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==Anabolism==

==Anabolism==

合成代谢 【Fernando标记】

合成代谢

{{further|Anabolism}}

{{further|Anabolism}}

Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. In general, the complex molecules that make up cellular structures are constructed step-by-step from small and simple 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 is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. In general, the complex molecules that make up cellular structures are constructed step-by-step from small and simple 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 the source of constructed molecules in their cells. [[Autotroph]]s such as plants can construct the complex organic molecules in cells such as polysaccharides and proteins from simple molecules like [[carbon dioxide]] and water.  [[Heterotroph]]s, 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 inorganic oxidation reactions.<ref name=":0" />

Anabolism in organisms can be different according to the source of constructed molecules in their cells. [[Autotroph]]s such as plants can construct the complex organic molecules in cells such as polysaccharides and proteins from simple molecules like [[carbon dioxide]] and water.  [[Heterotroph]]s, 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 inorganic oxidation reactions.<ref name=":0" />

Anabolism in organisms can be different according to the source of constructed molecules in their cells. Autotrophs such as plants can construct the complex organic molecules in 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 inorganic oxidation reactions.

Anabolism in organisms can be different according to the source of constructed molecules in their cells. Autotrophs such as plants can construct the complex organic molecules in 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 inorganic oxidation reactions.

生物体的合成代谢可以根据其细胞中构建分子的来源而有所不同。植物等自养生物可以从二氧化碳和水等简单分子中构建复杂的细胞有机分子，如多糖和蛋白质。而异养生物则需要更复杂的物质来源，如单糖和氨基酸，才能产生这些复杂的分子。生物可以根据其能量的最终来源进一步分类: 光自养生物和光异养生物从光中获得能量，而化能自养生物和化能异养生物从无机氧化反应中获得能量。

生物体的合成代谢根据其细胞中构建分子的来源而有所不同。植物等自养生物可以在细胞中利用二氧化碳和水等简单分子中构建复杂的有机分子（如多糖和蛋白质）。而异养生物则需要更复杂的物质来源（如单糖和氨基酸）才能产生这些复杂的分子。生物可以根据其能量的最终来源进一步分类: 光自养生物和光异养生物从光中获得能量，而化能自养生物和化能异养生物从无机氧化反应中获得能量。

===Carbon fixation===

===Carbon fixation===

[[File:Plagiomnium affine laminazellen.jpeg|thumb|Plant cells (bounded by purple walls) filled with chloroplasts (green), which are the site of photosynthesis]]

[[File:Plagiomnium affine laminazellen.jpeg|thumb|Plant cells (bounded by purple walls) filled with chloroplasts (green), which are the site of photosynthesis]]

Photosynthesis is the synthesis of carbohydrates from sunlight and [[carbon dioxide]] (CO₂). 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 centre]]s, as described above, to convert CO₂ into [[glycerate 3-phosphate]], which can then be converted into glucose. This carbon-fixation reaction is carried out by the enzyme [[RuBisCO]] as part of the [[Calvin cycle|Calvin&nbsp;– Benson cycle]].<ref>{{cite journal | vauthors = Miziorko HM, Lorimer GH | title = Ribulose-1,5-bisphosphate carboxylase-oxygenase | journal = Annual Review of Biochemistry | volume = 52 | issue =  | pages = 507–35 | year = 1983 | pmid = 6351728 | doi = 10.1146/annurev.bi.52.070183.002451 }}</ref> Three types of photosynthesis occur in plants, [[C3 carbon fixation]], [[C4 carbon fixation]] and [[Crassulacean acid metabolism|CAM photosynthesis]]. These differ by the route that carbon dioxide takes to the Calvin cycle, with C3 plants fixing CO₂ directly, while C4 and CAM photosynthesis incorporate the CO₂ into other compounds first, as adaptations to deal with intense sunlight and dry conditions.<ref>{{cite journal | vauthors = Dodd AN, Borland AM, Haslam RP, Griffiths H, Maxwell K | title = Crassulacean acid metabolism: plastic, fantastic | journal = Journal of Experimental Botany | volume = 53 | issue = 369 | pages = 569–80 | date = April 2002 | pmid = 11886877 | doi = 10.1093/jexbot/53.369.569 | doi-access = free }}</ref>

Photosynthesis is the synthesis of carbohydrates from sunlight and [[carbon dioxide]] (CO₂). 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 centre]]s, as described above, to convert CO₂ into [[glycerate 3-phosphate]], which can then be converted into glucose. This carbon-fixation reaction is carried out by the enzyme [[RuBisCO]] as part of the [[Calvin cycle|Calvin&nbsp;– Benson cycle]].<ref>{{cite journal | vauthors = Miziorko HM, Lorimer GH | title = Ribulose-1,5-bisphosphate carboxylase-oxygenase | journal = Annual Review of Biochemistry | volume = 52 | issue =  | pages = 507–35 | year = 1983 | pmid = 6351728 | doi = 10.1146/annurev.bi.52.070183.002451 }}</ref> Three types of photosynthesis occur in plants, [[C3 carbon fixation]], [[C4 carbon fixation]] and [[Crassulacean acid metabolism|CAM photosynthesis]]. These differ by the route that carbon dioxide takes to the Calvin cycle, with C3 plants fixing CO₂ directly, while C4 and CAM photosynthesis incorporate the CO₂ into other compounds first, as adaptations to deal with intense sunlight and dry conditions.<ref>{{cite journal | vauthors = Dodd AN, Borland AM, Haslam RP, Griffiths H, Maxwell K | title = Crassulacean acid metabolism: plastic, fantastic | journal = Journal of Experimental Botany | volume = 53 | issue = 369 | pages = 569–80 | date = April 2002 | pmid = 11886877 | doi = 10.1093/jexbot/53.369.569 | doi-access = free }}</ref>

Photosynthesis is the synthesis of carbohydrates from sunlight and carbon dioxide (CO₂). 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 CO₂ into glycerate 3-phosphate, which can then be converted into glucose. This carbon-fixation reaction is carried out by the enzyme RuBisCO as part of the Calvin – Benson cycle.[64] 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 CO2 directly, while C4 and CAM photosynthesis incorporate the CO2 into other compounds first, as adaptations to deal with intense sunlight and dry conditions.

