Question

explain the 3 steps of the source and sink hypothesis

Answer 1

Answer:

The source and sink hypothesis is the concept, which explains relationship of source- sink that is involved in the translocation of photosynthetic products in plants.

In plants there is a movement of various substances (sucrose, amino acids, vitamins, etc) from regions of supply (source) to a region of utilization / storage (sink).

Substances translocation mainly takes place in phloem bi- directionally always source to sink.

The amount of sugar translocated in phloem is relatively high and transport depends on hydrostatic pressure.

3 major steps mainly involved translocation pathway:

1. Phloem loading :

Sucrose produce in the mesophyll cells (source) is actively loaded using ATP into the sieve tubes in the phloem of the vein-let of the leaf through transfer cells. This is called Phloem loading.

2. Mass flow: In this process, phloem increases solute potential of the sieve tube and decreases water potential of the sieve tube. Then the water enters into sieve tubes by endosmosis from the xylem. Therefore it creates high hydrostatic pressure. Sucrose solution then transported to storage organ / growing point (sink) actively / passively by mass flow.

3. Phloem unloading: At this storage organ / growing point (sink) actively through transfer cells is called Phloem unloading.

Source Mesophyll cells Sink storage organ Sink growing point

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