Wednesday, October 5, 2011

Translocation in Plants

(A) Mass-Flow/Pressure Flow Hypothesis:

 1.   The mass-flow/pressure flow hypothesis:
-   postulates that dissolved sugar moves in phloem
-   by mean of pressure gradient
-   which exists between the source and sink.
2.   The photosynthetic cells in:
      -   leaves = common source of sugars
      -   roots = sinks.
3.   At the leaves
-   sucrose is actively transported
-   from mesophyll cells
-   to companion cell
-   into sieve tube
-   against its concentration gradient
-   process = phloem loading.
-   high conc of sucrose à lowers cell water potentialw)↓.
-   water - drawn into sieve tube
-   from nearby xylem vessel
-   creating a high hydrostatic pressure (HP)
-   forces the bulk/mass flow of the phloem sap
-   towards the sink.

Phloem loading and unloading of sucrose

4.   At the root:
-   sucrose is actively transported
-   from the sieve tube
-   into the companion cell
-   into a root cell.
-   process = phloem unloading.
5.   Loading (at the source) and unloading of sugar (at the sink)
      - require energy derived from ATP.

(B) In Electro-Osmosis Mechanism:
  • potential diff develops across sieve plate
  • by companion cell (actively transport K+ into sieve tube).
  • K+ accumulate at one end of sieve plate 
  • creates a potential diff between sieve plate.
  • caused K+ speed across sieve plate
  • water + dissolved sucrose follow (attracted by +ve charge).
  • water in phloem moves by osmosis 
  • as accumulation of K+ lower Ψw in sieve tube (compared to next cell).

K+ accumulate at one end of sieve plate creates a potential diff between sieve plate

(C) In Cytoplasmic Streaming Mechanism:
  • water + dissolved compounds (in phloem sap) 
  • move + circulate together
  • in one direction (in sieve tube)
  • it’s slow + depends on metabolic energy/due to their kinetic energy.
  • circulation slow down at sieve plate
  • and forced out from cytoplasmicc streaming (thru pores)
  • to cytoplasmic streaming of next sieve tube.

Circulation slow down at sieve plate

(D) In Peristaltic Wave Mechanism:
  • sieve tube is filled with fine cytoplasmic filaments
  • continuous from sieve tube to the next
  • thru pores of sieve plate.
  • contain phloem sap tube constrict + relax alternately
  • pushing sap from one sieve tube to the next.
  • constriction + relaxation/peristaltic movement form a pattern of wave = peristaltic wave
  • can be at diff speed + in opposite direction (in sieve tube)
  • depends on metabolic energy/ATP.

Phloem sap tube constrict + relax alternately

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