Describe in details and give reference to the assignment, Phloem loading from chloroplast to sieve

Answer 1

Phloem loading is the process by which sugars produced in the leaves during photosynthesis are transported to other parts of the plant, such as the roots, stems, and flowers. One of the mechanisms of phloem loading involves the movement of sugars from the chloroplasts to the sieve elements of the phloem.

During photosynthesis, sugars are produced in the chloroplasts of the leaf cells. These sugars are then transported to the companion cells located next to the sieve elements in the phloem. The loading of sugars into the phloem involves several steps, including the active transport of sugars across the cell membrane and the establishment of a concentration gradient that drives the movement of sugars into the sieve elements.

One of the key players in the phloem loading process is the enzyme sucrose phosphate synthase (SPS), which catalyzes the conversion of sucrose into sucrose-phosphate. This conversion is thought to be important for the movement of sugars from the chloroplasts to the sieve elements.

Several studies have examined the role of chloroplasts in phloem loading, including a study by Rennie and Turgeon (2009) that demonstrated a direct connection between the chloroplasts and the sieve elements in the phloem. This study provided evidence that sugars produced in the chloroplasts are transported directly to the sieve elements, bypassing the companion cells.

Overall, phloem loading from chloroplast to sieve is a complex process that involves the coordination of various cellular and molecular mechanisms. Further research is needed to fully understand the details of this process and its regulation in plants.

Reference:
Rennie, E. A., and Turgeon, R. (2009). A comprehensive picture of phloem loading strategies. Proceedings of the National Academy of Sciences of the United States of America, 106(37), 14162-14167.