Photosynthesis is the synthesis of carbohydrates from sunlight and carbon dioxide (CO₂). 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 CO₂ into glycerate 3-phosphate, which can then be converted into glucose. This carbon-fixation reaction is carried out by the enzyme RuBisCO as part of the Calvin – Benson cycle.[64] 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 CO2 directly, while C4 and CAM photosynthesis incorporate the CO2 into other compounds first, as adaptations to deal with intense sunlight and dry conditions.

光合作用是由阳光和二氧化碳(CO₂)合成碳水化合物。在植物中，蓝藻和藻类，产生氧气的光合作用分解水，产生氧气作为废物。如上所述，这一过程利用光合反应中心产生的ATP和NADPH。将CO₂转化为三磷酸甘油酯，然后再转化为葡萄糖。这个固碳反应是由RuBisCO酶作为卡尔文-本森Calvin – Benson 循环的一部分进行的。植物有三种类型的光合作用:C3固碳、C4固碳和CAM光合作用。不同之处在于二氧化碳进入卡尔文循环的路径不同，C3植物直接固定二氧化碳，而C4和CAM植物首先将二氧化碳吸收到其他化合物中，以适应强烈的阳光和干燥的环境。

光合作用就是依靠阳光和二氧化碳(CO₂)合成碳水化合物。在植物中，蓝细菌和藻类的含氧光合作用使水分解，排出氧气。如前所述，这一过程利用光合反应中心产生的ATP和NADPH将CO₂转化为三磷酸甘油酯，然后再转化为葡萄糖。这个固碳反应作为卡尔文-本森Calvin – Benson 循环的一部分是在RuBisCO酶的催化下进行的。植物有三种类型的光合作用:C3固碳、C4固碳和CAM光合作用。它们的不同之处在于二氧化碳进入卡尔文循环的路径不同，C3植物直接固定二氧化碳，而C4和CAM植物首先将二氧化碳吸收到其他化合物中，以适应强烈的阳光和干燥的环境。

In photosynthetic [[prokaryote]]s the mechanisms of carbon fixation are more diverse. Here, carbon dioxide can be fixed by the Calvin&nbsp;– Benson cycle, a [[Reverse Krebs cycle|reversed citric acid]] cycle,<ref>{{cite journal | vauthors = Hügler M, Wirsen CO, Fuchs G, Taylor CD, Sievert SM | title = Evidence for autotrophic CO2 fixation via the reductive tricarboxylic acid cycle by members of the epsilon subdivision of proteobacteria | journal = Journal of Bacteriology | volume = 187 | issue = 9 | pages = 3020–7 | date = May 2005 | pmid = 15838028 | pmc = 1082812 | doi = 10.1128/JB.187.9.3020-3027.2005 }}</ref> or the carboxylation of acetyl-CoA.<ref>{{cite journal | vauthors = Strauss G, Fuchs G | title = Enzymes of a novel autotrophic CO2 fixation pathway in the phototrophic bacterium Chloroflexus aurantiacus, the 3-hydroxypropionate cycle | journal = European Journal of Biochemistry | volume = 215 | issue = 3 | pages = 633–43 | date = August 1993 | pmid = 8354269 | doi = 10.1111/j.1432-1033.1993.tb18074.x }}</ref><ref>{{cite journal | vauthors = Wood HG | title = Life with CO or CO2 and H2 as a source of carbon and energy | journal = FASEB Journal | volume = 5 | issue = 2 | pages = 156–63 | date = February 1991 | pmid = 1900793 | doi = 10.1096/fasebj.5.2.1900793 | s2cid = 45967404 }}</ref> Prokaryotic [[Chemotroph|chemoautotrophs]] also fix CO₂ through the Calvin–Benson cycle, but use energy from inorganic compounds to drive the reaction.<ref>{{cite journal | vauthors = Shively JM, van Keulen G, Meijer WG | title = Something from almost nothing: carbon dioxide fixation in chemoautotrophs | journal = Annual Review of Microbiology | volume = 52 | issue =  | pages = 191–230 | year = 1998 | pmid = 9891798 | doi = 10.1146/annurev.micro.52.1.191 }}</ref>

In photosynthetic [[prokaryote]]s the mechanisms of carbon fixation are more diverse. Here, carbon dioxide can be fixed by the Calvin&nbsp;– Benson cycle, a [[Reverse Krebs cycle|reversed citric acid]] cycle,<ref>{{cite journal | vauthors = Hügler M, Wirsen CO, Fuchs G, Taylor CD, Sievert SM | title = Evidence for autotrophic CO2 fixation via the reductive tricarboxylic acid cycle by members of the epsilon subdivision of proteobacteria | journal = Journal of Bacteriology | volume = 187 | issue = 9 | pages = 3020–7 | date = May 2005 | pmid = 15838028 | pmc = 1082812 | doi = 10.1128/JB.187.9.3020-3027.2005 }}</ref> or the carboxylation of acetyl-CoA.<ref>{{cite journal | vauthors = Strauss G, Fuchs G | title = Enzymes of a novel autotrophic CO2 fixation pathway in the phototrophic bacterium Chloroflexus aurantiacus, the 3-hydroxypropionate cycle | journal = European Journal of Biochemistry | volume = 215 | issue = 3 | pages = 633–43 | date = August 1993 | pmid = 8354269 | doi = 10.1111/j.1432-1033.1993.tb18074.x }}</ref><ref>{{cite journal | vauthors = Wood HG | title = Life with CO or CO2 and H2 as a source of carbon and energy | journal = FASEB Journal | volume = 5 | issue = 2 | pages = 156–63 | date = February 1991 | pmid = 1900793 | doi = 10.1096/fasebj.5.2.1900793 | s2cid = 45967404 }}</ref> Prokaryotic [[Chemotroph|chemoautotrophs]] also fix CO₂ through the Calvin–Benson cycle, but use energy from inorganic compounds to drive the reaction.<ref>{{cite journal | vauthors = Shively JM, van Keulen G, Meijer WG | title = Something from almost nothing: carbon dioxide fixation in chemoautotrophs | journal = Annual Review of Microbiology | volume = 52 | issue =  | pages = 191–230 | year = 1998 | pmid = 9891798 | doi = 10.1146/annurev.micro.52.1.191 }}</ref>

In photosynthetic prokaryotes 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 CO₂ through the Calvin–Benson cycle, but use energy from inorganic compounds to drive the reaction.

In photosynthetic prokaryotes 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 CO₂ through the Calvin–Benson cycle, but use energy from inorganic compounds to drive the reaction.

