The two CO2 transporters, NDH-I3 and NDH-I4, are based on
plastoquinine oxidoreductase NADPH dehydrogenase respiratory complexes.
Both CO2 transporters are present in β-cyanobacteria while α-
cyanobacteria possess only one of them or even lack them completely.
Since CO2 can cross the plasma membrane in a passive way, the main activity
of these transporters is hydration of CO2 by the CpX or CpY proteins
which perform carbonic anhydrase (CA)-like reactions