A: Phloem transports food (sucrose) from the source to the sink in the form of sap. Source (leaf) and sink (other parts and storage organs) relationship is variable. Phloem sap consists water, sucrose, other sugars, hormones and amino acids. The translocation in phoem is bidirectional as source-sink relationship
is variable. The transport in phloem is explained by pressure flow hypothesis. It was proposed by Munch.
It is also called Mass flow hypothesis. It occurs in 2 steps.
(1) Loading (2) Unloading.
Loading of Phloem:
1. Glucose in the mesophyll cells (source) is converted to sucrose.
2. Sucrose moves to companion cells.
3. It moves from there to sieve tube cells by active transport.
4. Concentration of sap in these cells become hypertonic.
5. Water from Xylem cells moves by osmosis into sieve tube cells.
6. Mass and Pressure increase.
Unloading:
1. As mass increases in sieve tube cells, pressure flow begins.
2. Sugars in the sink are used up.
3. Sap in the cells become hypotonic.
4. Water moves out of the phoem.
5. Water potential gradient is established between sink and source.
6. Sugars are unloaded. They move into the cells which use sugar.
No comments:
Post a Comment