Symplastic loading is passive, and by definition does not require ATP for sucrose loading into the phloem. In apoplastic loaders, by contrast, ATP is required for sucrose loading into the phloem, which is provided through the Sucrose Synthase catalyzed breakdown of some of the sucrose actively symported into the sieve element companion cell (SE-CC) complexes. Is it then plausible that Sucrose Synthase would be found in the SE-CC complexes of apoplastic loaders and not in the SE-CC complexes of symplastic loaders?
Please see below the reference:
Daloso, D. M., Williams, T. C., Antunes, W. C., Pinheiro, D. P., Müller, C., Loureiro, M. E., & Fernie, A. R. (2016). Guard cell‐specific upregulation of sucrose synthase 3 reveals that the role of sucrose in stomatal function is primarily energetic. New Phytologist, 209(4), 1470-1483.
https://doi.org/10.1016/j.biotechadv.2015.11.003
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