C photosynthesis (also called glycine shuttle and photorespiratory CO pump) is a CCM that works by making use of &ndash; as opposed to avoiding &ndash; photorespiration. It performs carbon refixation by delaying the breakdown of photorespired glycine, so that the molecule is shuttled from the mesophyll into the bundle sheath. Once there, the glycine is decarboxylated in mitochondria as usual, releasing CO and concentrating it to triple the usual concentration.
Although C photosynthesis is traditionally understood as an intermediate step between C and C, a wide variety of plant lineages do end up in the C stage without further evolving, showing that it is an evolutionary steady state of its own. C may be easier to engineer into crops, as the phenotype requires fewer anatomical changes to produce.