The binding of bicarbonate ions to washed chloroplast grana.

Radioactive labelling techniques show that isolated broken chloroplasts can take up HCO3-in the dark. There are two pools of binding sites for this ion on, or within, the thylakoid membranes. A smaller, high affinity pool exists at a concentration of one HCO3-bound per 380-400 chlorophyll molecules. Removal of HCO3-bound in this pool requires special conditions and results in greater than 90% inhibition of oxygen evolution. The inhibition is fully reversed when HCO3-is added back. HCO3-bound in the small pool does not necessarily exchange with free HCO3-in the dark or in light. Evidence presented suggests that this site is very near the site of action of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea. A second, much larger, pool of HCO3-binding sites also exists in a concentration approaching that of the bulk chlorophyll. These sites have a much lower affinity for HCO3-, and their function has not yet been determined.