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A Handbook of Biology

Sugars leave sieve tube

for metabolism and

storage; water follows

by osmosis

=High

Phloem

turgor

pressure

Root

Sugars enter sieve tubes;

water follows by osmosis

Sugar solution flows

to regions of low

turgor pressure

Tip of stem

Sugars leave sieve tubes;

water follows by osmosis

Diagrammatic representation of phloem loading and unloading in mass-flow

hypothesis



Sucrose is moved into the companion cells and then into the living

phloem sieve tube by active transport (loading). It produces a hypertonic

condition in phloem (water potential decreases). Sieve tube cells form

long columns with holes in sieve plates. Cytoplasmic strands pass

through the holes in the sieve plates, so forming continuous filaments.

(AIPMT 2007)



Water in the adjacent xylem moves into the phloem by osmosis. As

osmotic pressure/hydrostatic pressure builds up, the phloem sap moves

to areas of lower osmotic pressure (sink).



The sucrose from the phloem sap actively moves into the cells. The

cells convert the sugar into energy, starch, or cellulose (complex

carbohydrates).



As sugars are removed, osmotic pressure decreases (water potential

increases) and water moves out of the phloem.

ÏDËÑTÏFÏÇÅTÏØÑ ØF THË TÏSSÜË THÅT TRÅÑSPØRTS FØØD (GÏRDLÏÑG ËXPËRÏMËÑT)



Carefully remove a ring of bark (including phloem layer) from a tree

trunk.



After a few weeks, the portion of the bark above the ring on the stem

becomes swollen. This is due to the absence of downward movement of

food.

(AIPMT 2015)



This shows that phloem is the tissue responsible for translocation of food;

and that transport takes place in one direction, i.e., towards the roots.