在光合作用的原核生物中，碳固定的机制更加多样。在这里，二氧化碳可以通过Calvin– Benson循环、反向三羧酸循环或乙酰辅酶 a 的羧化作用得到固定。原核化能自养生物也通过Calvin– Benson环固定 CO < sub > 2 </sub > ，但利用无机化合物的能量来驱动反应。

在能够光合作用的原核生物中，碳固定的机制更加多样。对它们而言，二氧化碳可以通过Calvin– Benson循环、反向三羧酸循环或乙酰辅酶A的羧化作用得到固定。原核化能自养生物也通过Calvin– Benson环固定CO < sub > 2 </sub >，但它们利用无机化合物的能量来驱动反应。

===Carbohydrates and glycans===

===Carbohydrates and glycans===

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 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 a futile cycle.

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 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 a futile cycle.

在碳水化合物合成代谢过程中，简单的有机酸可转化为葡萄糖等单糖，再用于合成淀粉等多糖。由丙酮酸、乳酸、甘油、3-磷酸甘油酸和氨基酸等化合物生成葡萄糖称为葡萄糖异生。糖异生作用通过一系列中间产物将丙酮酸转化为葡萄糖-6-磷酸，其中许多中间产物与糖酵解过程相同。然而，这一途径并不是简单的糖酵解逆向运行，因为有几个步骤是由非糖酵解酶催化的。这是很重要的，因为它允许葡萄糖的形成和分解被分别调节，并防止两条途径在无效循环中同时运行。

在碳水化合物合成代谢过程中，简单的有机酸可转化为葡萄糖等单糖，再用于合成淀粉等多糖。由丙酮酸、乳酸、甘油、3-磷酸甘油酸和氨基酸等化合物生成葡萄糖称为葡萄糖异生。糖异生作用通过一系列中间产物将丙酮酸转化为葡萄糖-6-磷酸，其中许多中间产物与糖酵解过程相同。然而，这一途径并不是简单的糖酵解逆向运行，因为有几个步骤是由非糖酵解酶催化的。这很重要，因为它使得葡萄糖的形成和分解可以分别被调节，而且防止了两条途径在无效循环中同时运行。

Although fat is a common way of storing energy, in [[vertebrate]]s such as humans the [[fatty acid]]s 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.<ref name=Ensign>{{cite journal | vauthors = Ensign SA | title = Revisiting the glyoxylate cycle: alternate pathways for microbial acetate assimilation | journal = Molecular Microbiology | volume = 61 | issue = 2 | pages = 274–6 | date = July 2006 | pmid = 16856935 | doi = 10.1111/j.1365-2958.2006.05247.x | s2cid = 39986630 }}</ref> As a result, after long-term starvation, vertebrates need to produce [[Ketone body|ketone bodies]] from fatty acids to replace glucose in tissues such as the brain that cannot metabolize fatty acids.<ref>{{cite journal | vauthors = Finn PF, Dice JF | title = Proteolytic and lipolytic responses to starvation | journal = Nutrition | volume = 22 | issue = 7–8 | pages = 830–44 | year = 2006 | pmid = 16815497 | doi = 10.1016/j.nut.2006.04.008 }}</ref> 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.<ref name=Ensign/><ref name=Kornberg>{{cite journal | vauthors = Kornberg HL, Krebs HA | title = Synthesis of cell constituents from C2-units by a modified tricarboxylic acid cycle | journal = Nature | volume = 179 | issue = 4568 | pages = 988–91 | date = May 1957 | pmid = 13430766 | doi = 10.1038/179988a0 | s2cid = 40858130 | bibcode = 1957Natur.179..988K }}</ref> Other than fat, glucose is stored in most tissues, as an energy resource available within the tissue through glycogenesis which was usually being used to maintained glucose level in blood.<ref>{{cite journal|last1=Evans|first1=Rhys D.|last2=Heather|first2=Lisa C. | name-list-style = vanc |date=June 2016|title=Metabolic pathways and abnormalities|journal=Surgery (Oxford)|volume=34|issue=6|pages=266–272|doi=10.1016/j.mpsur.2016.03.010|issn=0263-9319|url=https://ora.ox.ac.uk/objects/uuid:84c0a8e7-38e9-4de2-ba19-9f129a07987a}}</ref>  

Although fat is a common way of storing energy, in [[vertebrate]]s such as humans the [[fatty acid]]s 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.<ref name=Ensign>{{cite journal | vauthors = Ensign SA | title = Revisiting the glyoxylate cycle: alternate pathways for microbial acetate assimilation | journal = Molecular Microbiology | volume = 61 | issue = 2 | pages = 274–6 | date = July 2006 | pmid = 16856935 | doi = 10.1111/j.1365-2958.2006.05247.x | s2cid = 39986630 }}</ref> As a result, after long-term starvation, vertebrates need to produce [[Ketone body|ketone bodies]] from fatty acids to replace glucose in tissues such as the brain that cannot metabolize fatty acids.<ref>{{cite journal | vauthors = Finn PF, Dice JF | title = Proteolytic and lipolytic responses to starvation | journal = Nutrition | volume = 22 | issue = 7–8 | pages = 830–44 | year = 2006 | pmid = 16815497 | doi = 10.1016/j.nut.2006.04.008 }}</ref> 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.<ref name=Ensign/><ref name=Kornberg>{{cite journal | vauthors = Kornberg HL, Krebs HA | title = Synthesis of cell constituents from C2-units by a modified tricarboxylic acid cycle | journal = Nature | volume = 179 | issue = 4568 | pages = 988–91 | date = May 1957 | pmid = 13430766 | doi = 10.1038/179988a0 | s2cid = 40858130 | bibcode = 1957Natur.179..988K }}</ref> Other than fat, glucose is stored in most tissues, as an energy resource available within the tissue through glycogenesis which was usually being used to maintained glucose level in blood.<ref>{{cite journal|last1=Evans|first1=Rhys D.|last2=Heather|first2=Lisa C. | name-list-style = vanc |date=June 2016|title=Metabolic pathways and abnormalities|journal=Surgery (Oxford)|volume=34|issue=6|pages=266–272|doi=10.1016/j.mpsur.2016.03.010|issn=0263-9319|url=https://ora.ox.ac.uk/objects/uuid:84c0a8e7-38e9-4de2-ba19-9f129a07987a}}</ref>  

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 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 the tissue through glycogenesis which was usually being used to maintained glucose level in blood.  

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 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 the tissue through glycogenesis which was usually being used to maintained glucose level in blood.  

