  Plants create sugar in the mesophyll cells of their leaves by photosynthesis.
This sugar, mostly sucrose, has to be loaded via the bundle sheath into the
phloem vascular system (the sieve elements), where it is distributed to growing
parts of the plant. We analyze the feasibility of a particular loading
mechanism, active symplasmic loading, also called the polymer trap mechanism,
where sucrose is transformed into heavier sugars, such as raffinose and
stachyose, in the intermediary-type companion cells bordering the sieve
elements in the minor veins of the phloem. Keeping the heavier sugars from
diffusing back requires that the plasmodesmata connecting the bundle sheath
with the intermediary cell act as extremely precise filters, which are able to
distinguish between molecules that differ by less than 20% in size. In our
modeling, we take into account the coupled water and sugar movement across the
relevant interfaces, without explicitly considering the chemical reactions
transforming the sucrose into the heavier sugars. Based on the available data
for plasmodesmata geometry, sugar concentrations and flux rates, we conclude
that this mechanism can in principle function. We find that the water flow
through the plasmodesmata, which has not been quantified before, contributes
only 10-20% to the sucrose flux into the intermediary cells, while the main
part is transported by diffusion. On the other hand, the subsequent sugar
translocation into the sieve elements would very likely be carried
predominantly by bulk water flow through the plasmodesmata. Thus, in contrast
to apoplasmic loaders, all the necessary water for phloem translocation would
be supplied in this way with no need for additional water uptake across the
plasma membranes of the phloem.
