Mass Transport in Plants

• What is transpiration in plants?

  The loss of water from the plant's surface, especially the leaves.
• How is the xylem adapted for the transport of water?

  Xylem has no end walls, creating an uninterrupted tube, and contains lignin for support.
• How does water move through the leaf?

  Water moves down the water potential gradient from air spaces through the stomata when they are open.
• What replaces the water lost by diffusion from the air spaces in the leaf?

  Water evaporating from the moist cell walls of mesophyll cells replaces the lost water.
• How does water move from the mesophyll cells?

  Water lost from mesophyll cells is replaced by water from the xylem via cell walls or cytoplasm.
• What does cohesive mean in the context of water movement?

  Cohesive refers to hydrogen bonds between water molecules.
• What does adhesive mean in the context of water movement?

  Adhesive refers to hydrogen bonds between water molecules and the cell wall.
• Describe the process of transpiration.

  • Water evaporates from moist cell walls.
  • Accumulates in spaces between the leaf and the cell.
  • When stomata open, water moves out down the water potential gradient.
• How does water move up the xylem?

  Water moves up the xylem due to negative pressure created by transpiration, which pulls water into the leaf.
• What role does cohesion play in water movement up the xylem?

  Cohesion creates tension between water molecules, allowing them to be pulled upwards together.
• What role does adhesion play in water movement up the xylem?

  Adhesion allows water molecules to stick to the walls of the xylem, helping to pull the water column upwards.
• What are the two mechanisms that allow water to move against gravity?

  • Root pressure: Water diffuses into roots due to lower water potential.
  • Cohesion tension: Water molecules attract each other, moving together in the same direction.
• What are the four main factors that affect transpiration rate?

  1. Humidity: Negative correlation; higher humidity reduces water potential gradient.
  2. Temperature: Positive correlation; higher temperature increases evaporation.
  3. Wind: Positive correlation; more wind maintains water potential gradient.
  4. Light intensity: Positive correlation; more light opens stomata for CO2 intake.
• What is the purpose of companion cells in the phloem?

  They provide ATP required for active transport of organic substances.
• How is the phloem adapted for the transport of solutes?

  Phloem has sieve tube elements with perforated end walls and companion cells for ATP production.
• What is a source cell in plants?

  A source cell is a photosynthesizing leaf cell.
• What is a sink cell in plants?

  A sink cell is a respiring cell.
• What is translocation in plants?

  Translocation is the transport of assimilates from the source to the sink.
• How does sucrose transport from the source to the sieve tube element? (translocation)


  1. Companion cell actively transports hydrogen ions into surrounding cells.
  2. Creates a hydrogen ion gradient.
  3. Hydrogen ions move back into the companion cell via co-transporter protein.
  4. Sucrose is transported into the companion cell against its concentration gradient.
  5. Sucrose moves through plasmodesmata to sieve tube elements.
• Describe the mass flow hypothesis of translocation.


  1. Sucrose is actively transported from companion cells into phloem.
  2. Reduces water potential of phloem.
  3. Water enters sieve tube elements from xylem via osmosis.
  4. Increases hydrostatic pressure in phloem near source.
  5. Sucrose moves down pressure gradient to sink.
  6. Sugars diffuse into companion and sink cells; some water moves into xylem.
  7. Removal of water decreases volume in sieve tube, lowering hydrostatic pressure.
• What evidence supports cohesion-tension theory?

  • Diameter changes during the day; plants shrink due to tension.
  • Broken xylem vessels cannot pull water up, indicating water works under tension.
• What evidence supports mass transport?

  • Pressure in sieve tubes shown by sap release when cut.
  • Higher sucrose concentration in leaves than roots.
  • Downward flow in phloem ceases at night.
  • Increases in sucrose levels in leaves followed by similar increases in phloem.
  • Companion cells have many mitochondria for ATP production.
  • Sap flows quicker near leaves when aphids cause flow.
  • Metabolic inhibitors stop translocation, indicating active transport.
• What evidence is against mass transport?

  • Unclear function of sieve plates may hinder mass flow.
  • Not all solutes move at the same speed.
  • Sucrose delivered at similar rates to all regions, contrary to mass flow predictions.
• What is the ringing experiment and its conclusion?


  • A ring of bark removes phloem, leaving xylem intact.
  • Bulge forms above the ring with higher sugar concentration, indicating downward flow of sugars.
• What is the tracer experiment and its conclusion?


  • Radioactive tracers track movement of organic substances.
  • Sugars produced during photosynthesis are detected, highlighting phloem location.