虽然脂肪是储存能量的一种常见方式，但在脊椎动物(如人类)体内储存的脂肪酸不能通过葡萄糖异生作用转化为葡萄糖，因为这些生物不能将乙酰辅酶a转化为丙酮酸;植物有必要的酶催化机制，而动物没有。因此，在长期饥饿后，脊椎动物需要从脂肪酸中产生酮体，以取代大脑等不能代谢脂肪酸的组织中的葡萄糖。在其他生物体如植物和细菌中，这个代谢问题是通过乙醛酸循环来解决的。乙醛酸循环绕过三羧酸循环中的脱羧步骤，并允许乙酰辅酶a转化为草酰乙酸，在那里它可以用来生产葡萄糖。除了脂肪，葡萄糖作为一种能量资源储存在大多数组织中，通过糖化通常被用于维持血液中的葡萄糖水平。

虽然脂肪是储存能量的一种常见方式，但在脊椎动物(如人类)体内储存的脂肪酸不能通过葡萄糖异生作用转化为葡萄糖，因为这些生物不能将乙酰辅酶A转化为丙酮酸;植物有必要的酶催化机制，而动物没有。因此，在长期饥饿后，脊椎动物需要从脂肪酸中产生酮体，以取代大脑等不能代谢脂肪酸的组织中的葡萄糖。在其他生物体如植物和细菌中，这个代谢问题是靠乙醛酸循环来解决的。乙醛酸循环绕过三羧酸循环中的脱羧步骤，并将乙酰辅酶a转化为草酰乙酸，在那里它可以用来生产葡萄糖。除了脂肪，葡萄糖也作为一种能量资源储存在大多数组织中，一般会通过它的糖化来维持血液中的葡萄糖水平。

Polysaccharides and [[glycan]]s 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 the substrate can be acceptors, the polysaccharides produced can have straight or branched structures.<ref>{{cite book |last1=Freeze|first1=Hudson H. | name-list-style = vanc | chapter =Glycosylation Precursors|date=2015|url=http://www.ncbi.nlm.nih.gov/books/NBK453043/| title = Essentials of Glycobiology|editor-last=Varki|editor-first=Ajit|edition=3rd|place=Cold Spring Harbor (NY)|publisher=Cold Spring Harbor Laboratory Press|pmid=28876856|access-date=2020-07-08|last2=Hart|first2=Gerald W.|last3=Schnaar|first3=Ronald L.|doi=10.1101/glycobiology.3e.005 |doi-broken-date=1 November 2020 |editor2-last=Cummings|editor2-first=Richard D.|editor3-last=Esko|editor3-first=Jeffrey D.|editor4-last=Stanley|editor4-first=Pamela }}</ref> The polysaccharides produced can have structural or metabolic functions themselves, or be transferred to lipids and proteins by enzymes called [[oligosaccharyltransferase]]s.<ref>{{cite journal | vauthors = Opdenakker G, Rudd PM, Ponting CP, Dwek RA | title = Concepts and principles of glycobiology | journal = FASEB Journal | volume = 7 | issue = 14 | pages = 1330–7 | date = November 1993 | pmid = 8224606 | doi = 10.1096/fasebj.7.14.8224606 | s2cid = 10388991 }}</ref><ref>{{cite journal | vauthors = McConville MJ, Menon AK | title = Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review) | journal = Molecular Membrane Biology | volume = 17 | issue = 1 | pages = 1–16 | year = 2000 | pmid = 10824734 | doi = 10.1080/096876800294443 | doi-access = free }}</ref>

Polysaccharides and [[glycan]]s 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 the substrate can be acceptors, the polysaccharides produced can have straight or branched structures.<ref>{{cite book |last1=Freeze|first1=Hudson H. | name-list-style = vanc | chapter =Glycosylation Precursors|date=2015|url=http://www.ncbi.nlm.nih.gov/books/NBK453043/| title = Essentials of Glycobiology|editor-last=Varki|editor-first=Ajit|edition=3rd|place=Cold Spring Harbor (NY)|publisher=Cold Spring Harbor Laboratory Press|pmid=28876856|access-date=2020-07-08|last2=Hart|first2=Gerald W.|last3=Schnaar|first3=Ronald L.|doi=10.1101/glycobiology.3e.005 |doi-broken-date=1 November 2020 |editor2-last=Cummings|editor2-first=Richard D.|editor3-last=Esko|editor3-first=Jeffrey D.|editor4-last=Stanley|editor4-first=Pamela }}</ref> The polysaccharides produced can have structural or metabolic functions themselves, or be transferred to lipids and proteins by enzymes called [[oligosaccharyltransferase]]s.<ref>{{cite journal | vauthors = Opdenakker G, Rudd PM, Ponting CP, Dwek RA | title = Concepts and principles of glycobiology | journal = FASEB Journal | volume = 7 | issue = 14 | pages = 1330–7 | date = November 1993 | pmid = 8224606 | doi = 10.1096/fasebj.7.14.8224606 | s2cid = 10388991 }}</ref><ref>{{cite journal | vauthors = McConville MJ, Menon AK | title = Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review) | journal = Molecular Membrane Biology | volume = 17 | issue = 1 | pages = 1–16 | year = 2000 | pmid = 10824734 | doi = 10.1080/096876800294443 | doi-access = free }}</ref>

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 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 enzymes called oligosaccharyltransferases.

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 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 enzymes called oligosaccharyltransferases.

多糖和聚糖是由糖基转移酶从活性糖-磷酸盐供体如尿苷二磷酸葡萄糖(UDP-Glc)依次加入到生长中的多糖的受体羟基上形成的单糖。由于底物环上的任何羟基都可以作为受体，所以产生的多糖可以具有直链或支链结构。产生的多糖本身具有结构或代谢功能，或通过被称为低聚糖转移酶的酶转移到脂质和蛋白质中。

多糖和聚糖是在糖基转移酶作用下，将单糖从活性糖-磷酸盐供体（如尿苷二磷酸葡萄糖(UDP-Glc)）依次加入到生长中的多糖的受体羟基上形成的。由于底物环上的任何羟基都可以作为受体，所以产生的多糖会有直链或支链结构。产生的多糖本身具有结构或代谢功能，还可以通过低聚糖转移酶转移到脂质和蛋白质中。

Simplified version of the [[steroid synthesis pathway with the intermediates isopentenyl pyrophosphate (IPP), dimethylallyl pyrophosphate (DMAPP), geranyl pyrophosphate (GPP) and squalene shown. Some intermediates are omitted for clarity.]]

