymes of fatty acid biosynthesis are divided into two groups: in animals and fungi, all these fatty acid synthase reactions are carried out by a single multifunctional type I protein, while in plant plastids and bacteria separate type II enzymes perform each step in
700-473: A definition similar to "all theropods closer to birds than to Deinonychus ", with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus . Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier , who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight , and
800-442: A group called Paraves . Some basal members of Deinonychosauria, such as Microraptor , have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal , have been able to glide, or both. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that
900-634: A group of warm-blooded vertebrates constituting the class Aves ( Latin: [ˈaveːs] ), characterised by feathers , toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart , and a strong yet lightweight skeleton . Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich . There are over 11,000 living species and they are split into 44 orders . More than half are passerine or "perching" birds. Birds have wings whose development varies according to species;
1000-458: A hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to transfer hydrogen atoms to their substrates. Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD /NADH form is more important in catabolic reactions, while NADP /NADPH
1100-424: A large bird about 1 metre (3.3 ft) tall. Because of its age and geographic location, it was argued that this unnamed species may have been close to the ancestry of both cariamids and phorusrhacids . However, a subsequent study published by West et al. (2019) reinterpreted this specimen as a fossil of an unnamed large-bodied member of a non-cariamiform genus Vegavis . In 2024, two ungual phalanx specimens from
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#17327831847171200-558: A large group of compounds that contain fatty acids and glycerol ; a glycerol molecule attached to three fatty acids by ester linkages is called a triacylglyceride . Several variations of the basic structure exist, including backbones such as sphingosine in sphingomyelin , and hydrophilic groups such as phosphate in phospholipids . Steroids such as sterol 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
1300-404: A larger increase in the entropy of their environments. The metabolism of a cell achieves this by coupling the spontaneous processes of catabolism to the non-spontaneous processes of anabolism. In thermodynamic terms, metabolism maintains order by creating disorder. As the environments of most organisms are constantly changing, the reactions of metabolism must be finely regulated to maintain
1400-472: A membrane. Pumping protons out of the mitochondria creates a proton concentration difference across the membrane and generates an electrochemical gradient . This force drives protons back into the mitochondrion through the base of an enzyme called ATP synthase . The flow of protons makes the stalk subunit rotate, causing the active site of the synthase domain to change shape and phosphorylate adenosine diphosphate —turning it into ATP. Chemolithotrophy
1500-591: A set of xenobiotic-metabolizing enzymes. In humans, these include cytochrome P450 oxidases , UDP-glucuronosyltransferases , and glutathione S -transferases . This system of enzymes acts in three stages to firstly oxidize the xenobiotic (phase I) and then conjugate water-soluble groups onto the molecule (phase II). The modified water-soluble xenobiotic can then be pumped out of cells and in multicellular organisms may be further metabolized before being excreted (phase III). In ecology , these reactions are particularly important in microbial biodegradation of pollutants and
1600-459: A short ancestral pathway, the duplication and then divergence of entire pathways as well as the recruitment of pre-existing enzymes and their assembly into a novel reaction pathway. The relative importance of these mechanisms is unclear, but genomic studies have shown that enzymes in a pathway are likely to have a shared ancestry, suggesting that many pathways have evolved in a step-by-step fashion with novel functions created from pre-existing steps in
1700-406: A sister group, the order Crocodilia , contain the only living representatives of the reptile clade Archosauria . During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica . However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and
1800-530: A source of energy, while switching between carbon fixation and the fermentation of organic compounds. In many organisms, the capture of solar energy is similar in principle to oxidative phosphorylation, as it involves the storage of energy as a proton concentration gradient. This proton motive force then drives ATP synthesis. The electrons needed to drive this electron transport chain come from light-gathering proteins called photosynthetic reaction centres . Reaction centers are classified into two types depending on
1900-581: A suborder within Gruiformes , both morphological and genetic studies show that it belongs to a separate group of birds, Australaves , whose other living members are Falconidae , Psittaciformes and Passeriformes . This proposal has been confirmed by a 2014 study of whole genomes of 48 representative bird species. This analysis shows that the Cariamiformes are basal among extant Australaves , while falcons are next most basal; in combination with
