Question

Describe Loading and unloading of sieve tubes.

Answer 1

Translocation of organic
solutes such as sucrose (i.e.,
photosynthetic) takes place
through sieve tube elements
of phloem from supply end
(or source) to consumption
end (or sink). But, before this
translocation of sugars could
proceed, the soluble sugars
must be transferred from
mesophyll cells to sieve tube
elements of the respective
leaves.
This transfer of sugars
(photosynthetic) from
mesophyll cells to sieve tube
elements in the leaf is called
as phloem loading. On the
other hand, the transfer of
sugars (photosynthetic)
from sieve tube elements to
the receiver cells of
consumption end (i.e., sink
organs) is called as phloem
unloading. Both are energy
requiring processes.
Phloem Loading:
As a result of
photosynthesis, the sugars
such as sucrose produced in
mesophyll cells move to the
sieve tubes of smallest veins
of the leaf either directly or
through only 2-3 cells
depending upon the leaf
anatomy. Consequently, the
concentration of sugars
increases in sieve tubes in
comparison to the
surrounding mesophyll cells.