transport in plants

• Main substances transported in plants are water and organic substances
• Substances transported in plants

  • Water
  • Organic substances
• Structure of phloem

  • Consists of sieve tube elements and companion cells
• Sieve tube elements

  • Living cells with no nucleus, few organelles, and perforated end walls for mass transport of fluids
• Companion cells

  • Provide ATP required for active transport of organic substances into sieve tube elements
• Structure of xylem

  • Consists of dead, hollow cells stacked on each other, containing no organelles or end walls, strengthened with lignin for transporting water and mineral ions
• Transport of water in plants

  Occurs from soil to root hair cells, then moves to xylem through simplast or apoplast pathways
• Root hair cell adaptation for water absorption

  Thin walls and long protrusions for maximizing absorption by osmosis
• Simplast pathway

  Water moves through the cytoplasm of cells towards the xylem by osmosis
• Apoplast pathway

  Water moves through cell walls due to cohesive forces, transporting water faster with little resistance
• Plants have adaptations for high or low water quantities
• Gas exchange in plants occurs through stomata
• Guard cells determine the opening and closing of stomata to prevent excessive water loss
• Extreme xerophytes have adaptations to reduce water loss, found in environments with limited water like deserts or sandy areas
• Marin grass is an example of a xerophyte with adaptations like curled leaves and sunken stomata to reduce water loss
• Adaptations of plants for transpiration

  • Curls and hair-like structures to trap moist air and increase humidity
  • Sunken stomata to increase local humidity and reduce water potential gradients
  • Thicker cuticle to reduce water loss by evaporation
  • Longer root network to absorb more water by osmosis
• Hydrophytes


  • Plants living in or on water with adaptations for excess water
  • Short roots, thin to no waxy cuticle, stomata permanently open on top surface of leaf
  • Large and wide leaves on the surface of the water to ensure enough light absorption for photosynthesis
• Factors affecting the rate of transpiration

  1. Higher light intensity causes stomata to open more

  2. Higher temperature increases evaporation

  3. Higher humidity reduces water potential gradient and transpiration

  4. Higher wind increases transpiration
• Water movement up the xylem

  1. Water moves up from the roots against gravity due to cohesion tension theory

  2. Cohesion of water creates a continuous column in the xylem

  3. Adhesion of water to xylem walls strengthens water movement

  4. Root pressure from osmosis increases pressure and pushes water up

  5. Cohesion tension theory: water evaporates from leaves, creating negative pressure, pulling up more water due to cohesion and adhesion, resulting in water movement up the xylem
• Xylem water transport

  Water is stuck to the walls of the xylem as the water column is being pulled up, pulling the water column upwards and the walls of the xylem inwards, making the walls closer together and the Lumen narrower. The tension of the water being pulled upwards in a continuous column causes the xylem to be narrower, enhancing the impact of adhesion. Root pressure also pushes the water up, creating a continuous column of water moving up the xylem against gravity
• Phloem translocation

  Transport of organic substances produced in photosynthesis through the phloem. Requires energy and is an active process. Involves mass flow from the source (where organic substances are made) to the sink (where they are used in respiration). Source cell has a lower water potential due to sucrose production, causing water to move into the cell. Sink cell has a higher water potential, causing water to leave the cell. Hydrostatic pressure increases at the source cell and decreases at the sink cell, forcing liquids through the phloem towards the sink cell
• Translocation stages

  Photosynthesizing cell (source cell) produces organic substances like sucrose. Sucrose diffuses down its concentration gradient into the companion cell. Active co-transport of sucrose with hydrogen ions from the companion cell into the sieve tube element. Sucrose in the sieve tube element decreases water potential, causing water from the xylem to move into the phloem. Sucrose is used in respiration at the sink, causing more sucrose to be actively transported into the sink cell, decreasing water potential and causing water to move back into the xylem. Movement of solutes down the pressure gradient causes the solution to move through the phloem