Different models for sugar transport have been proposed through the years. Look through different editions of college texts to get some idea of the diversity of ideas. In the Eurekalerts, there is an announcement about a paper in this week’s PNAS with strong evidence supporting one specific hypothesis:
The theory of transporting sugar, the polymer trap model, was first proposed in 1991 by Robert Turgeon, Cornell professor of plant biology. He is also the senior author of the latest research published in the Dec. 4 issue of the Proceedings of the National Academy of Sciences. Ashlee McCaskill, Ph.D. ’07, who worked in Turgeon’s lab, is the paper’s lead author.
Turgeon’s theory suggested that as sucrose, a form of sugar, accumulates in leaves as a product of photosynthesis, it diffuses into the plant’s tubelike transport tissue, called phloem, along with other nutrients to move to other areas of the plant. Once in the phloem, small molecules of sucrose polymerize, or combine, to form larger, more complex sugars, which become too large to flow back into the leaf. The polymerized sugars are then forced to move away from the leaf to parts of the plant where they may be used or stored.
To prove the theory, Turgeon and McCaskill genetically engineered a plant closely related to a member of the figwort family, purple mullein (Verbascum phoeneceum L.), so that two genes involved with polymerizing sucrose into larger molecules were silenced. When they did so, sugars backed up in the leaves.
AP teachers in particular will want to be aware of this paper and model for transport.