2000-717: A time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction . They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds , parrots, and other species are popular as pets. Guano (bird excrement)
2100-413: A vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of functional groups of atoms and their bonds within molecules. This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions. These group-transfer intermediates are called coenzymes . Each class of group-transfer reactions
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#17327831847172200-419: Is oxidative stress . Here, processes including oxidative phosphorylation and the formation of disulfide bonds during protein folding produce reactive oxygen species such as hydrogen peroxide . These damaging oxidants are removed by antioxidant metabolites such as glutathione and enzymes such as catalases and peroxidases . Living organisms must obey the laws of thermodynamics , which describe
2300-571: Is a type of metabolism found in prokaryotes where energy is obtained from the oxidation of inorganic compounds . These organisms can use hydrogen , reduced sulfur compounds (such as sulfide , hydrogen sulfide and thiosulfate ), ferrous iron (Fe(II)) or ammonia as sources of reducing power and they gain energy from the oxidation of these compounds. These microbial processes are important in global biogeochemical cycles such as acetogenesis , nitrification and denitrification and are critical for soil fertility . The energy in sunlight
2400-401: Is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells. Nicotinamide adenine dinucleotide (NAD ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as
2500-446: Is called gluconeogenesis . Gluconeogenesis converts pyruvate to glucose-6-phosphate through a series of intermediates, many of which are shared with glycolysis . However, this pathway is not simply glycolysis run in reverse, as several steps are catalyzed by non-glycolytic enzymes. This is important as it allows the formation and breakdown of glucose to be regulated separately, and prevents both pathways from running simultaneously in
2600-540: Is called ornithology . Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs . Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians . Birds are descendants of the primitive avialans (whose members include Archaeopteryx ) which first appeared during the Late Jurassic . According to recent estimates, modern birds ( Neornithes ) evolved in
2700-409: Is captured by plants , cyanobacteria , purple bacteria , green sulfur bacteria and some protists . This process is often coupled to the conversion of carbon dioxide into organic compounds, as part of photosynthesis, which is discussed below. The energy capture and carbon fixation systems can, however, operate separately in prokaryotes, as purple bacteria and green sulfur bacteria can use sunlight as
2800-405: Is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. These coenzymes are therefore continuously made, consumed and then recycled. One central coenzyme is adenosine triphosphate (ATP), the energy currency of cells. This nucleotide is used to transfer chemical energy between different chemical reactions. There
2900-645: Is carried out by the enzyme RuBisCO as part of the Calvin–Benson cycle . Three types of photosynthesis occur in plants, C3 carbon fixation , C4 carbon fixation and CAM photosynthesis . These differ by the route that carbon dioxide takes to the Calvin cycle, with C3 plants fixing CO 2 directly, while C4 and CAM photosynthesis incorporate the CO 2 into other compounds first, as adaptations to deal with intense sunlight and dry conditions. In photosynthetic prokaryotes
3000-485: Is composed of a phosphate attached to a ribose or deoxyribose sugar group which is attached to a nitrogenous base . Nucleic acids are critical for the storage and use of genetic information, and its interpretation through the processes of transcription and protein biosynthesis . This information is protected by DNA repair mechanisms and propagated through DNA replication . Many viruses have an RNA genome , such as HIV , which uses reverse transcription to create
3100-525: Is early Eocene taxon Paleopsilopterus itaboraiensis . An isolated femur from the Cape Lamb Member of the Lopez de Bertodano Formation , Vega Island , Antarctica was briefly described as a cariamiform femur in 2006. This specimen, which dates to the late Cretaceous period 66 million years ago, was originally reported as indistinguishable from the femurs of modern seriemas , and belonging to
Cariamiformes - Misplaced Pages Continue
3200-429: Is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry. The first classification of birds
3300-477: Is its primary structure . Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a huge variety of proteins. Proteins are made from amino acids that have been activated by attachment to a transfer RNA molecule through an ester bond. This aminoacyl-tRNA precursor is produced in an ATP -dependent reaction carried out by an aminoacyl tRNA synthetase . This aminoacyl-tRNA
3400-635: Is likely due to their efficacy . In various diseases, such as type II diabetes , metabolic syndrome , and cancer , normal metabolism is disrupted. The metabolism of cancer cells is also different from the metabolism of normal cells, and these differences can be used to find targets for therapeutic intervention in cancer. Most of the structures that make up animals, plants and microbes are made from four basic classes of molecules : amino acids , carbohydrates , nucleic acid and lipids (often called fats ). As these molecules are vital for life, metabolic reactions either focus on making these molecules during