Simplified version of the [[steroid synthesis pathway with the intermediates isopentenyl pyrophosphate (IPP), dimethylallyl pyrophosphate (DMAPP), geranyl pyrophosphate (GPP) and squalene shown. Some intermediates are omitted for clarity.]]

Fatty acids are made by [[fatty acid synthase]]s that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acyl group, reduce it to an alcohol, [[dehydration reaction|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,<ref>{{cite journal | vauthors = Chirala SS, Wakil SJ | title = Structure and function of animal fatty acid synthase | journal = Lipids | volume = 39 | issue = 11 | pages = 1045–53 | date = November 2004 | pmid = 15726818 | doi = 10.1007/s11745-004-1329-9 | s2cid = 4043407 }}</ref> while in plant [[plastid]]s and bacteria separate type II enzymes perform each step in the pathway.<ref>{{cite journal | vauthors = White SW, Zheng J, Zhang YM | title = The structural biology of type II fatty acid biosynthesis | journal = Annual Review of Biochemistry | volume = 74 | issue =  | pages = 791–831 | year = 2005 | pmid = 15952903 | doi = 10.1146/annurev.biochem.74.082803.133524 }}</ref><ref>{{cite journal | vauthors = Ohlrogge JB, Jaworski JG | title = Regulation of Fatty Acid Synthesis | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 48 | issue =  | pages = 109–136 | date = June 1997 | pmid = 15012259 | doi = 10.1146/annurev.arplant.48.1.109 | s2cid = 46348092 }}</ref>

Fatty acids are made by [[fatty acid synthase]]s that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acyl group, reduce it to an alcohol, [[dehydration reaction|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,<ref>{{cite journal | vauthors = Chirala SS, Wakil SJ | title = Structure and function of animal fatty acid synthase | journal = Lipids | volume = 39 | issue = 11 | pages = 1045–53 | date = November 2004 | pmid = 15726818 | doi = 10.1007/s11745-004-1329-9 | s2cid = 4043407 }}</ref> while in plant [[plastid]]s and bacteria separate type II enzymes perform each step in the pathway.<ref>{{cite journal | vauthors = White SW, Zheng J, Zhang YM | title = The structural biology of type II fatty acid biosynthesis | journal = Annual Review of Biochemistry | volume = 74 | issue =  | pages = 791–831 | year = 2005 | pmid = 15952903 | doi = 10.1146/annurev.biochem.74.082803.133524 }}</ref><ref>{{cite journal | vauthors = Ohlrogge JB, Jaworski JG | title = Regulation of Fatty Acid Synthesis | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 48 | issue =  | pages = 109–136 | date = June 1997 | pmid = 15012259 | doi = 10.1146/annurev.arplant.48.1.109 | s2cid = 46348092 }}</ref>

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 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 the pathway.

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 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 the pathway.

脂肪酸是由脂肪酸合成酶聚合后还原乙酰辅酶A单位制成的。脂肪酸中的酰基链通过一系列反应得以延伸，这些反应包括:添加酰基，将其还原为醇，脱水成烯烃基团，然后再还原为烷烃基团。脂肪酸生物合成的酶分为两类：在动物和真菌中，所有这些脂肪酸合成酶的反应都是由单一的多功能I型蛋白来完成的，而在植物的质体和细菌中，则由单独的II型酶来完成途径中的每一步。

脂肪酸是由脂肪酸合成酶聚合并还原乙酰辅酶A单元制成的。脂肪酸中的酰基链通过一些反应的循环得以延伸，这些反应包括:添加酰基，将其还原为醇，脱水成烯烃基团，然后再还原为烷烃基团。脂肪酸生物合成的酶可以分为两类：在动物和真菌中，所有脂肪酸合成酶的反应都是由单一的多功能I型蛋白来完成的，而在植物的质体和细菌中，则由单独的II型酶来完成每一步。

[[Terpene]]s and [[terpenoid|isoprenoids]] are a large class of lipids that include the [[carotenoid]]s and form the largest class of plant [[natural product]]s.<ref>{{cite journal | vauthors = Dubey VS, Bhalla R, Luthra R | title = An overview of the non-mevalonate pathway for terpenoid biosynthesis in plants | journal = Journal of Biosciences | volume = 28 | issue = 5 | pages = 637–46 | date = September 2003 | pmid = 14517367 | doi = 10.1007/BF02703339 | url = http://www.ias.ac.in/jbiosci/sep2003/637.pdf | url-status = dead | s2cid = 27523830 | archive-url = https://web.archive.org/web/20070415213325/http://www.ias.ac.in/jbiosci/sep2003/637.pdf | df =  | archive-date = 15 April 2007 }}</ref> These compounds are made by the assembly and modification of [[isoprene]] units donated from the reactive precursors [[isopentenyl pyrophosphate]] and [[dimethylallyl pyrophosphate]].<ref name=Kuzuyama>{{cite journal | vauthors = Kuzuyama T, Seto H | title = Diversity of the biosynthesis of the isoprene units | journal = Natural Product Reports | volume = 20 | issue = 2 | pages = 171–83 | date = April 2003 | pmid = 12735695 | doi = 10.1039/b109860h }}</ref> These precursors can be made in different ways. In animals and archaea, the [[mevalonate pathway]] produces these compounds from acetyl-CoA,<ref>{{cite journal | vauthors = Grochowski LL, Xu H, White RH | title = Methanocaldococcus jannaschii uses a modified mevalonate pathway for biosynthesis of isopentenyl diphosphate | journal = Journal of Bacteriology | volume = 188 | issue = 9 | pages = 3192–8 | date = May 2006 | pmid = 16621811 | pmc = 1447442 | doi = 10.1128/JB.188.9.3192-3198.2006 }}</ref> while in plants and bacteria the [[non-mevalonate pathway]] uses pyruvate and [[glyceraldehyde 3-phosphate]] as substrates.<ref name=Kuzuyama/><ref>{{cite journal | vauthors = Lichtenthaler HK | title = The 1-Deoxy-D-Xylulose-5-Phosphate Pathway of Isoprenoid Biosynthesis in Plants | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 50 | issue =  | pages = 47–65 | date = June 1999 | pmid = 15012203 | doi = 10.1146/annurev.arplant.50.1.47 }}</ref> One important reaction that uses these activated isoprene donors is [[steroid biosynthesis|sterol biosynthesis]]. Here, the isoprene units are joined together to make [[squalene]] and then folded up and formed into a set of rings to make [[lanosterol]].<ref name=Schroepfer>{{cite journal | vauthors = Schroepfer GJ | title = Sterol biosynthesis | journal = Annual Review of Biochemistry | volume = 50 | issue =  | pages = 585–621 | year = 1981 | pmid = 7023367 | doi = 10.1146/annurev.bi.50.070181.003101 }}</ref> Lanosterol can then be converted into other sterol such as [[cholesterol]] and [[ergosterol]].<ref name=Schroepfer/><ref>{{cite journal | vauthors = Lees ND, Skaggs B, Kirsch DR, Bard M | title = Cloning of the late genes in the ergosterol biosynthetic pathway of Saccharomyces cerevisiae--a review | journal = Lipids | volume = 30 | issue = 3 | pages = 221–6 | date = March 1995 | pmid = 7791529 | doi = 10.1007/BF02537824 | s2cid = 4019443 }}</ref>

