75. The photosynthetic assimilation of atmospheric CO₂ by leaves yield sucrose and starch as end products of two gluconeogenic pathways that are physically separated. Which one of the following combination of cell organelles are involved in such physical separation of the process?
(1) Sucrose in cytosol and starch in mitochondria.
(2) Sucrose in chloroplasts and starch in cytosol.
(3) Sucrose in mitochondria and starch in cytosol.
(4) Sucrose in cytosol and starch in chloroplasts.

Introduction

In photosynthetic plant cells, the end products of CO₂ assimilation—sucrose and starch—are generated via two gluconeogenic pathways located in different cellular compartments. This compartmentalization is essential for coordinated carbohydrate production and export, optimizing plant growth and metabolism.

Explanation of Each Option

(1) Sucrose in cytosol and starch in mitochondria.

• Incorrect. Starch is synthesized and stored within chloroplasts, not mitochondria. Mitochondria are primarily involved in respiration, not carbohydrate storage.

(2) Sucrose in chloroplasts and starch in cytosol.

• Incorrect. Sucrose synthesis occurs in the cytosol after triose-phosphates are exported from chloroplasts, while starch is built up within the chloroplast stroma.

(3) Sucrose in mitochondria and starch in cytosol.

• Incorrect. Neither of these processes primarily involves mitochondria, and starch is never synthesized in the cytosol.

(4) Sucrose in cytosol and starch in chloroplasts.

• Correct. Triose-phosphates from photosynthetic carbon fixation are either directed toward starch synthesis in the chloroplast (the site of starch formation and storage) or exported to the cytosol, where they participate in the pathway leading to sucrose synthesis for translocation and metabolism.

Why Option (4) Is Correct

This answer matches the well-established cellular biochemistry of plant carbohydrate metabolism:

• Starch is synthesized and accumulated in the chloroplast stroma as a storage carbohydrate.

• Sucrose is synthesized in the cytosol and exported to other plant tissues for metabolism and/or storage.​

Cellular compartmentalization of these two pathways is a core principle in plant physiology and biochemistry, vital for academic and exam success in advanced plant sciences.