Translocation

• Translocation is an energy requiring process which serves as a means of transporting assimilates such as sucrose in the phloem between sources which release sucrose such as leaves and sinks e.g. roots and meristem which remove sucrose from the phloem
• Sucrose enters the phloem in a process known as active loading where companion cells use ATP to transport hydrogen ions into the surrounding tissue, thus creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cell
• It is a form of facilitated diffusion involving cotransporter proteins which allows the returning H+ ions to bring sucrose molecules into the companion cells, thus causing the concentration of sucrose in the companion cells to increase
• As a result of that, the sucrose diffuses out of the companion cells down the concentration gradient into the sieve tube elements through links known as plasmodesmata
• As sucrose enters the sieve tube elements, the water potential inside the tube is reduced, therefore causing water to enter via osmosis, as a result increasing the hydrostatic pressure of the sieve tube
• Therefore, water moves down the sieve tube from an area of higher pressure to an area of lower pressure
• Eventually, sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus increasing the water potential in the sieve tube
• This in turn means that water leaves the sieve tube by osmosis, as a result reducing the pressure in the phloem at the sink
• Therefore, in summary the mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where they are needed