Phloem Loading

Phloem loading is the process by which the sugars made by photosynthesis move out of the mesophyll cells of a mature leaf and into the sieve elements of the phloem, ready for long-distance transport, and it proceeds in three linked steps. First, because sieve tubes are only able to conduct sucrose, the starch or triose-phosphate that is actually synthesised inside the chloroplast during photosynthesis must first be converted, in the surrounding cytoplasm, into sucrose. Second, this newly made sucrose then moves the short distance from the mesophyll cells into the nearby sieve elements of the leaf's minor veins. Third, once inside the sieve t …