Lecture on the Function of Phloem and Transport of Sucrose

Introduction to Phloem

• Phloem is a type of vascular tissue in plants.
• Functions to transport substances like sucrose and amino acids.

Understanding Sucrose

• Sucrose is a disaccharide, a non-reducing sugar.
• Composed of alpha glucose and fructose linked by glycosidic bonds.

Source and Sink Concepts

• Source: Part of the plant producing sucrose/amino acids (e.g., mesophyll cells in leaves).
• Sink: Part of the plant receiving sucrose/amino acids (e.g., root cortex cells).
• Transport occurs from source to sink via phloem tissue.

Structure of Phloem

• Composed of companion cells and sieve tube elements.

Transport Mechanism

Active Loading of Sucrose

• Sucrose synthesized by mesophyll cells (source).
• Active loading: Sucrose transported into phloem sieve tube elements.
• Not simple diffusion, involves complex processes.

Water Potential and Osmosis

• Sucrose in phloem lowers water potential in sieve tube elements.
• Surrounding cells have higher water potential, causing water to enter phloem by osmosis.
• Increased water volume in sieve tubes creates pressure.

Pressure Gradient and Mass Flow

• Higher pressure at the source side; lower at the sink side.
• Mass flow: Movement of water and sucrose from high to low pressure areas.
• Sucrose reaches the sink and is unloaded.

Directionality of Transport

• Transport can be bidirectional (not simultaneously).
  • Top to bottom (e.g., leaves to roots).
  • Bottom to top (e.g., leaves to fruits).
• Same principles apply irrespective of direction:
  • Active loading of sucrose.
  • Lowering of water potential.
  • Water uptake by osmosis.
  • Creation of a pressure gradient.
  • Mass flow direction (in the direction of lower pressure).

Important Concepts for Exams

• Key terms include active loading, water potential, osmosis, pressure gradient, and mass flow.
• Further detail on active loading will be covered in the final video of the chapter.