[[Terpene]]s and [[terpenoid|isoprenoids]] are a large class of lipids that include the [[carotenoid]]s and form the largest class of plant [[natural product]]s.<ref>{{cite journal | vauthors = Dubey VS, Bhalla R, Luthra R | title = An overview of the non-mevalonate pathway for terpenoid biosynthesis in plants | journal = Journal of Biosciences | volume = 28 | issue = 5 | pages = 637–46 | date = September 2003 | pmid = 14517367 | doi = 10.1007/BF02703339 | url = http://www.ias.ac.in/jbiosci/sep2003/637.pdf | url-status = dead | s2cid = 27523830 | archive-url = https://web.archive.org/web/20070415213325/http://www.ias.ac.in/jbiosci/sep2003/637.pdf | df =  | archive-date = 15 April 2007 }}</ref> These compounds are made by the assembly and modification of [[isoprene]] units donated from the reactive precursors [[isopentenyl pyrophosphate]] and [[dimethylallyl pyrophosphate]].<ref name=Kuzuyama>{{cite journal | vauthors = Kuzuyama T, Seto H | title = Diversity of the biosynthesis of the isoprene units | journal = Natural Product Reports | volume = 20 | issue = 2 | pages = 171–83 | date = April 2003 | pmid = 12735695 | doi = 10.1039/b109860h }}</ref> These precursors can be made in different ways. In animals and archaea, the [[mevalonate pathway]] produces these compounds from acetyl-CoA,<ref>{{cite journal | vauthors = Grochowski LL, Xu H, White RH | title = Methanocaldococcus jannaschii uses a modified mevalonate pathway for biosynthesis of isopentenyl diphosphate | journal = Journal of Bacteriology | volume = 188 | issue = 9 | pages = 3192–8 | date = May 2006 | pmid = 16621811 | pmc = 1447442 | doi = 10.1128/JB.188.9.3192-3198.2006 }}</ref> while in plants and bacteria the [[non-mevalonate pathway]] uses pyruvate and [[glyceraldehyde 3-phosphate]] as substrates.<ref name=Kuzuyama/><ref>{{cite journal | vauthors = Lichtenthaler HK | title = The 1-Deoxy-D-Xylulose-5-Phosphate Pathway of Isoprenoid Biosynthesis in Plants | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 50 | issue =  | pages = 47–65 | date = June 1999 | pmid = 15012203 | doi = 10.1146/annurev.arplant.50.1.47 }}</ref> One important reaction that uses these activated isoprene donors is [[steroid biosynthesis|sterol biosynthesis]]. Here, the isoprene units are joined together to make [[squalene]] and then folded up and formed into a set of rings to make [[lanosterol]].<ref name=Schroepfer>{{cite journal | vauthors = Schroepfer GJ | title = Sterol biosynthesis | journal = Annual Review of Biochemistry | volume = 50 | issue =  | pages = 585–621 | year = 1981 | pmid = 7023367 | doi = 10.1146/annurev.bi.50.070181.003101 }}</ref> Lanosterol can then be converted into other sterol such as [[cholesterol]] and [[ergosterol]].<ref name=Schroepfer/><ref>{{cite journal | vauthors = Lees ND, Skaggs B, Kirsch DR, Bard M | title = Cloning of the late genes in the ergosterol biosynthetic pathway of Saccharomyces cerevisiae--a review | journal = Lipids | volume = 30 | issue = 3 | pages = 221–6 | date = March 1995 | pmid = 7791529 | doi = 10.1007/BF02537824 | s2cid = 4019443 }}</ref>

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, 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 together to make squalene and then folded up and formed into a set of rings to make lanosterol. Lanosterol can then be converted into other sterol such as cholesterol and ergosterol.

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, 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 together to make squalene and then folded up and formed into a set of rings to make lanosterol. Lanosterol can then be converted into other sterol such as cholesterol and ergosterol.

萜烯和异戊二烯是一大类脂类，包括类胡萝卜素，是最大的一类植物天然产品。这些化合物是由反应性前体焦磷酸异戊烯酯和焦磷酸二甲基烯丙基酯所捐献的异戊二烯单元组装和改性而成。这些前体可以通过不同的方式制造。在动物和古生物中，甲戊二酸途径从乙酰辅酶A产生这些化合物，而在植物和细菌中，非甲戊二酸途径使用丙酮酸和甘油醛3-磷酸作为底物。使用这些活化的异戊二烯供体的一个重要反应是固醇的生物合成。在这里，异戊二烯单元连接在一起，制成角鲨烯，然后折叠起来，形成一组环，制成羊毛固醇。羊毛固醇随后可转化为其他固醇，如胆固醇和麦角固醇。

萜烯和异戊二烯是一大类脂类，包括类胡萝卜素，也是最大的一类植物天然产品。这些化合物是由反应性前体焦磷酸异戊烯酯和焦磷酸二甲基烯丙基酯所提供的异戊二烯单元组装和改性而成。这些前体可以靠不同的途径制造。在动物和古生物中，甲戊二酸途径从乙酰辅酶A产生这些化合物，而在植物和细菌中，非甲戊二酸途径使用丙酮酸和甘油醛3-磷酸作为底物。使用这些活化的异戊二烯供体的一个重要反应是固醇的生物合成。在该反应中，异戊二烯单元连接在一起，制成角鲨烯，然后折叠起来形成一组环，制成羊毛固醇。羊毛固醇随后可转化为其他固醇，如胆固醇和麦角固醇。

===Proteins===

===Proteins===

Organisms vary in their ability to synthesize 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. All amino acids are synthesized from intermediates in glycolysis, 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.

