79. Which one of the following metabolites formed during Calvin-Benson cycle in chloroplast is involved in starch biosynthesis and can also be transported to cytosol?
(1) Triose phosphate
(2) Glyceraldehyde-3 phosphate
(3) Fructose-6-phosphate
(4) Ribulose 1,5-bisphosphate

Introduction

The Calvin-Benson cycle generates various sugar phosphates in plant chloroplasts. Among these, triose phosphate stands out for its central dual role: it serves as a building block for starch within the chloroplast and also moves to the cytosol for further metabolism into sucrose.

Explanation of Each Option

(1) Triose phosphate

• Correct. Triose phosphate refers to three-carbon intermediates (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate) produced in the Calvin cycle. It is used for starch synthesis in the chloroplast and is the only metabolite efficiently transported to the cytosol via the triose phosphate/phosphate translocator for sucrose production.

(2) Glyceraldehyde-3-phosphate

• Although this is a type of triose phosphate, the correct transport terminology in plant physiology is “triose phosphate,” encompassing both glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

(3) Fructose-6-phosphate

• Incorrect. Fructose-6-phosphate is a hexose phosphate involved in subsequent steps of carbohydrate metabolism but is not the main metabolite exported from the chloroplast.

(4) Ribulose 1,5-bisphosphate

• Incorrect. This is the CO₂-acceptor molecule regenerated during the Calvin cycle and not used for direct carbohydrate biosynthesis or transport out of the chloroplast.

Why Option (1) Is Correct

Triose phosphate uniquely connects starch synthesis in chloroplasts and sucrose synthesis in cytosol, signifying its essential role in daytime photosynthesis and carbohydrate allocation.