Lecture Notes: Active Loading of Sucrose

Overview of Transport in Plants

• Sucrose and amino acids transported from source to sink via phloem.
• Focus on active loading of sucrose for exams.

Active Loading Process

• More complex than simple diffusion.
• Involves three main components:
  • Mesophyll cells (source): Produce sucrose and amino acids.
  • Companion cells.
  • Phloem sieve tube elements.

Key Structures

• Mesophyll Cell

  • Source cell producing sucrose.
  • Important organelles (e.g., mitochondria) present but not depicted.

• Companion Cell

  • Adjacent to phloem sieve tube element.
  • Contains transport proteins:
    • Hydrogen ion-sucrose co-transporter: follows hydrogen ion gradient.
    • Proton pump (hydrogen ion pump): involved in active transport.

• Phloem Sieve Tube Element

  • Identified by sieve plates and limited cytoplasm.

Transport Proteins and Their Functions

• Proton Pump

  • Carries out active transport of hydrogen ions using ATP.
  • Creates a proton gradient (high concentration outside, low inside).

• Co-transporter

  • Transports hydrogen ions and sucrose simultaneously.
  • Follows hydrogen ion concentration gradient, not sucrose gradient.

Active Loading Steps

1. Active Transport by Proton Pump

   • ATP powers proton pump.
   • Hydrogen ions are actively transported from companion cell cytoplasm to cell wall.
   • Results in proton gradient.

2. Passive Diffusion through Co-transporter

   • Hydrogen ions diffuse back into companion cell.
   • Sucrose molecules follow hydrogen ions into companion cell regardless of their concentration gradient.
   • No ATP required for sucrose transport at this stage.

3. Diffusion into Phloem Sieve Tube Element

   • Sucrose diffuses into phloem sieve tube element through plasmodesmata.

Summary

• Active loading involves active transport and passive diffusion.
• Hydrogen ion concentration gradient is crucial for sucrose transport.
• Understanding of these processes is key for exams.