3
Question
Phloem unloading refers to
Phloem unloading refers to
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Solution
The correct option is B movement of solutes from companion cells into sink
Pressure flow hypothesis was proposed by Ernst Munch. It refers to the movement of substances in bulk or enmasse. Substances move like suspended silt moving through the flowing river. It is the accepted mechanism for the translocation of sugars. In the 1st phase, sugar produced in the leaves diffuse into the companion cells of the phloem through plasmodesmata. The remaining sucrose gets transported by active transport. The movement is not against the concentration gradient, but since sucrose is large in size, active transport is required.
In the 2nd phase, phloem loading occurs. It is the movement of sucrose from the companion cells to living sieve tube cells. ATP is consumed in this process, hence it is an active transport mechanism. The movement of sucrose occurs against the concentration gradient.
In the 3rd phase, water moves from xylem to phloem through osmosis. The 𝚿w is higher in xylem since there is less solute in the form of ions there. Phloem has a lot of solute, so 𝚿w is low. The active transport of sucrose into the phloem by companion cells makes the sap solution hypertonic. This causes the water to be drawn from the xylem via osmosis.
In the 4th phase, as the water flows from xylem to phloem, turgor pressure builds in the sieve tube cells, the phloem sap moves to areas of low pressure through perforated sieve plates.
In the 5th phase, phloem unloading occurs. The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc). It is done actively with the utilization of ATP. Sucrose is moved out of the phloem sap and into the cells that will use the sugar. Sugars moved out to the sink leads to the low osmotic pressure. Water moves out of the phloem back to the xylem.
Pressure flow hypothesis was proposed by Ernst Munch. It refers to the movement of substances in bulk or enmasse. Substances move like suspended silt moving through the flowing river. It is the accepted mechanism for the translocation of sugars. In the 1st phase, sugar produced in the leaves diffuse into the companion cells of the phloem through plasmodesmata. The remaining sucrose gets transported by active transport. The movement is not against the concentration gradient, but since sucrose is large in size, active transport is required.
In the 2nd phase, phloem loading occurs. It is the movement of sucrose from the companion cells to living sieve tube cells. ATP is consumed in this process, hence it is an active transport mechanism. The movement of sucrose occurs against the concentration gradient.
In the 3rd phase, water moves from xylem to phloem through osmosis. The 𝚿w is higher in xylem since there is less solute in the form of ions there. Phloem has a lot of solute, so 𝚿w is low. The active transport of sucrose into the phloem by companion cells makes the sap solution hypertonic. This causes the water to be drawn from the xylem via osmosis.
In the 4th phase, as the water flows from xylem to phloem, turgor pressure builds in the sieve tube cells, the phloem sap moves to areas of low pressure through perforated sieve plates.
In the 5th phase, phloem unloading occurs. The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc). It is done actively with the utilization of ATP. Sucrose is moved out of the phloem sap and into the cells that will use the sugar. Sugars moved out to the sink leads to the low osmotic pressure. Water moves out of the phloem back to the xylem.
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