Translocation

Translocation                                                                                         

• Sucrose is loaded in to the phloem by active processes
• ATP is used by companion cells to actively transport H+ out of thier cytoplasm and in to surrounding tissue, setting up a diffusion gradient
• H+ diffuse back in to the companion cells using cotransporter proteins which allow H+ to bring sucrose in to the companion cell
• As conc. of sucrose builds up inside the companion cell, they diffuse in to the sieve tube elements via plasmodesmata

Source (releases sucrose in to phloem); sucrose entering sieve tube reduces WP, as a result water moves in by osmosis increasing hydrostatic pressure

Sink (removes sucrose from the phloem); sucrose is used in cells surrounding the phloem for respiration etc. which reduces sucrose conc. Sucrose diffuses in to surrounding cells, increasing WP in sieve tube so water moves in to surrounding cells by osmosis. This reduces hydrostatic pressure in the phloem at the sink.

Translocation                                                                                                               

• Sucrose is loaded in to the phloem by active processes
• ATP is used by companion cells to actively transport H+ out of thier cytoplasm and in to surrounding tissue, setting up a diffusion gradient
• H+ diffuse back in to the companion cells using cotransporter proteins which allow H+ to bring sucrose in to the companion cell
• As conc. of sucrose builds up inside the companion cell, they diffuse in to the sieve tube elements via plasmodesmata

Source (releases sucrose in to phloem); sucrose entering sieve tube reduces WP, as a result water moves in by osmosis increasing hydrostatic pressure

Sink (removes sucrose from the phloem); sucrose is used in cells surrounding the phloem for respiration etc. which reduces sucrose conc. Sucrose diffuses in to surrounding cells, increasing WP in sieve tube so water moves in to surrounding cells by osmosis. This reduces hydrostatic pressure in the phloem at the sink.