Organisms vary in their ability to synthesize 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. All amino acids are synthesized from intermediates in glycolysis, 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.

生物体合成这20种常见氨基酸的能力各不相同。大多数细菌和植物都能合成这20种氨基酸，但哺乳动物只能合成11种非必需氨基酸，所以必须从食物中获得9种必需氨基酸。所有的氨基酸都是由糖酵解、三羧酸循环或磷酸戊糖途径的中间产物合成的。氮由谷氨酸和谷氨酰胺提供。无敏感性氨基酸的合成取决于适当的α-酮酸的形成，然后该酸被转氨基形成氨基酸。

生物体合成这20种常见氨基酸的能力各不相同。大多数细菌和植物都能合成这20种氨基酸，但哺乳动物只能合成11种非必需氨基酸，因此必须从食物中获得9种必需氨基酸。所有的氨基酸都是由糖酵解、三羧酸循环或磷酸戊糖途径的中间产物合成的。氮由谷氨酸和谷氨酰胺提供。非关键氨基酸的合成取决于适当的α-酮酸的形成，它可以转氨基形成氨基酸。

Amino acids are made into proteins by being joined together in a chain of [[peptide bond]]s. Each different protein has a unique sequence of amino acid residues: this 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 [[Adenosine triphosphate|ATP]]-dependent reaction carried out by an [[aminoacyl tRNA synthetase]].<ref>{{cite journal | vauthors = Ibba M, Söll D | title = The renaissance of aminoacyl-tRNA synthesis | journal = EMBO Reports | volume = 2 | issue = 5 | pages = 382–7 | date = May 2001 | pmid = 11375928 | pmc = 1083889 | doi = 10.1093/embo-reports/kve095 | url = http://www.molcells.org/home/journal/include/downloadPdf.asp?articleuid={A158E3B4-2423-4806-9A30-4B93CDA76DA0} | url-status = dead | archive-url = https://web.archive.org/web/20110501181419/http://www.molcells.org/home/journal/include/downloadPdf.asp?articleuid=%7BA158E3B4-2423-4806-9A30-4B93CDA76DA0%7D | df =  | archive-date = 1 May 2011 }}</ref> This aminoacyl-tRNA 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]].<ref>{{cite journal | vauthors = Lengyel P, Söll D | title = Mechanism of protein biosynthesis | journal = Bacteriological Reviews | volume = 33 | issue = 2 | pages = 264–301 | date = June 1969 | pmid = 4896351 | pmc = 378322 | doi = 10.1128/MMBR.33.2.264-301.1969 }}</ref>

Amino acids are made into proteins by being joined together in a chain of [[peptide bond]]s. Each different protein has a unique sequence of amino acid residues: this 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 [[Adenosine triphosphate|ATP]]-dependent reaction carried out by an [[aminoacyl tRNA synthetase]].<ref>{{cite journal | vauthors = Ibba M, Söll D | title = The renaissance of aminoacyl-tRNA synthesis | journal = EMBO Reports | volume = 2 | issue = 5 | pages = 382–7 | date = May 2001 | pmid = 11375928 | pmc = 1083889 | doi = 10.1093/embo-reports/kve095 | url = http://www.molcells.org/home/journal/include/downloadPdf.asp?articleuid={A158E3B4-2423-4806-9A30-4B93CDA76DA0} | url-status = dead | archive-url = https://web.archive.org/web/20110501181419/http://www.molcells.org/home/journal/include/downloadPdf.asp?articleuid=%7BA158E3B4-2423-4806-9A30-4B93CDA76DA0%7D | df =  | archive-date = 1 May 2011 }}</ref> This aminoacyl-tRNA 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]].<ref>{{cite journal | vauthors = Lengyel P, Söll D | title = Mechanism of protein biosynthesis | journal = Bacteriological Reviews | volume = 33 | issue = 2 | pages = 264–301 | date = June 1969 | pmid = 4896351 | pmc = 378322 | doi = 10.1128/MMBR.33.2.264-301.1969 }}</ref>

Amino acids are made into proteins by being joined together in a chain of peptide bonds. Each different protein has a unique sequence of amino acid residues: this 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 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.

Amino acids are made into proteins by being joined together in a chain of peptide bonds. Each different protein has a unique sequence of amino acid residues: this 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 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.

===Nucleotide synthesis and salvage===

===Nucleotide synthesis and salvage===

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 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.

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 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.

核苷酸由氨基酸、二氧化碳和甲酸在需要大量代谢能量的途径中生成。因此，大多数生物体都有有效的系统来挽救预先形成的核苷酸。嘌呤以核苷的形式合成(碱基附着在核糖上)。腺嘌呤和鸟嘌呤都是由一磷酸核苷肌苷前体合成的，它是由甘氨酸、谷氨酰胺和天冬氨酸的原子合成的，还有从辅酶四氢叶酸转移的甲酸酯。另一方面，嘧啶是由谷氨酰胺和天门冬氨酸形成的磷酸基合成的。

核苷酸由氨基酸、二氧化碳和甲酸在需要大量代谢能量的途径中生成。因此，大多数生物体都有有效的系统来挽救预先形成的核苷酸。嘌呤以核苷的形式合成(碱基附着在核糖上)。腺嘌呤和鸟嘌呤都是由一磷酸核苷肌苷前体合成的，而前体是由甘氨酸、谷氨酰胺和天冬氨酸的原子合成的，从辅酶四氢叶酸转移来的甲酸酯也是如此。另一方面，嘧啶是由磷酸基合成的，而磷酸基是由谷氨酰胺和天门冬氨酸形成的。

==Xenobiotics and redox metabolism==

==Xenobiotics and redox metabolism==【Fernando标记】

异种生物学和氧化还原代谢

异种生物学和氧化还原代谢

{{further|Xenobiotic metabolism|Drug metabolism|Alcohol metabolism|Antioxidant}}

{{further|Xenobiotic metabolism|Drug metabolism|Alcohol metabolism|Antioxidant}}

All organisms are constantly exposed to compounds that they cannot use as foods and 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 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 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.

All organisms are constantly exposed to compounds that they cannot use as foods and 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 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 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.

