Page 26 - 11-Bio-14 Transport

14. Transport eLearn.Punjab

short distance transport (involving 2 - 3 cells). Thus sucrose diffuses through plasmodesmata to
sieve tube cell or sieve element, raising the concentration of sucrose in it. (Fig. 14.12) The pathway
taken by sucrose is symplast in most cases; but is some, apoplastic movement does take place.
The sucrose is actively transported to the sieve elements. (3) The water moves by osmosis from the
nearby xylem in the leaf vein. This increases the hydrostatic pressure of the sieve tube element.

(4) Hydrostatic pressure moves the sucrose and other substances in the sieve tube cells, and moves
to sinks e.g. fruits and roots. In the storage sinks, such as sugar beet root and sugarcane stem,
sucrose is removed into apoplast prior to entering symplast of the sink.

(5) Water moves out of sieve tube cell by osmosis, lowering the hydrostatic pressure.

In symplastic pathway, sucrose (or sugars) move through plasmodesmata to the receiver cell. Thus
according to pressure flow theory, the pressure gradient is established as a consequence of entry
of sugars in the sieve elements at the source; and removal of sugars (sucrose) at the sink (Fig.
14.13). The energy driven entry of sugars in sieve tube elements, generate high osmotic pressure
in the sieve tube elements of the source causing a steep drop in the water potential.

(6) The presence of sieve plates greatly increases the resistance along the pathway and results in
the generation and maintenance of a substantial pressure gradient in the sieve elements between
source and sink.

The sieve element’s contents are physically pushed along the transportation pathway by bulk flow,
much like water flowing through a garden hose.

The pressure flow theory accounts for the mass flow of molecules within phloem. It may be noted
that the transpertation of photosynthate or carbohydrates from the mesophyll cells to phloem
tissue involves diffusion and active transport (carrier mediated transport). Then in phloem tissue
(sieve tubes) the movement of materials is according to pressure flow theory.

Again in the sink cells when the sugar and the carbohydrates are passed from the phloem tissue,
diffusion and carrier mediated transport, either passive or active, takes place, (see table 14.1).

26 V: 1.1