Translocation: is the transport of assimilates throughout the plant, in the phloem tissue. A source releases sucrose into the phloem. A sink removes sucrose from the phloem.

How does sucrose enter the phloem? : Sucrose is loaded into the phloem by an active process. The companion cells use ATP to transport hydrogen ions out of their cytoplasm and into surrounding tissues. This sets up a diffusion gradient and the hydrogen ions diffuse back into the companion cells . This diffusion occurs through special cotransporter proteins. These proteins enable the hydrogen ions to bring sucrose molecules into the companion cells. As the concentration of sucrose builds up inside the companion cells, they diffuse into the sieve tube elements through the numerous plasmodesmata.

Movement of sucrose along the phloem:

Sucrose is actively loaded into the sieve tube element and reduces the water potential.
Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element .
Water moves down the sieve tube from higher hydrostatic pressure at the source to lower hydrostatic pressure at the sink.
Sucrose is removed from the sieve tube element by surrounding cells, increasing the water potential of the sieve tube element
Water moves out the sieve tube and reduces the hydrostatic pressure.

This process produces a flow of water along the phloem. This flow carries sucrose and other assimilates along the phloem. This is called mass flow.

Evidence for the mechanism of translocation:

radioactively labelled CO2 appears in the phloem.
ringing a tree to remove the phloem results in sugars collecting above the ring.
an aphid feeding on a plant stem can be used to show that the mouthparts are taking food from the phloem
the companion cells have many mitochondria.
translocation can be stopped by using a metabolic poison that inhibits the formation of ATP.
the rate of flow of sugars in the phloem is so high that energy must be needed to drive the flow.
the pH of the companion cells is higher than that of surrounding cells( because of CO2).
the concentration of sucrose is higher in the source than in the sink.

Evidence against the mechanism of translocation:

not all solutes in the phloem sap move at the same rate.
sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to areas with a low concentration.
the role of sieve plates is unclear.

	Created by Megan Vann over 9 years ago

A Level AS OCR Biology Note on OCR AS Biology F211 Transport in plants., created by Megan Vann on 05/12/2014.