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.