Cambridge A-Level Biology 9700 – Transport Mechanisms
Transport Mechanisms
Phloem Loading and Unloading
In higher plants, the movement of organic nutrients (assimilates) occurs primarily in the phloem. Assimilates such as sucrose and amino acids are dissolved in the watery sap and travel from regions of production (sources) to regions of utilization or storage (sinks) through specialised conducting cells called sieve tubes.
Key statement: Assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes.
Key Concepts
Source: Tissue that produces or releases assimilates (e.g., mature leaves).
Sink: Tissue that consumes or stores assimilates (e.g., roots, developing fruits, young leaves).
Phloem loading: Active transport of sugars into the sieve‑tube elements at the source, raising the osmotic pressure of the sap.
Pressure‑flow hypothesis: The osmotic gradient generated by loading creates a hydrostatic pressure that drives bulk flow of sap toward the sink.
Phloem unloading: Removal of assimilates from the sieve tube at the sink, often by diffusion or active transport, lowering the pressure and allowing continued flow.
Steps in the Pressure‑Flow Process
Photosynthesis produces sucrose in mesophyll cells.
Sucrose is actively transported into companion cells and then into sieve‑tube elements (phloem loading).
Water follows by osmosis, increasing turgor pressure in the sieve tube at the source.
The pressure gradient drives the bulk flow of sap toward regions of lower pressure (the sink).
At the sink, sucrose is removed from the sap (phloem unloading), water exits, and pressure drops.
Continuing pressure differences maintain a continuous stream of assimilates.
Comparison of Sources and Sinks
Feature
Source
Sink
Typical Organs
Mature leaves, photosynthetic tissues
Roots, developing fruits, young leaves, storage organs
Metabolic Activity
Net production of carbohydrates
Net consumption or storage of carbohydrates
Phloem Loading Mechanism
Active (energy‑requiring) transport of sucrose
Often passive diffusion or active uptake depending on sink type
Osmotic Pressure in Sieve Tubes
High (due to high solute concentration)
Low (after solute removal)
Suggested diagram: Phloem loading at a source leaf and unloading at a sink root, illustrating the pressure‑flow mechanism.
Important Points for Examination
State clearly that assimilates dissolved in water move from sources to sinks in phloem sieve tubes.
Explain how active loading creates a high osmotic potential at the source.
Describe how the resulting pressure gradient drives bulk flow (pressure‑flow hypothesis).
Identify typical source and sink tissues in a flowering plant.
Link the role of companion cells in loading and unloading processes.