3500-563: Is needed, or back to glucose in the Cori cycle . An alternative route for glucose breakdown is the pentose phosphate pathway , which produces less energy but supports anabolism (biomolecule synthesis). This pathway reduces the coenzyme NADP to NADPH and produces pentose compounds such as ribose 5-phosphate for synthesis of many biomolecules such as nucleotides and aromatic amino acids . Fats are catabolized by hydrolysis to free fatty acids and glycerol. The glycerol enters glycolysis and
3600-503: Is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor. Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs , i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex . The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. After
3700-478: Is only a small amount of ATP in cells, but as it is continuously regenerated, the human body can use about its own weight in ATP per day. ATP acts as a bridge between catabolism and anabolism . Catabolism breaks down molecules, and anabolism puts them together. Catabolic reactions generate ATP, and anabolic reactions consume it. It also serves as a carrier of phosphate groups in phosphorylation reactions. A vitamin
3800-411: Is produced in response to rises in blood glucose levels . Binding of the hormone to insulin receptors on cells then activates a cascade of protein kinases that cause the cells to take up glucose and convert it into storage molecules such as fatty acids and glycogen . The metabolism of glycogen is controlled by activity of phosphorylase , the enzyme that breaks down glycogen, and glycogen synthase ,
3900-546: Is scarce, or when cells undergo metabolic stress. Lipids are the most diverse group of biochemicals. Their main structural uses are as part of internal and external biological membranes , such as the cell membrane . Their chemical energy can also be used. Lipids contain a long, non-polar hydrocarbon chain with a small polar region containing oxygen. Lipids are usually defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as ethanol , benzene or chloroform . The fats are
4000-516: Is synonymous to Avifilopluma. † Scansoriopterygidae † Eosinopteryx † Jinfengopteryx † Aurornis † Dromaeosauridae † Troodontidae Avialae Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs and, more specifically, members of Maniraptora , a group of theropods which includes dromaeosaurids and oviraptorosaurs , among others. As scientists have discovered more theropods closely related to birds,
4100-617: 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 likely due to their early appearance in evolutionary history , and their retention
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4200-529: Is the set of life -sustaining chemical reactions in organisms . The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the conversion of food to building blocks of proteins , lipids , nucleic acids , and some carbohydrates ; and the elimination of metabolic wastes . These enzyme -catalyzed reactions allow organisms to grow and reproduce, maintain their structures , and respond to their environments. The word metabolism can also refer to
4300-422: Is the synthesis of carbohydrates from sunlight and carbon dioxide (CO 2 ). In plants, cyanobacteria and algae, oxygenic photosynthesis splits water, with oxygen produced as a waste product. This process uses the ATP and NADPH produced by the photosynthetic reaction centres , as described above, to convert CO 2 into glycerate 3-phosphate , which can then be converted into glucose. This carbon-fixation reaction
4400-495: Is then a substrate for the ribosome , which joins the amino acid onto the elongating protein chain, using the sequence information in a messenger RNA . Nucleotides are made from amino acids, carbon dioxide and formic acid in pathways that require large amounts of metabolic energy. Consequently, most organisms have efficient systems to salvage preformed nucleotides. Purines are synthesized as nucleosides (bases attached to ribose ). Both adenine and guanine are made from
4500-423: 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 and 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 rapidly—and they also allow
4600-535: Is used by many scientists including adherents to the PhyloCode . Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils , and assigning them, instead, to the broader group Avialae, on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives. Gauthier and de Queiroz identified four different definitions for
4700-429: Is used in anabolic reactions. Inorganic elements play critical roles in metabolism; some are abundant (e.g. sodium and potassium ) while others function at minute concentrations. About 99% of a human's body weight is made up of the elements carbon , nitrogen , calcium , sodium , chlorine , potassium , hydrogen , phosphorus , oxygen and sulfur . Organic compounds (proteins, lipids and carbohydrates) contain
4800-722: The Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non- ornithuran dinosaurs. Many social species preserve knowledge across generations ( culture ). Birds are social, communicating with visual signals, calls, and songs , and participating in such behaviour as cooperative breeding and hunting, flocking , and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous , usually for one breeding season at
