Question

Arrange the following events in a correct order that explains the mass flow of materials in the phloem.

(I) Water diffuses into the sieve tube elements.
(II) Cells of the leaves produce sugar by photosynthesis.
(III) Solutes are actively transported into the sieve elements.
(IV) Sugar is transported from cell to cell in the leaf.
(V) Sugar moves down the stem.

• A. (ii) – (iv) – (iii) – (i) – (v)
• B. (ii) – (iv) – (i) – (iii) – (v)
• C. (i) – (ii) – (iii) – (iv) – (v)
• D. (iv) – (ii) – (i) – (iii) – (v)

Answer 1

The correct option is A (ii) – (iv) – (iii) – (i) – (v)

As per Munch's mass flow hypothesis, organic substances flow in a solution form from sieve elements due to the development of osmotically generated pressure gradient between source and a sink.
Mesophyll cells synthesise organic substances which are loaded into the sieve tube elements. They absorb water from the near by xylem elements. It produces a high turgor pressure that causes the flow of substances to the region of low turgor pressure which is the stem. This movement goes on till it reaches the sink where the food is stored.

So the correct order of the events that take place in the mass flow (translocation) in the phloem is: (ii) - (iv) - (iii) - (i) - (v).

(ii) Cells of the leaves produce sugar by photosynthesis.
(iv) Sugar is transported from cell to cell in the leaf.
(iii) Solutes are actively transported into the sieve elements.
(i) Water diffuses into the sieve tube elements.
(v) Sugar moves down the stem.