Editing Metabolic pathway (section)

===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>