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source-to-sink (1 Viewer)
- Thread starter beiiishey
- Start date
Lori.
Member
- Joined
- May 29, 2006
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- Female
- HSC
- 2006
Translocation is a bit confusing, because it isnt yet known exactly how materials are moved in phloem.
There are two theories;
One explains translocation as the Pressure-Flow system (this is copied from HSC online - hsc.csu.edu.au):
"The pressure-flow mechanism (or Source to Sink) is a model for phloem transport now widely accepted.
The model has the following steps.
Step 1: Sugar is loaded into the phloem tube from the sugar source, e.g. the leaf (active transport)
Step 2: Water enters by osmosis due to a high solute concentration in the phloem tube. Water pressure is now raised at this end of the tube.
Step 3: At the sugar sink, where sugar is taken to be used or stored, it leaves the phloem tube. Water follows the sugar, leaving by osmosis and thus the water pressure in the tube drops.
The building up of pressure at the source end, and the reduction of pressure at the sink end, causes water to flow from source to sink. As sugar is dissolved in the water, it flows at the same rate as the water. Sieve tubes between phloem cells allow the movement of the phloem sap to continue relatively unimpeded. "
The other theory states that the sieve and companion cells generate the moving force of the materials, so they actively move the materials by diffusing them through the sieve-plates/cells.
There are two theories;
One explains translocation as the Pressure-Flow system (this is copied from HSC online - hsc.csu.edu.au):
"The pressure-flow mechanism (or Source to Sink) is a model for phloem transport now widely accepted.
The model has the following steps.
Step 1: Sugar is loaded into the phloem tube from the sugar source, e.g. the leaf (active transport)
Step 2: Water enters by osmosis due to a high solute concentration in the phloem tube. Water pressure is now raised at this end of the tube.
Step 3: At the sugar sink, where sugar is taken to be used or stored, it leaves the phloem tube. Water follows the sugar, leaving by osmosis and thus the water pressure in the tube drops.
The building up of pressure at the source end, and the reduction of pressure at the sink end, causes water to flow from source to sink. As sugar is dissolved in the water, it flows at the same rate as the water. Sieve tubes between phloem cells allow the movement of the phloem sap to continue relatively unimpeded. "
The other theory states that the sieve and companion cells generate the moving force of the materials, so they actively move the materials by diffusing them through the sieve-plates/cells.