所有的生物都不断地接触到在它们不能用作食物的化合物中，如果它们在细胞中积累起来就会有害，因为它们没有新陈代谢功能。这些具有潜在破坏性的化合物被称为异生物质。合成药物、天然毒物和抗生素等异生物质是通过一系列异生物质代谢酶来解毒的。在人类中，这些酶包括细胞色素 P450氧化酶、UDP-葡萄糖醛酸转移酶和谷胱甘肽 s- 转移酶。这套酶系统的作用分为三个阶段，首先氧化异生物质(第一阶段) ，然后将水溶性基团共轭到分子上(第二阶段)。经过修饰的水溶性异生物质随后可以从细胞中泵出，在多细胞生物中，可以在排出之前进一步代谢(第三期)。在生态学中，这些反应在微生物对污染物的生物降解以及污染土地和溢油的生物修复中尤为重要。这些微生物反应中有许多是与多细胞生物共享的，但是由于难以置信的微生物种类多样性，这些微生物能够处理比多细胞生物更广泛的异生物质，甚至能够降解有机氯化合物等持久性有机污染物。

所有的生物都不断地接触到它们不能进食的化合物，如果它们在细胞中积累这些化合物就会被伤害，因为它们没有新陈代谢功能。这些具有潜在破坏性的化合物被称为异生物质。合成药物、天然毒物和抗生素等异生物质是通过一系列异生物质代谢酶来解毒的。在人体中，这些酶包括细胞色素 P450氧化酶、UDP-葡萄糖醛酸转移酶和谷胱甘肽 s- 转移酶。这套酶系统的作用分为三个阶段，首先氧化异生物质(第一阶段) ，然后将水溶性基团共轭到分子上(第二阶段)。经过降解的水溶性异生物质随后会从细胞中泵出，对多细胞生物来说，还会在排出之前进一步代谢(第三阶段)。在生态学中，这些反应在微生物对污染物的生物降解以及污染土地和溢油的生物修复中尤为重要。这些微生物反应中有许多是与多细胞生物相同的，但是由于微生物种类的惊人多样性，这些微生物能够处理比多细胞生物更广泛的异生物质，甚至能够降解有机氯化合物等持久性有机污染物。

A related problem for [[aerobic organism]]s is [[oxidative stress]].<ref name=Davies>{{cite journal | vauthors = Davies KJ | title = Oxidative stress: the paradox of aerobic life | journal = Biochemical Society Symposium | volume = 61 | issue =  | pages = 1–31 | year = 1995 | pmid = 8660387 | doi = 10.1042/bss0610001 }}</ref> Here, processes including [[oxidative phosphorylation]] and the formation of [[disulfide bond]]s during [[protein folding]] produce [[reactive oxygen species]] such as [[hydrogen peroxide]].<ref>{{cite journal | vauthors = Tu BP, Weissman JS | title = Oxidative protein folding in eukaryotes: mechanisms and consequences | journal = The Journal of Cell Biology | volume = 164 | issue = 3 | pages = 341–6 | date = February 2004 | pmid = 14757749 | pmc = 2172237 | doi = 10.1083/jcb.200311055 }}</ref> These damaging oxidants are removed by [[antioxidant]] metabolites such as [[glutathione]] and enzymes such as [[catalase]]s and [[peroxidase]]s.<ref name=Sies>{{cite journal | vauthors = Sies H | title = Oxidative stress: oxidants and antioxidants | journal = Experimental Physiology | volume = 82 | issue = 2 | pages = 291–5 | date = March 1997 | pmid = 9129943 | doi = 10.1113/expphysiol.1997.sp004024 | s2cid = 20240552 | url = http://ep.physoc.org/cgi/reprint/82/2/291.pdf | access-date = 9 March 2007 | url-status = dead | archive-url = https://web.archive.org/web/20090325001126/http://ep.physoc.org/cgi/reprint/82/2/291.pdf | archive-date = 25 March 2009 }}</ref><ref name=Vertuani>{{cite journal | vauthors = Vertuani S, Angusti A, Manfredini S | title = The antioxidants and pro-antioxidants network: an overview | journal = Current Pharmaceutical Design | volume = 10 | issue = 14 | pages = 1677–94 | year = 2004 | pmid = 15134565 | doi = 10.2174/1381612043384655 | s2cid = 43713549 }}</ref>

A related problem for [[aerobic organism]]s is [[oxidative stress]].<ref name=Davies>{{cite journal | vauthors = Davies KJ | title = Oxidative stress: the paradox of aerobic life | journal = Biochemical Society Symposium | volume = 61 | issue =  | pages = 1–31 | year = 1995 | pmid = 8660387 | doi = 10.1042/bss0610001 }}</ref> Here, processes including [[oxidative phosphorylation]] and the formation of [[disulfide bond]]s during [[protein folding]] produce [[reactive oxygen species]] such as [[hydrogen peroxide]].<ref>{{cite journal | vauthors = Tu BP, Weissman JS | title = Oxidative protein folding in eukaryotes: mechanisms and consequences | journal = The Journal of Cell Biology | volume = 164 | issue = 3 | pages = 341–6 | date = February 2004 | pmid = 14757749 | pmc = 2172237 | doi = 10.1083/jcb.200311055 }}</ref> These damaging oxidants are removed by [[antioxidant]] metabolites such as [[glutathione]] and enzymes such as [[catalase]]s and [[peroxidase]]s.<ref name=Sies>{{cite journal | vauthors = Sies H | title = Oxidative stress: oxidants and antioxidants | journal = Experimental Physiology | volume = 82 | issue = 2 | pages = 291–5 | date = March 1997 | pmid = 9129943 | doi = 10.1113/expphysiol.1997.sp004024 | s2cid = 20240552 | url = http://ep.physoc.org/cgi/reprint/82/2/291.pdf | access-date = 9 March 2007 | url-status = dead | archive-url = https://web.archive.org/web/20090325001126/http://ep.physoc.org/cgi/reprint/82/2/291.pdf | archive-date = 25 March 2009 }}</ref><ref name=Vertuani>{{cite journal | vauthors = Vertuani S, Angusti A, Manfredini S | title = The antioxidants and pro-antioxidants network: an overview | journal = Current Pharmaceutical Design | volume = 10 | issue = 14 | pages = 1677–94 | year = 2004 | pmid = 15134565 | doi = 10.2174/1381612043384655 | s2cid = 43713549 }}</ref>

A related problem for aerobic organisms 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.

A related problem for aerobic organisms 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.

==Thermodynamics of living organisms==

==Thermodynamics of living organisms==【Fernando标记】

生命有机体的热力学

生命有机体的热力学

{{further|Biological thermodynamics}}

{{further|Biological thermodynamics}}