Module 3: Translocation

These free OCR A Level Biology Translocation revision notes have been written for specification point 3.1.3(f).

Translocation

Translocation is the movement of dissolved substances (called assimilates) in the phloem sap from a source (e.g. leaves, tubers) to sinks (e.g. roots, fruits, tubers).

Translocation: The Pressure Flow Hypothesis

Loading (at the source):

1. Companion cells use ATP to transport H⁺ into the surrounding source tissues.
2. H⁺ diffuse back into the companion cells through cotransporter proteins alongside sucrose.
3. Sucrose diffuses down its concentration gradient into the sieve tube elements via the plasmodesmata from the companion cells.
4. This lowers the water potential inside sieve tubes.
5. Water enters from adjacent xylem into this area down its water potential gradient by osmosis, increasing the hydrostatic pressure.

Mass flow (through phloem):

• Higher hydrostatic pressure at the source pushes the phloem sap toward the sink, where the pressure is lower.
• This is a bulk flow mechanism.

Unloading (at the sink):

1. Sucrose is removed from the phloem by companion cells using active transport, or through the plasmodesmata by diffusion.
2. The removal of sucrose from the phloem increases the water potential in the phloem at the sink.
3. Water moves to a lower water potential back into the xylem by osmosis, lowering the hydrostatic pressure at the sink.
4. Sucrose is used in respiration or stored.