4900-669: The Tiaojishan Formation of China, which has been dated to the late Jurassic period ( Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi , Xiaotingia zhengi , and Aurornis xui . The well-known probable early avialan, Archaeopteryx , dates from slightly later Jurassic rocks (about 155 million years old) from Germany . Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on
5000-426: The bioremediation of contaminated land and oil spills. Many of these microbial reactions are shared with multicellular organisms, but due to the incredible diversity of types of microbes these organisms are able to deal with a far wider range of xenobiotics than multicellular organisms, and can degrade even persistent organic pollutants such as organochloride compounds. A related problem for aerobic organisms
5100-503: The chloroplast . These protons move back through the membrane as they drive the ATP synthase, as before. The electrons then flow through photosystem I and can then be used to reduce the coenzyme NADP . This coenzyme can enter the Calvin cycle or be recycled for further ATP generation. Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. In general,
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#17327831847175200-467: The control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway (the flux through the pathway). For example, an enzyme may show large changes in activity (i.e. it is highly regulated) but if these changes have little effect on the flux of a metabolic pathway, then this enzyme is not involved in the control of the pathway. There are multiple levels of metabolic regulation. In intrinsic regulation,
5300-441: The cytoskeleton , a system of scaffolding that maintains the cell shape. Proteins are also important in cell signaling , immune responses , cell adhesion , active transport across membranes, and the cell cycle . Amino acids also contribute to cellular energy metabolism by providing a carbon source for entry into the citric acid cycle ( tricarboxylic acid cycle ), especially when a primary source of energy, such as glucose ,
5400-406: The last universal common ancestor . This universal ancestral cell was prokaryotic and probably a methanogen that had extensive amino acid, nucleotide, carbohydrate and lipid metabolism. The retention of these ancient pathways during later evolution may be the result of these reactions having been an optimal solution to their particular metabolic problems, with pathways such as glycolysis and
5500-550: The mevalonate pathway produces these compounds from acetyl-CoA, while in plants and bacteria the non-mevalonate pathway uses pyruvate and glyceraldehyde 3-phosphate as substrates. One important reaction that uses these activated isoprene donors is sterol biosynthesis . Here, the isoprene units are joined to make squalene and then folded up and formed into a set of rings to make lanosterol . Lanosterol can then be converted into other sterols such as cholesterol and ergosterol . Organisms vary in their ability to synthesize
5600-426: 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
5700-407: The stomach and pancreas , and in salivary glands . The amino acids or sugars released by these extracellular enzymes are then pumped into cells by active transport proteins. Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. Carbohydrates are usually taken into cells after they have been digested into monosaccharides such as glucose and fructose . Once inside,
5800-428: The 20 common amino acids. Most bacteria and plants can synthesize all twenty, but mammals can only synthesize eleven nonessential amino acids, so nine essential amino acids must be obtained from food. Some simple parasites , such as the bacteria Mycoplasma pneumoniae , lack all amino acid synthesis and take their amino acids directly from their hosts. All amino acids are synthesized from intermediates in glycolysis,
5900-428: The appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer. The integument evolved into complex, pennaceous feathers . The oldest known paravian (and probably the earliest avialan) fossils come from
6000-509: The birds that descended from them. Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition
6100-431: The brain that cannot metabolize fatty acids. In other organisms such as plants and bacteria, this metabolic problem is solved using the glyoxylate cycle , which bypasses the decarboxylation step in the citric acid cycle and allows the transformation of acetyl-CoA to oxaloacetate , where it can be used for the production of glucose. Other than fat, glucose is stored in most tissues, as an energy resource available within
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#17327831847176200-439: The cell for energy. M. tuberculosis can also grow on the lipid cholesterol as a sole source of carbon, and genes involved in the cholesterol-use pathway(s) have been validated as important during various stages of the infection lifecycle of M. tuberculosis . Amino acids are either used to synthesize proteins and other biomolecules, or oxidized to urea and carbon dioxide to produce energy. The oxidation pathway starts with
6300-441: The citric acid cycle producing their end products highly efficiently and in a minimal number of steps. The first pathways of enzyme-based metabolism may have been parts of purine nucleotide metabolism, while previous metabolic pathways were a part of the ancient RNA world . Many models have been proposed to describe the mechanisms by which novel metabolic pathways evolve. These include the sequential addition of novel enzymes to
