Translocation of Solutes in the Phloem

Introduction

• End goal: Describe translocation of solutes in the phloem
• Focus on:
  • Active loading
  • Mass flow

Photosynthesis and Glucose Conversion

• Plants produce glucose during photosynthesis in the leaves
• Glucose is needed by all parts of the plant for respiration
• Glucose is converted to sucrose (a disaccharide) because:
  • Sucrose is less reactive than glucose

Assimilates

• Assimilates: Molecules made during photosynthesis (e.g., sucrose, amino acids)
• Transported in the phloem
• Translocation: Movement of assimilates in the phloem

Source and Sink Concept

• Sources: Regions where assimilates are produced (e.g., photosynthesizing leaves, storage organs like tubers)
• Sinks: Regions where assimilates are needed (e.g., roots, storage organs when refilling, growing regions like shoots)

Structure of the Phloem

• Understanding the structure is crucial for the translocation process

Active Loading at the Source

• Sucrose is loaded into the phloem actively:
  • Companion Cell Membrane:
    • Protein uses ATP to pump hydrogen ions out of the cytoplasm to the cell wall
    • Process: Active transport creating a concentration gradient
    • Hydrogen ions flow back into cell with sucrose through co-transporter protein
  • Companion Cells:
    • High concentration of sucrose due to transport process
    • Sucrose diffuses to sieve tube element cells lowering water potential

Mass Flow and Osmosis

• Water moves into sieve tube element by osmosis, increasing hydrostatic pressure
• Phloem sap moves towards sink: Mass Flow

At the Sink

• Sucrose moves out and is converted:
  • To glucose for respiration
  • To starch in storage organs
• Water potential increases as sucrose leaves, water moves out by osmosis
• Water may re-enter xylem, joining transpiration stream

Evidence Supporting Active Model

• Rate of sucrose flow in phloem is faster than diffusion
• Inhibition of companion cell mitochondria stops translocation

Conclusion

• Understanding translocation involves understanding both active loading and mass flow in the phloem
• Key factors include sucrose transport, role of companion cells, and the source-sink relationship