Lecture Notes: Plant Transport - Sugar Transport in Phloem

Overview of Phloem Transport

• Phloem transports sugars and organic materials throughout the plant.
• Directionality: Unlike xylem (one-directional, roots to upward), phloem translocates materials in multiple directions.
• Sap: Phloem contains sap, which is sugar dissolved in water, along with hormones and amino acids.

Structure of Phloem

• Cells:
  • Sieve Cells: Cells with sieves between them.
  • Sieve Tube Elements: More efficient, found in angiosperms.
• Phloem cells are living cells, unlike xylem cells, to perform active transport.
• Companion Cells: Parenchyma cells connected by plasmodesmata; support sieve cells in metabolism and structure.

Pressure Flow Hypothesis

• Describes sugar transport in phloem using water pressure for bulk flow.
• Source: Location where sugars are produced. Sugars enter phloem by active transport, making the solution hypertonic.
• Sink: Location where sugars are needed. Sugars exit the phloem by active transport.

Phloem Loading and Unloading

• Phloem Loading (at Source):
  • Active transport of sugars into phloem, making it hypertonic.
  • Water enters phloem passively from xylem (osmosis via plasmodesmata).
  • Creates osmotic pressure, causing bulk flow through sieve plates.
• Phloem Unloading (at Sink):
  • Active transport of sugars out of phloem, making it hypotonic.
  • Water exits phloem passively back to xylem.

Functionality Comparison

• Active transport at source and sink differentiates phloem function from xylem.
• Phloem is flexible in directional transport, supported by active sugar transport.
• Phloem relies on xylem for water supply, emphasizing the collaborative function of plant vascular systems.

Conclusion

• Phloem transport is a crucial part of plant transport systems, complementing xylem function.
• Completion of plant transport lectures.