Section 3B Concise

Transport in Flowering Plants

• Xylem Bulk Flow

  • Water and minerals enter via root epidermis.

  • Bulk flow: fluid movement due to pressure difference.

  • Transpiration: evaporation replaces water lost from leaves.

• Root Pressure

  • Mineral ions pumped into xylem at night.

  • Generates root pressure, pushing xylem sap.

  • Results in guttation: water droplets on leaf edges.

• Cohesion-Tension Hypothesis

  • Transpiration creates a pull for xylem sap ascent.

  • Cohesion of water molecules helps transmit this pull.

  • Water vapor diffuses out of leaves, creating negative pressure.

• Ascent of Xylem Sap

  • Adhesion to cellulose in xylem walls offsets gravity.

  • Cohesion allows water molecules to pull each other along.

  • Thick secondary walls prevent collapse under negative pressure.

• Stomatal Regulation

  • Stomata open/close for gas exchange and water conservation.

  • Guard cells control stomatal diameter via turgor pressure.

  • Turgor change due to K+ uptake/loss regulates stomatal movements.

• Adaptations to Reduce Water Loss

  • Xerophytes adapted to arid climates (e.g., cacti).

  • CAM: CO2 uptake at night to minimize daytime transpiration.

  • Thicker cuticles and specialized structures reduce water loss.

• Phloem Transport

  • Sugars transported by phloem through translocation.

  • Sugar sources (e.g., mature leaves) vs. sugar sinks (e.g., roots).

  • Sugar loading into sieve-tube elements requires active transport.

• Pressure Flow Mechanism

  • Phloem sap moves by bulk flow via pressure flow.

  • Loading sugar creates positive pressure, pushing sap forward.

  • Water recycles from sinks back to sources via xylem.