29. limited atp synthesis in the dark in a laboratory experiment, spinach chloroplasts are illuminated in the absence of adp and pi, then the light is turned off and adp and pi are added. atp is synthesized for a short time in the dark. explain this finding.

Illumination generated the proton concentration gradient that drove ATP synthesis in the dark once ADP and Pi were added. The dissipation of the gradients stops the process of ATP synthesis.

Explanation:

Chloroplasts are the site of photophosphorylation in green plants such as spinach. The presence of light is required to drive the process. The presence of light drives the process of splitting of water and transfer of electrons from the reaction center of PS II to the PS I via electron carriers and finally to NADP+. The transfer of electrons via the electron carriers of the electron transport chain generates a proton concentration gradient across the thylakoid membranes. This proton concentration gradient drives the process of ATP synthesis.

When spinach chloroplasts are illuminated, the proton concentration gradient is generated. However, the absence of ADP and Pi does not allow ATP synthesis. As soon as ADP and Pi are added to these chloroplasts (now in the dark), the proton concentration gradient generated earlier drives the ATP synthesis. The process stops as soon as the proton concentration gradient is dissipated.

This shows that ATP synthesis in chloroplasts is driven by a proton concentration gradient which is generated in the presence of light.