6400-416: The citric acid cycle, or the pentose phosphate pathway. Nitrogen is provided by glutamate and glutamine . Nonessensial amino acid synthesis depends on the formation of the appropriate alpha-keto acid, which is then transaminated to form an amino acid. Amino acids are made into proteins by being joined in a chain of peptide bonds . Each different protein has a unique sequence of amino acid residues: this
6500-637: The coenzyme nicotinamide adenine dinucleotide (NAD ) into NADH. Macromolecules cannot be directly processed by cells. Macromolecules must be broken into smaller units before they can be used in cell metabolism. Different classes of enzymes are used to digest these polymers. These digestive enzymes include proteases that digest proteins into amino acids, as well as glycoside hydrolases that digest polysaccharides into simple sugars known as monosaccharides . Microbes simply secrete digestive enzymes into their surroundings, while animals only secrete these enzymes from specialized cells in their guts , including
6600-534: The complex molecules that make up cellular structures are constructed step-by-step from smaller and simpler precursors. Anabolism involves three basic stages. First, the production of precursors such as amino acids , monosaccharides , isoprenoids and nucleotides , secondly, their activation into reactive forms using energy from ATP, and thirdly, the assembly of these precursors into complex molecules such as proteins , polysaccharides , lipids and nucleic acids . Anabolism in organisms can be different according to
6700-488: The construction of cells and tissues, or on breaking them down and using them to obtain energy, by their digestion. These biochemicals can be joined to make polymers such as DNA and proteins , essential macromolecules of life. Proteins are made of amino acids arranged in a linear chain joined by peptide bonds . Many proteins are enzymes that catalyze the chemical reactions in metabolism. Other proteins have structural or mechanical functions, such as those that form
6800-665: The earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and bird evolution , though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers. † Coelurus † Ornitholestes † Ornithomimosauria † Alvarezsauridae † Oviraptorosauria Paraves Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used
6900-594: The early Eocene strata in Antarctica have been identified as those of a cariamiform, possibly of a phorusrachid. Molecular phylogenetic studies have shown that Cariamiformes is basal to the Falconiformes, Psittaciformes and Passeriformes: Cariamiformes (seriemas and relatives) [REDACTED] Falconiformes (falcons) [REDACTED] Psittaciformes (parrots) [REDACTED] Passeriformes (songbirds) [REDACTED] Bird Birds are
7000-445: The electrons removed from organic molecules in areas such as the citric acid cycle are transferred to oxygen and the energy released is used to make ATP. This is done in eukaryotes by a series of proteins in the membranes of mitochondria called the electron transport chain . In prokaryotes , these proteins are found in the cell's inner membrane . These proteins use the energy from reduced molecules like NADH to pump protons across
7100-683: The end of the reaction catalyzed. Metal micronutrients are taken up into organisms by specific transporters and bind to storage proteins such as ferritin or metallothionein when not in use. Catabolism is the set of metabolic processes that break down large molecules. These include breaking down and oxidizing food molecules. The purpose of the catabolic reactions is to provide the energy and components needed by anabolic reactions which build molecules. The exact nature of these catabolic reactions differ from organism to organism, and organisms can be classified based on their sources of energy, hydrogen, and carbon (their primary nutritional groups ), as shown in
7200-469: The enzyme that makes it. These enzymes are regulated in a reciprocal fashion, with phosphorylation inhibiting glycogen synthase, but activating phosphorylase. Insulin causes glycogen synthesis by activating protein phosphatases and producing a decrease in the phosphorylation of these enzymes. The central pathways of metabolism described above, such as glycolysis and the citric acid cycle, are present in all three domains of living things and were present in
7300-650: The exchange of electrolytes between the extracellular fluid and the cell's fluid, the cytosol . Electrolytes enter and leave cells through proteins in the cell membrane called ion channels . For example, muscle contraction depends upon the movement of calcium, sodium and potassium through ion channels in the cell membrane and T-tubules . Transition metals are usually present as trace elements in organisms, with zinc and iron being most abundant of those. Metal cofactors are bound tightly to specific sites in proteins; although enzyme cofactors can be modified during catalysis, they always return to their original state by
7400-517: The fact that the two most basal branches of Afroaves ( New World vultures plus Accipitriformes , and owls ) are also predatory, it is inferred that the common ancestor of 'core landbirds' ( Telluraves ) was an apex predator . However, some researchers like Darren Naish feel that this assessment is biased towards the more well known, predatory representatives of the clade, and indeed at least one form, Strigogyps , appears to have been herbivorous. The earliest known unambiguous member of this group
