78. The following statements were made with the assumption that the concentration of 3 phosphoglycerate is high inside chloroplasts of an actively photosynthesizing leaf.
A. There will be high concentration of triose phosphate in the chloroplast.
B. The activity of ADP-glucose pyrophosphorylase will be inhibited.
C. The carbon flow will be diverted from sucrose to starch.
D. Starch synthesis will be inhibited and carbon flow will be more towards sucrose synthesis.
Which one of the following combinations of above statements is correct?
(1) A and B                      (2) B and D
(3) C and D                         (4) A and C

Introduction

Photosynthetically active leaves dynamically manage metabolite levels and carbon flow. The concentrations of intermediates such as 3-phosphoglycerate (3-PGA) and triose phosphate inside the chloroplast determine whether carbon is directed into starch or sucrose, a process crucial for plant energy management and growth.

Explanation of Each Statement

A. There will be high concentration of triose phosphate in the chloroplast.

• Correct. High 3-PGA signals active Calvin cycle turnover, leading to elevated amounts of triose phosphate (glyceraldehyde-3-phosphate/DHAP) via the reduction phase.

B. The activity of ADP-glucose pyrophosphorylase will be inhibited.

• Incorrect. ADP-glucose pyrophosphorylase (AGPase) activity is actually activated by high concentrations of 3-PGA, promoting starch synthesis.

C. The carbon flow will be diverted from sucrose to starch.

• Correct. With abundant triose phosphate and 3-PGA in the chloroplast, more carbon will remain in the chloroplast for starch synthesis rather than being exported for sucrose synthesis in the cytosol.

D. Starch synthesis will be inhibited and carbon flow will be more towards sucrose synthesis.

• Incorrect. High 3-PGA leads to starch synthesis, not its inhibition; carbon is directed away from sucrose production in this context.

Why Option (4) A and C is Correct

• Both statements accurately reflect central regulatory principles in photosynthetic carbon metabolism: high 3-PGA promotes triose phosphate build-up and redirects carbon into starch via activation of ADP-glucose pyrophosphorylase.