BIOL10004 Chapter Notes - Chapter 18: Vascular Bundle, Electrochemical Gradient, Water Potential
29 Jun 2018
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Translocation of Assimilates
Raw materials for growth of plants come from transport of assimilates
oEnd product of photosynthesis
oMain sources of assimilates are leaves
oNon-photosynthetic tissues act as sinks
oImporters of assimilates (sugars)
oStorage organ, imports and exports carbohydrates
Can act as both source and sink
oSinks supplied by nearest source
Pathway of assimilate transport
Assimilated transport
oMoves down the plant
oPhloem is assimilate-transporting tissue
Sinks to which assimilates moved are out the roots
What substances are transported in phloem
Phloem sap collected from cut stems
oConcentrated solution of sucrose
Concentration of 0.5 moles
oSucrose is a non-reducing sugar
Rate of phloem transport
Rate of phloem transport
Measured by tracing speed of pulse of radioactively labelled assimilates
Transport in phloem involves living cells
Rate of transport sensitive to temperature
Concentration of sucrose and hydrostatic pressure greater near source than at sink
oGradient exists
Mass-flow mechanism of transport
Transport of sugars from a source
Mesophyll cells of a leaf to sink
Involves 3 processes
1. Phloem loading
Transport from mesophyll to sieve tubes in veins
Involve step requiring metabolic energy
Pumps sucrose against concentration gradient
Many crop plats require polymerisation of smaller sucrose molecules
Into larger ones
Cannot diffuse back through plasmodesmata
2. Long-distance transport of sucrose
In stem
In root phloem
3. Phloem unloading
Transport from sieve tubes
To cells at root tip
Pathways of loading sucrose
1. From photosynthesising mesophyll cell to sieve tubes
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Document Summary
Raw materials for growth of plants come from transport of assimilates o. End product of photosynthesis: main sources of assimilates are leaves, non-photosynthetic tissues act as sinks o o o. Assimilated transport: moves down the plant o. Sinks to which assimilates moved are out the roots. Phloem sap collected from cut stems o o. Measured by tracing speed of pulse of radioactively labelled assimilates. Concentration of sucrose and hydrostatic pressure greater near source than at sink: gradient exists. Mesophyll cells of a leaf to sink. Transport from mesophyll to sieve tubes in veins. Many crop plats require polymerisation of smaller sucrose molecules. Cannot diffuse back through plasmodesmata: long-distance transport of sucrose. 7: show numerous plasmodesmata connections with adjacent mesophyll cells. Concentration of sugars in photosynthesising cell is 50mm. Plants with apoplastic pathway, loading sucrose is accompanied by ph increase. Indicates that sucrose uptake is associated with proton transport. Driving force for active movement is membrane-bound atpase.
