Looking for classes? Ksquare Career Institute, Bengaluru →
The translocation of organic solutes in sieve tube members is supported by -
A
Root pressure and transpiration pull
B
P-proteins
C
Mass flow involving a carrier and ATP
D
Cytoplasmic streaming
Detailed Solution
The accepted mechanism for the translocation of sugars from source to sink is the pressure flow or mass flow hypothesis.
At the source (for example a leaf), sucrose is moved into the companion cells and then into the sieve tube cells by active transport, using carrier proteins and the energy of ATP. This is phloem loading.
Loading makes the phloem sap hypertonic, so water enters the sieve tubes from the adjacent xylem by osmosis and builds up a high turgor pressure.
This pressure pushes the sap in bulk through the sieve tubes towards the sink, where the pressure is lower.
At the sink, the sugar is again removed actively (phloem unloading), and water moves out.
Root pressure and transpiration pull are concerned with the ascent of water in xylem.
So translocation in sieve tube members is supported by mass flow involving a carrier and ATP.
At the source (for example a leaf), sucrose is moved into the companion cells and then into the sieve tube cells by active transport, using carrier proteins and the energy of ATP. This is phloem loading.
Loading makes the phloem sap hypertonic, so water enters the sieve tubes from the adjacent xylem by osmosis and builds up a high turgor pressure.
This pressure pushes the sap in bulk through the sieve tubes towards the sink, where the pressure is lower.
At the sink, the sugar is again removed actively (phloem unloading), and water moves out.
Root pressure and transpiration pull are concerned with the ascent of water in xylem.
So translocation in sieve tube members is supported by mass flow involving a carrier and ATP.