Anaplerotic reactions, a term coined by Hans Kornberg and originating from the Greek [http://en.wiktionary.org/wiki/%E1%BC%80%CE%BD%CE%AC#Ancient_Greek ἀνά]= up and [http://en.wiktionary.org/wiki/%CF%80%CE%BB%CE%B7%CF%81%CF%8C%CF%89 πληρόω]= to fill, are chemical reactions that form intermediates of a metabolic pathway. Examples of such are found in the citric acid cycle (TCA cycle). In normal function of this cycle for respiration, concentrations of TCA intermediates remain constant; however, many biosynthetic reactions also use these molecules as a substrate. Anaplerosis is the act of replenishing TCA cycle intermediates that have been extracted for biosynthesis (in what are called anaplerotic reactions).
The TCA cycle is a hub of metabolism, with central importance in both energy production and biosynthesis. Therefore, it is crucial for the cell to regulate concentrations of TCA cycle metabolites in the mitochondria. Anaplerotic flux must balance cataplerotic flux in order to retain homeostasis of cellular metabolism.