7500-414: The fatty acids are broken down by beta oxidation to release acetyl-CoA, which then is fed into the citric acid cycle. Fatty acids release more energy upon oxidation than carbohydrates. Steroids are also broken down by some bacteria in a process similar to beta oxidation, and this breakdown process involves the release of significant amounts of acetyl-CoA, propionyl-CoA, and pyruvate, which can all be used by
7600-451: The first avialans were omnivores . The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It
7700-484: The first stage, large organic molecules, such as proteins , polysaccharides or lipids , are digested into their smaller components outside cells. Next, these smaller molecules are taken up by cells and converted to smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy. Finally, the acetyl group on acetyl-CoA is oxidized to water and carbon dioxide in the citric acid cycle and electron transport chain , releasing more energy while reducing
7800-405: The form of water-soluble messengers such as hormones and growth factors and are detected by specific receptors on the cell surface. These signals are then transmitted inside the cell by second messenger systems that often involved the phosphorylation of proteins. A very well understood example of extrinsic control is the regulation of glucose metabolism by the hormone insulin . Insulin
7900-529: The major route of breakdown is glycolysis , in which glucose is converted into pyruvate . This process generates the energy-conveying molecule NADH from NAD , and generates ATP from ADP for use in powering many processes within the cell. Pyruvate is an intermediate in several metabolic pathways, but the majority is converted to acetyl-CoA and fed into the citric acid cycle , which enables more ATP production by means of oxidative phosphorylation . This oxidation consumes molecular oxygen and releases water and
8000-506: The majority of the carbon and nitrogen; most of the oxygen and hydrogen is present as water. The abundant inorganic elements act as electrolytes . The most important ions are sodium , potassium , calcium , magnesium , chloride , phosphate and the organic ion bicarbonate . The maintenance of precise ion gradients across cell membranes maintains osmotic pressure and pH . Ions are also critical for nerve and muscle function, as action potentials in these tissues are produced by
8100-680: The mechanisms of carbon fixation are more diverse. Here, carbon dioxide can be fixed by the Calvin–Benson cycle, a reversed citric acid cycle, or the carboxylation of acetyl-CoA. Prokaryotic chemoautotrophs also fix CO 2 through the Calvin–Benson cycle, but use energy from inorganic compounds to drive the reaction. In carbohydrate anabolism, simple organic acids can be converted into monosaccharides such as glucose and then used to assemble polysaccharides such as starch . The generation of glucose from compounds like pyruvate , lactate , glycerol , glycerate 3-phosphate and amino acids
8200-482: The metabolic pathway self-regulates to respond to changes in the levels of substrates or products; for example, a decrease in the amount of product can increase the flux through the pathway to compensate. This type of regulation often involves allosteric regulation of the activities of multiple enzymes in the pathway. Extrinsic control involves a cell in a multicellular organism changing its metabolism in response to signals from other cells. These signals are usually in
8300-512: The most abundant biological molecules, and fill numerous roles, such as the storage and transport of energy ( starch , glycogen ) and structural components ( cellulose in plants, chitin in animals). The basic carbohydrate units are called monosaccharides and include galactose , fructose , and most importantly glucose . Monosaccharides can be linked together to form polysaccharides in almost limitless ways. The two nucleic acids, DNA and RNA , are polymers of nucleotides . Each nucleotide
8400-420: The nature of photosynthetic pigment present, with most photosynthetic bacteria only having one type, while plants and cyanobacteria have two. In plants, algae, and cyanobacteria, photosystem II uses light energy to remove electrons from water, releasing oxygen as a waste product. The electrons then flow to the cytochrome b6f complex , which uses their energy to pump protons across the thylakoid membrane in
8500-523: The only known groups without wings are the extinct moa and elephant birds . Wings, which are modified forelimbs , gave birds the ability to fly, although further evolution has led to the loss of flight in some birds , including ratites , penguins , and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds , have further evolved for swimming. The study of birds
8600-402: The outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle , an ossification of fused tail vertebrae. In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size. Around 95 million years ago, they evolved
8700-412: The pathway. Terpenes and isoprenoids are a large class of lipids that include the carotenoids and form the largest class of plant natural products . These compounds are made by the assembly and modification of isoprene units donated from the reactive precursors isopentenyl pyrophosphate and dimethylallyl pyrophosphate . These precursors can be made in different ways. In animals and archaea,
8800-674: The precursor nucleoside inosine monophosphate, which is synthesized using atoms from the amino acids glycine , glutamine , and aspartic acid , as well as formate transferred from the coenzyme tetrahydrofolate . Pyrimidines , on the other hand, are synthesized from the base orotate , which is formed from glutamine and aspartate. All organisms are constantly exposed to compounds that they cannot use as foods and that would be harmful if they accumulated in cells, as they have no metabolic function. These potentially damaging compounds are called xenobiotics . Xenobiotics such as synthetic drugs , natural poisons and antibiotics are detoxified by
