Translocation in the Phloem. Phloem transport. A highly specialized process for redistributing: Photosynthesis products Other organic compounds (metabolites, hormones) some mineral nutrients Redistributed from SOURCE SINK. Phloem transport: Sources and sinks. Source :
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Translocation in the Phloem
A system of vascular tissue runs through all higher plants.
It evolved as a response to the increase in the size of plants, which caused an progressing separation of roots and leaves in space.
The phloem is the tissue that translocates assimilates from mature leaves to growing or storage organs and roots.
Photosynthesis provides a
New growth is a
Exactly what is transported in phloem?
The osmotic effect of a substance is tied to the number of particles in solution, so a millilitre of sucrose solution with the same osmolarity as glucose will be have twice the number carbon atoms and therefore about twice the energy.
Thus, for the same osmolarity, twice the energy can be transported per ml.
As a non-reducing sugar, sucrose is less reactive and more likely to survive the journey in the phloem.
Invertase (sucrase) is the only enzyme that will touch it and this is unlikely to be present in the phloem sieve tubes.
The mechanism of phloem transport
The Pressure-Flow Model
Translocation is thought to move at 1 meter per hour
Physiological process of loading sucrose into the phloem
Phloem and xylem are coupled in an osmotic system that transports sucrose and circulates water.
Physiological process of unloading sucrose from the phloem into the sink
The pressure-flow process
Build-up of pressure at thesource and release of pressure at the sink causes source-to-sink flow.
At the source phloem loading causes high solute concentrations. y decreases, so water flows into the cells increasing hydrostatic pressure.
At the sink y is lower outside the cell due to unloading of sucrose. Osmotic loss of water releases hydrostatic pressure.Xylem vessels recycle water from the sink to the source.