Acronym for the tricarboxylic acid or Kreb's cycle(after the Sheffield scientist who discovered it). It converts citric acid, through a series of reactions to a four carbon sugar, which is then acetylated back to citric acid again. In the process, NAD and FAD are reduced (as well as ADP and GDP phosphorylated) so that oxidative phosphorylation can be carried out.

This not only provides reduced cofactors for the cell, but many amino acids are synthesised from the intermediates of the cycle.It is therefore the centre of the cell's metabolic net, with compounds feeding in and being taken out at all stages.