9.4 Translocation

Translocation

Energy-Requiring (active) process transporting assimilates, particularly sucrose between the sources (e.g. leaves) and sinks (e.g. roots, meristem)

Sources

• Green leaves and stems

• Storage organs (tubers, tap roots)

  • Unload their stores at beginning of growth period

• Food stores in seeds when they germinate

Sinks

• Roots that are growing and/or actively absorbing mineral ions

• Meristems that are actively dividing

• Any parts of the plan that are laying down food stores e.g. developing seeds, fruits, or storage organs

Why is sucrose the main carb transported rather than glucose?

Not used in metabolism as readily as glucose so is therefore less likely to be metabolized during transport process

• Is also more soluble

Phloem loading - Symplast route (conc. of sucrose low)

• Passive route (uses diffusion)

• Sucrose moves thru plasmodesmata down conc. g.

• Sucrose ends up in sieve elements & water follows by osmosis

• Creates a pressure of water that moves sucrose thru phloem by mass flow

Phloem loading - Apoplast Route 

In companion cells, sucrose moved into cytoplasm across CSM in an active process

1. Proton Pump actively pumps H⁺  out of companion cell into surrounding tissue using ATP 

2. H⁺ move down conc. gradient & return to companion cell w. sucrose via a co-transporter protein 

3. Sucrose diffuses into sieve tube elements

Adaptations of companion cells

• Many infoldings in cell membranes, gives increased SA for active transport of sucrose into cell cytoplasm

• Many mitochondria, supplies ATP needed for transport pumps

What happens as a result of built up sucrose in companion cell & sieve tube element?

Water moves in by osmosis

• Build up of turgor pressure 

• Water carrying assimilates moves into tubes of sieve elements

  • Reduces pressure in companion cells

• Water moves up or down plant by mass flow to areas of lower pressure (sinks)