The Mass Flow Hypothesis is the leading explanation for translocation, proposing that sap moves due to a hydrostatic pressure gradient. This gradient is created by the difference in solute concentration between the source and the sink.
At the source, the active accumulation of solutes lowers the water potential () within the phloem sieve tubes. This causes water to move from the adjacent xylem into the phloem by osmosis, significantly increasing the internal hydrostatic pressure.
At the sink, solutes are removed for use or storage, which increases the water potential in the phloem. Water then leaves the phloem and returns to the xylem, resulting in a drop in hydrostatic pressure at the sink end.
The resulting pressure difference () forces the entire column of phloem sap to flow en masse toward the sink, carrying all dissolved solutes at the same velocity.
Active loading is the mechanism by which sucrose enters the phloem against its concentration gradient, requiring metabolic energy in the form of ATP. This process primarily occurs in the companion cells associated with sieve tube elements.
Companion cells use ATP-powered proton pumps to actively transport hydrogen ions () out of their cytoplasm and into the surrounding cell wall space. This creates a steep electrochemical gradient of protons across the cell membrane.
Protons then diffuse back into the companion cell through specialized cotransporter proteins. As they move down their gradient, they carry sucrose molecules with them against the sucrose concentration gradient.
Once inside the companion cell, sucrose moves into the sieve tube elements via cytoplasmic connections called plasmodesmata, where it then contributes to the osmotic pressure required for mass flow.
| Feature | Xylem Transport (Transpiration) | Phloem Transport (Translocation) |
|---|---|---|
| Substances | Water and mineral ions | Organic solutes (sucrose, amino acids) |
| Direction | Unidirectional (roots to leaves) | Bidirectional (source to sink) |
| Mechanism | Cohesion-tension (passive) | Mass flow (active loading) |
| Tissue State | Dead cells (vessels/tracheids) | Living cells (sieve tubes/companion cells) |
| Energy Source | Solar energy (evaporation) | Metabolic energy (ATP) |
Identify Source and Sink: In exam questions, always determine the physiological state of the plant. For example, in early spring, a potato tuber is a source (releasing stored starch as sucrose), but in late summer, it is a sink (storing excess sugar).
Explain the Gradient: When describing mass flow, always link the active loading of sucrose to the lowering of water potential, the subsequent entry of water by osmosis, and the resulting increase in hydrostatic pressure.
Evaluate the Evidence: Be prepared to discuss why the mass flow hypothesis is a theory. Supporting evidence includes the high pressure in phloem (sap oozing when cut), while contradicting evidence includes the fact that different solutes move at different speeds, which mass flow alone cannot explain.
Terminology Precision: Use the term 'assimilates' for phloem contents and 'hydrostatic pressure' for the driving force. Avoid saying 'sugars move by osmosis'; water moves by osmosis, which then creates the pressure for sugar movement.
Ringing Experiments: Removing a ring of bark (which contains the phloem) while leaving the xylem intact demonstrates that sugars cannot move past the break, leading to a bulge of sugar-rich fluid above the ring. This confirms phloem's role in sugar transport.
Tracer Studies: Using radioactive isotopes like allows scientists to track the movement of sucrose through the plant using autoradiography. This provides visual evidence of the source-to-sink pathway and the speed of trans
Metabolic Inhibitors: Applying chemicals that stop ATP production (like cyanide) halts trans This proves that the loading process is an active, energy-dependent biological mechanism rather than a purely physical one.
