Photosynthesis by Isolated Pea Chloroplasts

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Abstract
When added singly to chloroplasts isolated from young pea (Pisum sativum) leaves, both PPi and small quantities (0.2 mM) of ADP inhibit photosynthesis, but when added together they cause a marked stimulation. ATP (at 0.2 mM) is less inhibitory (or not inhibitory) when added alone, but like ADP, stimulates when added in the presence of PPi. This behavior is in contrast to that of spinach (Spinacia oleracea) chloroplasts which are normally stimulated rather than inhibited by PPi and which are largely unresponsive to exogenous adenylates. The inhibitory behavior of PPi with pea chloroplasts was observed under conditions where external hydrolysis to Pi is negligible. The exchange of organic and PPi across the chloroplast envelope may be more rapid in chloroplasts from young pea leaves than in chloroplasts from spinach and interaction between these 2 processes could account for the principal observations.