CO₂ Assimilation by Chloroplasts illuminated on Filter Paper

Abstract

CHLOROPLAST suspensions from Spinacea oleracea or Pisum sativum assimilate CO₂ in the light at rates comparable with those observed with intact leaves^1–3. With most preparations the major photosynthetic products are phosphoglyceric acid (PGA) and dihydroxyacetone phosphate (DHAP)^2,4,5. Although the synthesis of these compounds is consistent with the operation of the photosynthetic carbon reduction (PCR) cycle⁶ in the isolated plastids it is still not clear why these particular intermediates accumulate. It has been shown⁵ that the outer membranes of the chloroplasts are permeable to both PGA and DHAP. Thus a large proportion of the carbon fixed into these compounds may diffuse into the external medium. The amount of carbon lost from the chloroplasts will presumably depend on the ratio of the volume of external medium to that of the aqueous phase of the chloroplasts. For chloroplast suspensions containing 100 µg chlorophyll per ml. this ratio would be over 100 : 1, contrasting markedly with the fluid environment of the chloroplasts in vivo, where the ratio of the volume of non-chloroplastic cytoplasm to that of the chloroplasts is usually less than one. It seemed possible, therefore, that more of the PGA and DHAP formed might be metabolized further if the volume of medium in the experimental sample was reduced to a minimum by layering the chloroplasts on filter paper.