Translocation

What is translocation

The transport of assimilates such as sucrose from a source e.g. leaves to a sink e.g. roots via the phloem

Why does water move from xylem to phloem

Phloem has high sucrose concentration, as it is actively loaded into phloem sieve tube elements , so there is a low water potential in phloem meaning water moves from xylem to phloem via osmosis

How is sucrose loaded into phloem

1. Companion cells use ATP to actively transport H⁺ ions out of their cytoplasm via proton pumps

2. This creates a H⁺ diffusion gradient

3. H⁺ ions will then move back into the companion cell down their gradient via a co-transporter protein

4. As H⁺ ions move back in through co-transporter proteins, it drives the transport of sucrose, which is in the leaves, into cell at the same time

5. Sucrose conc. builds up in companion cells so diffuse into sieve tube element via plasmodesmata

Movement of sucrose in phloem

• At source sucrose enters sieve tube element, so water potential is reduced. Water is then drawn into sieve tube element by osmosis which increases hydrostatic pressure forcing sap to move alone sieve tube

• At sink sucrose is used by cells to be converted to starch or used in respiration meaning water potential increases, so water is drawn out of sieve tube element by osmosis which lowers hydrostatic pressure

What is mass flow

Mass flow is the bulk movement of phloem sap (water + sucrose) from a region of high hydrostatic pressure to a region of low hydrostatic pressure. Essentially the loading and unloading of sucrose

What evidence is there for mass movement

• Through microscopy we can see companion cells adaptations for active transport

• If mitochondria of companion cells are poisoned translocation stopped

• Flows of sugar is 10,000 times faster than if it was diffusion alone