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 potential (Ψ_w)↓.

-   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 Ψ_w 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