8900-536: The previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs , contributed to this ambiguity. The consensus view in contemporary palaeontology is that the flying theropods, or avialans , are the closest relatives of the deinonychosaurs , which include dromaeosaurids and troodontids . Together, these form
9000-430: The removal of the amino group by a transaminase . The amino group is fed into the urea cycle , leaving a deaminated carbon skeleton in the form of a keto acid . Several of these keto acids are intermediates in the citric acid cycle, for example α- ketoglutarate formed by deamination of glutamate . The glucogenic amino acids can also be converted into glucose, through gluconeogenesis . In oxidative phosphorylation,
9100-452: The same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. They assigned other names to the other groups. Lizards & snakes Turtles Crocodiles Birds Under the fourth definition Archaeopteryx , traditionally considered one of
9200-551: The second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering. Avialans diversified into a wide variety of forms during the Cretaceous period. Many groups retained primitive characteristics , such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). Increasingly stiff tails (especially
9300-627: The source of constructed molecules in their cells. Autotrophs such as plants can construct the complex organic molecules in their cells such as polysaccharides and proteins from simple molecules like carbon dioxide and water. Heterotrophs , on the other hand, require a source of more complex substances, such as monosaccharides and amino acids, to produce these complex molecules. Organisms can be further classified by ultimate source of their energy: photoautotrophs and photoheterotrophs obtain energy from light, whereas chemoautotrophs and chemoheterotrophs obtain energy from oxidation reactions. Photosynthesis
9400-425: The substrate can be acceptors, the polysaccharides produced can have straight or branched structures. The polysaccharides produced can have structural or metabolic functions themselves, or be transferred to lipids and proteins by the enzymes oligosaccharyltransferases . Fatty acids are made by fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by
9500-729: The sum of all chemical reactions that occur in living organisms, including digestion and the transportation of substances into and between different cells, in which case the above described set of reactions within the cells is called intermediary (or intermediate) metabolism. Metabolic reactions may be categorized as catabolic —the breaking down of compounds (for example, of glucose to pyruvate by cellular respiration ); or anabolic —the building up ( synthesis ) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized into metabolic pathways , in which one chemical
9600-826: The table below. Organic molecules are used as a source of hydrogen atoms or electrons by organotrophs , while lithotrophs use inorganic substrates. Whereas phototrophs convert sunlight to chemical energy , chemotrophs depend on redox reactions that involve the transfer of electrons from reduced donor molecules such as organic molecules , hydrogen , hydrogen sulfide or ferrous ions to oxygen , nitrate or sulfate . In animals, these reactions involve complex organic molecules that are broken down to simpler molecules, such as carbon dioxide and water. Photosynthetic organisms, such as plants and cyanobacteria , use similar electron-transfer reactions to store energy absorbed from sunlight. The most common set of catabolic reactions in animals can be separated into three main stages. In
9700-407: The tissue through glycogenesis which was usually being used to maintained glucose level in blood. Polysaccharides and glycans are made by the sequential addition of monosaccharides by glycosyltransferase from a reactive sugar-phosphate donor such as uridine diphosphate glucose (UDP-Glc) to an acceptor hydroxyl group on the growing polysaccharide. As any of the hydroxyl groups on the ring of
9800-480: The transfer of heat and work . The second law of thermodynamics states that in any isolated system , the amount of entropy (disorder) cannot decrease. Although living organisms' amazing complexity appears to contradict this law, life is possible as all organisms are open systems that exchange matter and energy with their surroundings. Living systems are not in equilibrium , but instead are dissipative systems that maintain their state of high complexity by causing
9900-465: The waste product carbon dioxide. When oxygen is lacking, or when pyruvate is temporarily produced faster than it can be consumed by the citric acid cycle (as in intense muscular exertion), pyruvate is converted to lactate by the enzyme lactate dehydrogenase , a process that also oxidizes NADH back to NAD for re-use in further glycolysis, allowing energy production to continue. The lactate is later converted back to pyruvate for ATP production where energy
10000-466: Was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae . Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy . Phylogenetic taxonomy places Aves in the clade Theropoda as an infraclass or a subclass, more recently a subclass. Aves and
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