Editing Glycolysis (section)

===Conversion of pyruvate into oxaloacetate for the citric acid cycle===
Pyruvate molecules produced by glycolysis are [[active transport|actively transported]] across the inner [[Mitochondrion|mitochondrial]] membrane, and into the matrix where they can either be [[Redox|oxidized]] and combined with [[coenzyme A]] to form {{chem2|CO2}}, acetyl-CoA, and NADH,<ref name=voet /> or they can be [[carboxylated]] (by [[pyruvate carboxylase]]) to form [[oxaloacetate]]. This latter reaction "fills up" the amount of oxaloacetate in the citric acid cycle, and is therefore an [[anaplerotic reaction]] (from the Greek meaning to "fill up"), increasing the cycle's capacity to metabolize acetyl-CoA when the tissue's energy needs (e.g. in [[Cardiac muscle|heart]] and [[Skeletal striated muscle|skeletal muscle]]) are suddenly increased by activity.<ref name=stryer3>{{cite book | vauthors = Stryer L | title = Biochemistry |chapter= Citric acid cycle. |edition= Fourth |location= New York |publisher= W.H. Freeman and Company|date= 1995 |pages= 509–527, 569–579, 614–616, 638–641, 732–735, 739–748, 770–773 |isbn= 0-7167-2009-4 }}</ref>
In the [[citric acid cycle]] all the intermediates (e.g. citrate, iso-citrate, alpha-ketoglutarate, succinate, fumarate, malate and oxaloacetate) are regenerated during each turn of the cycle. Adding more of any of these intermediates to the mitochondrion therefore means that that additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other. Hence the addition of oxaloacetate greatly increases the amounts of all the citric acid intermediates, thereby increasing the cycle's capacity to metabolize acetyl CoA, converting its acetate component into {{chem2|CO2}} and water, with the release of enough energy to form 11 [[Adenosine triphosphate|ATP]] and 1 [[Guanosine triphosphate|GTP]] molecule for each additional molecule of acetyl CoA that combines with oxaloacetate in the cycle.<ref name=stryer3 />

To cataplerotically remove oxaloacetate from the citric cycle, [[malate]] can be transported from the mitochondrion into the cytoplasm, decreasing the amount of oxaloacetate that can be regenerated.<ref name=stryer3 /> Furthermore, citric acid intermediates are [[Citric acid cycle#Citric acid cycle intermediates serve as substrates for biosynthetic processes|constantly used to form a variety of substances such as the purines, pyrimidines and porphyrins]].<ref name=stryer3 />