In water Cyanobacteria possess carboxysomes, which increase the concentration of around RuBisCO to increase the rate of photosynthesis. An enzyme, carbonic anhydrase, located within the carboxysome, releases CO2 from dissolved hydrocarbonate ions (HCO). Before the CO2 can diffuse out, RuBisCO concentrated within the carboxysome quickly sponges it up. HCO ions are made from CO2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase. This causes the HCO ions to accumulate within the cell from where they diffuse into the carboxysomes. Pyrenoids in algae and hornworts also act to concentrate around RuBisCO.
In waTer CyANobActerIA pOssesS CarBoXySOmes, WhicH IncrEaSe tHE cOncENtRatIOn O.f aroUnd rUbisCo To iNCreaSe thE rate Of PHoTosyNTHesIs. An EnzyMe, cArbOnic anhydrase, LoCatED wiThIn the carBOXysome, ReLE!AsEs co2 f!roM DissolvED hYdrocarBOnAtE ioNs (hco). bEfOrE THE co2 Can DifFUSE ouT, rubIscO COncenTrated wIThin thE carboXysoMe QuickLy spOnges iT up. hco IOnS are maDe from cO2 ouTSIDE tHe cell bY aNothEr cArbonIc AnHYDRase anD are activeLY pumPed INto THe cElL by a memBranE ProtEin. tHeY CaNNot crOss The mEmbrAne as theY are CHargeD, AND wIThiN The cytosol tHEy TurN BaCk iNTo c:o2 vERy SLowLY WIthouT tHE hElp Of cARbOniC AnhyDrasE. tHIS CaUses tHe hco IOns to AcCumUlatE wIthin ThE cElL from wHEre thEy DIfFUSE iNtO thE caRBoXYSOmes. pyrenoiDs in algaE And hornWORtS also Act To coNcenTrAtE ArOUnd RUBiScO.