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Yes in entire system no in single sieve element should

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Unformatted text preview: of water leading to osmosis of water OUT of the phloem and maintenance of the pressure gradient leaf sale outload this is a root sale inload • Bidirectional? – Yes, in entire system – No, in single sieve element should be a companion cell here Translocation Phloem Loading • Photosynthate from mesophyll to sieve elements – Triose-phosphate transported from chloroplast (Calvin-Benson-Bassham cycle) to cytosol and converted to sucrose – Sucrose then moves (diffuses along gradient) from mesophyll leaf tissues to vicinity of sieve element – Sucrose actively transported into sieve element Translocation Phloem Loading: 2 or 3 ways Apoplastic loading Movement around, not through membranes Apoplastic Symplastic diffusion Symplastic loading Movement through membranes and plasmodesmata Translocation Phloem Loading: apoplastic • Herbaceous species • Requires energy – Sugar 1.5 to 3X more concentrated in sieve element than in mesophyll or companion cells – Sucrose loading is driven by a proton gradient using ATP cell wall Plasma mem http://www.hort.purdue.edu/hort/courses/HORT301/ Translocation Phloem Loading: symplastic • Passive symplastic (e.g. apple): – Mesophyll produces and maintains high sugar concentrations driving diffusion and rest of translocation pathway • Symplastic with polymer trapping: – Sucrose (small) diffuses to companion cell – Lo...
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