Lecture 5- Phloem Transport Mechanisms

Phloem Transport Mechanisms

Overview of Xylem and Phloem

  • Xylem:

    • Function: Responsible for transporting water and minerals in plants.

    • Composition: Primarily made of dead cells.

  • Phloem:

    • Function: Transports nutrients, mainly sugars (monosaccharides, disaccharides, polysaccharides).

    • Composition: Composed of living cells that include sieve-tube members and companion cells.

    • Photosynthate: Refers to products of photosynthesis, primarily glucose.

    • Storage of Glucose:

      • Glucose can be converted to starch (100% yield), contributing as a carbohydrate source.

Transport of Phloem Sap

  • Source-Sink Relationship:

    • Source Leaves :

    • Sends sugars to tissues located on the same end of the plant.

    • Sink:

    • Tissues that utilize or store these sugars.

  • Experimental Observation:

    • Radiolabeled Sugars: Applications of 14C radioactive carbon to leaves showed that labeled sugars were translocated to growing leaves on the same side of the plant.

Phloem Tissue Structure

  • Cell Types in Phloem:

    • Contains more than one cell type, specifically:

    • Sieve-tube Members: Long, living cells that serve as conduits for sap transport.

    • Companion Cells: Aids in the transport of photosynthates, maintaining the function of the sieve-tube elements.

  • Sieve-Tube Elements:

    • Individual cells that form the sieve tube in phloem tissues, characterized by the presence of sieve plates.

Morphology of Sieve Plates and Transport

  • Structure Observations:

    • Longitudinal Section Analysis:

    • Electron micrographs reveal the vertical passage of sugars through pores in the walls between sieve-tube members.

    • Pores: Formed by plasmodesmata in the companion cells that allow intercellular communication and transport.

Sucrose and its Components

  • Sucrose Structure:

    • Composed of α-glucose and β-fructose.

    • Glycosidic bond formation:

    • Specifically illustrated as an alpha 1-2 glycosidic bond.

Mechanism of Phloem Transport

  • Loading Mechanisms:

    • Symplastic Loading: Involves the transport of photosynthates from one side of a plasma membrane to another within the cell via plasmodesmata.

    • Apoplastic Loading: Transport occurs across cell walls, utilizing ATP for movement of substances.

Pressure-Flow Theory

  • Mechanism Description:

    • The pressure-flow mechanism outlines how high sugar concentrations in sieve-tube elements create osmotic pressures that draw water from the xylem into the phloem.

    • Pressure generation occurs under the following conditions:

      • When water enters sieve-tube elements from areas of high sugar concentration.

      • As a consequence of the rigid cell walls that prevent them from collapsing during pressure variations.

Source-to-Sink Analogies

  • Analogous Models:

    • The transport from source to sink can be compared to several models:

    • A conveyor belt, where items are loaded at one end and unloaded at the other.

    • A line of people entering a movie theater.

Implications of Phloem Disruption

  • Experiment: Leaf-cutter bees severing a vertical file of sieve elements results in the cessation of phloem transport both above and below the cut due to:

    • Rapid formation of P-protein and callose plugs that block the flow in the sieve tubes.

Aphid Feeding Observations

  • Feeding Mechanism:

    • Aphids utilize a stylet to penetrate sieve-tube elements, resulting in exuding sap droplets, providing real-time insights into phloem transport mechanisms.

    • Analyzing such interactions helps understand phloem sap dynamics and plant responses to herbivory.