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Phloem Transport, Movement from sugar sources to Sugar Sinks, Bulk Flow by…
Phloem Transport
The products of photosynthesis are transported through phloem by process of translocation
Movement from sugar sources to Sugar Sinks
In angiosperms, seive-tube elements are channels for translocation
Phloem sap is an aqueous solutions that is high is sucrose
It travels from a sugar source to a sugar sink
Sugar sink is an organ that is a net consumer/depository of sugar, like roots, buds, and fruits
Sugar source is an organ that is a net producer of sugar, like mature leaves
Sugar must be loaded into seive-tube elements before being exported to sinks
depending on the species, sugar may move by symplastic and apoplastic pathways
Companion cells enhance solute covement between the apoplast and symplast
phloem loading requires active transport
proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose
at the sink, sugar molecules diffuse from the phleom to sink tissues and followed by water
Bulk Flow by Positive Pressure
Phloem sap moves through a seive tube by bulk flow driven by positive pressure called pressure flow
Phloem sap flows from sources, where pressure is high, to sinks, where pressure is low
A storage organ can be a sugar sink in the summer and sugar source in spring
