Overview
This lecture explains how assimilates like sucrose are transported through the phloem in plants, focusing on the processes of active loading and mass flow.
Translocation in the Phloem
- Translocation is the transport of assimilates (e.g., sucrose, amino acids) in the phloem from sources to sinks.
- Sources are sites where assimilates are produced or released (e.g., photosynthesizing leaves, storage organs).
- Sinks are regions where assimilates are required (e.g., roots, growing shoots, refilling storage organs).
Active Loading of Sucrose
- Sucrose is actively loaded into companion cells of the phloem using ATP.
- Companion cells use ATP to pump hydrogen ions out of the cytoplasm into the cell wall space (active transport).
- Hydrogen ions flow back into the companion cells via co-transporter proteins, bringing sucrose with them.
- Companion cells have many mitochondria for ATP and folded membranes to increase surface area.
Mass Flow Mechanism
- High sucrose concentration in sieve tube elements lowers their water potential.
- Water moves into sieve tube elements by osmosis, increasing hydrostatic pressure.
- Phloem sap (containing sucrose) moves toward sinks via mass flow (bulk movement).
- At the sink, sucrose exits the sieve tube element, increasing water potential and causing water to leave by osmosis.
Fate of Sucrose at the Sink
- Sucrose is converted to glucose for respiration or to starch in storage organs.
- Water leaving the sieve tube element may re-enter the xylem and join the transpiration stream.
Evidence for the Active Model
- The rate of sucrose flow in the phloem is faster than diffusion alone allows.
- Inhibiting companion cell mitochondria stops translocation.
Key Terms & Definitions
- Assimilate — molecules produced by photosynthesis, like sucrose or amino acids, transported in the phloem.
- Source — plant area where assimilates are produced or released.
- Sink — plant area where assimilates are used or stored.
- Active loading — ATP-driven process of moving sucrose into phloem companion cells.
- Mass flow — the bulk movement of phloem sap from high to low pressure.
- Phloem — plant tissue that transports assimilates.
- Osmosis — movement of water from high water potential to low water potential.
- Hydrostatic pressure — pressure exerted by a fluid due to water influx.
Action Items / Next Steps
- Review the video on phloem structure if not already completed.