77. From the following statements:
    (A) Triose phosphate is utilized for the synthesis of both starch and sucrose.
    (B) Triose phosphate is translocated to cytosol from chloroplast.
    (C) Triose phosphate is confined to chloroplast and is utilized for synthesis of starch only.
    (D) Triose phosphate is translocated from cytosol to chloroplast.
    Which one of the following combinations is correct regarding starch and sucrose synthesis during day time?

(1) A and B                  (2) B and C
(3) C and D                  (4) D and A

Introduction

Triose phosphate, a direct product of the Calvin cycle, serves as a pivotal intermediate in carbohydrate biosynthesis during photosynthesis. Its compartmentalization and transport define the pathways leading to either starch (chloroplast) or sucrose (cytosol) synthesis in plant cells.

Explanation of Each Statement

A. Triose phosphate is utilized for the synthesis of both starch and sucrose.

• Correct. Triose phosphates generated during the Calvin cycle can either be retained in the chloroplast for starch synthesis or exported to the cytosol for sucrose synthesis.

B. Triose phosphate is translocated to cytosol from chloroplast.

• Correct. During the day, some triose phosphates are exported from the chloroplast stroma to the cytosol via the triose phosphate/phosphate translocator, fueling sucrose biosynthesis.

C. Triose phosphate is confined to chloroplast and is utilized for synthesis of starch only.

• Incorrect. While some triose phosphate is retained for starch synthesis, a significant fraction is exported to the cytosol for sucrose formation; it is not exclusively confined to the chloroplast.

D. Triose phosphate is translocated from cytosol to chloroplast.

• Incorrect. The physiological direction is opposite—export is from the chloroplast to the cytosol, not vice versa.

Why Option (1) A and B is Correct

Both statements correctly describe the dual fate and export mechanism of triose phosphate, central to coordinating cellular carbohydrate production during photosynthetic periods in plants.