Translocation is the movement of dissolved substances (also called solutes or assimilates) through the phloem vessels. If you haven’t already, make sure to read about the structure of the phloem vessels here first so you’ve got a good grounding to take this topic further.
Source to Sink
Solutes move from a source to a sink. A source is anywhere that the solute is produced, for example sucrose is produced in leaves using glucose from photosynthesis. A sink is anywhere that the solute is used up, for example sucrose is converted to starch or to glucose (and fructose) by enzymes at areas such as the roots, meristems or fruit. The solute concentration is higher at the source than the sink, creating a concentration gradient from source to sink. However, some areas can be be both a source and a sink so don’t let that catch you out – this is partly why transport of solutes in the phloem goes in both directions rather than water transport in the xylem which only moves upwards.
Mass Flow Hypothesis
The mass flow hypothesis is just that – a hypothesis. It is currently the theory with the most evidence as to how translocation works, but research is ongoing. Science changes all the time! But let’s have a look at mass flow anyway, using sucrose as our example again.
Let’s begin at the source. For sucrose, this will predominantly be the leaves (but as I said in the previous section it could the roots if they are breaking down stores of starch). Sucrose first needs to be transported from the leaf tissue into the companion cells by active loading:
- H+ ions are actively transported from companion cells to the surrounding leaf tissue against a concentration gradient.
- H+ ions diffuse back into the companion cells down the concentration gradient through a co-transporter protein. To use the co-transporter protein, the H+ ions must bring sucrose with them. This is co-transport.
Loading from the companion cells into the sieve tubes themselves can actually happen by two methods (looking at two OCR A past papers I found a couple of conflicting exam questions – sort it out OCR – so best to know both). It can be passive or active. For passive loading, the sucrose simply diffuses from the companion cell into the sieve tube through plasmodesmata in the cell walls. Nice and simple. However, it can also be active, using much the same process as we have just looked at above. H+ ions are pumped out of the sieve tubes and into the companion cells by active transport. As the H+ ions diffuse back across, they bring sucrose with them using a co-transporter protein.
The loading of sucrose into the sieve tubes lowers the water potential in the sieve tubes, so water is drawn in by osmosis from the nearby xylem vessels and companion cells. The hydrostatic pressure (fluid pressure) is now high in the sieve tubes at the source.
At the sink, sucrose diffuses through plasmodesmata from the sieve tubes to the companion cells, then into the surrounding tissue where it is used up or stored. The water potential of the sieve tube increases because there is less sucrose, so water is forced out by osmosis into the xylem vessels. The hydrostatic pressure is low. The pressure gradient from source to sink is thought to be what drives translocation.
Well if nothing else you’ve probably learnt that plants are more complicated than they look! Here are some summary points to finish off this article:
- Translocation is the movement of solutes from source to sink through the phloem sieve tubes.
- Sucrose is actively loaded into companion cells using co-transport with H+ ions.
- There is a pressure gradient from source to sink which drives translocation. This is the mass flow hypothesis.