Editing Metabolic pathway (section)

==Major metabolic pathways==
{{for|additional infographics of major metabolic pathways|#External links}}
{{metabolic metro}}
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===Catabolic pathway (catabolism)===
A '''[[catabolic pathway]]''' is a series of reactions that bring about a net release of energy in the form of a high energy phosphate bond formed with the energy carriers adenosine diphosphate (ADP) and guanosine diphosphate (GDP) to produce adenosine triphosphate (ATP) and guanosine triphosphate (GTP), respectively.<ref name="Harvey"/>{{rp|91–93}} The net reaction is, therefore, thermodynamically favorable, for it results in a lower free energy for the final products.<ref name="Clarke">{{cite book| vauthors = Berg JM, Tymoczko JL, Stryer L |title=Biochemistry|date=2002|publisher=W. H. Freeman|location=New York, NY |isbn=978-0-7167-3051-4 |edition=5th |url=https://archive.org/details/biochemistrychap00jere}}</ref>{{rp|578–579}} A catabolic pathway is an exergonic system that produces chemical energy in the form of ATP, GTP, NADH, NADPH, FADH2, etc. from energy containing sources such as carbohydrates, fats, and proteins. The end products are often carbon dioxide, water, and ammonia. Coupled with an endergonic reaction of anabolism, the cell can synthesize new macromolecules using the original precursors of the anabolic pathway.<ref>{{cite book| vauthors = Raven PH, Evert RF, Eichhorn SE |title=Biology of plants|date=2011|publisher=Freeman|location=New York, NY|isbn=978-1-4292-1961-7|pages=100–106|edition=8th }}</ref> An example of a coupled reaction is the phosphorylation of [[Fructose 6-phosphate|fructose-6-phosphate]] to form the intermediate [[Fructose 1,6-bisphosphate|fructose-1,6-bisphosphate]] by the enzyme [[phosphofructokinase]] accompanied by the hydrolysis of ATP in the pathway of [[glycolysis]]. The resulting chemical reaction within the metabolic pathway is highly thermodynamically favorable and, as a result, irreversible in the cell.

<chem>Fructose-6-Phosphate + ATP -> Fructose-1,6-Bisphosphate + ADP</chem>

====Cellular respiration====
{{Main|Cellular respiration}}

A core set of energy-producing [[catabolic]] pathways occur within all living organisms in some form. These pathways transfer the energy released by breakdown of [[nutrient]]s into [[Adenosine triphosphate|ATP]] and other small molecules used for energy (e.g. [[Guanosine triphosphate|GTP]], [[NADPH]], [[FADH2|FADH<sub>2</sub>]]). All cells can perform [[anaerobic respiration]] by [[glycolysis]]. Additionally, most organisms can perform more efficient [[aerobic respiration]] through the [[citric acid cycle]] and [[oxidative phosphorylation]]. Additionally [[plant]]s, [[algae]] and [[cyanobacteria]] are able to use sunlight to [[anabolic]]ally synthesize compounds from non-living matter by [[photosynthesis]].
[[File:Gluconeogenese Schema 2.png|thumb|Gluconeogenesis mechanism|557x557px]]

===Anabolic pathway (anabolism)===
In contrast to catabolic pathways, '''[[anabolic pathways]]''' require an energy input to construct macromolecules such as polypeptides, nucleic acids, proteins, polysaccharides, and lipids. The isolated reaction of anabolism is unfavorable in a cell due to a positive [[Gibbs free energy]] (+Δ''G''). Thus, an input of chemical energy through a coupling with an [[exergonic reaction]] is necessary.<ref name="Nelson"/>{{rp|25–27}} The coupled reaction of the catabolic pathway affects the thermodynamics of the reaction by lowering the overall [[activation energy]] of an anabolic pathway and allowing the reaction to take place.<ref name="Nelson"/>{{rp|25}} Otherwise, an [[endergonic reaction]] is non-spontaneous.

An anabolic pathway is a biosynthetic pathway, meaning that it combines smaller molecules to form larger and more complex ones.<ref name="Clarke"/>{{rp|570}} An example is the reversed pathway of glycolysis, otherwise known as [[gluconeogenesis]], which occurs in the liver and sometimes in the kidney to maintain proper glucose concentration in the blood and supply the brain and muscle tissues with adequate amount of glucose. Although gluconeogenesis is similar to the reverse pathway of glycolysis, it contains four distinct enzymes([[pyruvate carboxylase]], [[phosphoenolpyruvate carboxykinase]], [[fructose 1,6-bisphosphatase]], [[glucose 6-phosphatase]]) from glycolysis that allow the pathway to occur spontaneously.<ref>{{cite book| vauthors = Berg JM, Tymoczko JL, Stryer L, Gatto GJ |title=Biochemistry|date=2012|publisher=W.H. Freeman|location=New York|isbn=978-1-4292-2936-4 |pages=480–482|edition=7th}}</ref>

===Amphibolic pathway (Amphibolism)===
[[File:Amphibolic Properties of the Citric Acid Cycle.gif|Amphibolic properties of the citric acid cycle|thumb|506x506px]]

An '''[[amphibolic|amphibolic pathway]]''' is one that can be either catabolic or anabolic based on the availability of or the need for energy.<ref name="Clarke"/>{{rp|570}} The currency of energy in a biological cell is [[adenosine triphosphate]] (ATP), which stores its energy in the [[High-energy phosphate|phosphoanhydride bonds]]. The energy is utilized to conduct biosynthesis, facilitate movement, and regulate active transport inside of the cell.<ref name="Clarke"/>{{rp|571}} Examples of amphibolic pathways are the citric acid cycle and the glyoxylate cycle. These sets of chemical reactions contain both energy producing and utilizing pathways.<ref name="Voet, Voet, Pratt"/>{{rp|572}} To the right is an illustration of the amphibolic properties of the TCA cycle.

The [[glyoxylate cycle|glyoxylate shunt pathway]] is an alternative to the [[Citric acid cycle|tricarboxylic acid (TCA) cycle]], for it redirects the pathway of TCA to prevent full oxidation of carbon compounds, and to preserve high energy carbon sources as future energy sources. This pathway occurs only in plants and bacteria and transpires in the absence of glucose molecules.<ref>{{cite book| veditors = Pray L, Relman DA, Choffnes ER <!--rapporteurs, Forum on Microbial Threat, Board on Global Health, Institute of Medicine of the National Academies-->|title=The science and applications of synthetic and systems biology workshop summary|date=2011|publisher=National Academies Press|location=Washington, D.C.|isbn=978-0-309-21939-6|page=135}}</ref>