Transport in Plants

• Plants require a transport system to ensure all cells receive a strong supply of the nutrients they require, especially important as a plant must be able to transport substances up their stem (against gravity)
• Structure of xylem:

  • Long, continuous columns made of dead tissue, allowing transportation of water
  • Contain pits, allowing water to move sideways between vessels
  • Thickened with a tough substance, providing structural support
• Structure of phloem:

  • Sieve tube elements transport sugars around the plant
  • Companion cells designed for active transport of sugars into tubes
  • Plasmodesmata allow flow of substances between cytoplasm of different cells
• Vascular system in the roots:

  • Consists of xylem and phloem
  • Xylem arranged in an X shape to provide resistance against force
  • Surrounded by endodermis, a water supply
• Vascular system in the stem:

  • Consists of xylem and phloem
  • Xylem on the inside of the bundle to provide support and flexibility, phloem on the outside
  • Layer of meristem cells that produce new xylem and phloem tissue when required
• Vascular system in the leaves:

  • Consists of xylem and phloem, forms the midrib and veins
  • Involved in transport and support
• Transpiration:

  • The evaporation of water from the leaves of a plant
  • Consequence of gaseous exchange; occurs when the plant opens the stomata to exchange oxygen and CO2
• Factors affecting the rate of transpiration:

  • Increased light increases transpiration
  • Increased temperature increases transpiration
  • Increased humidity decreases transpiration
  • Increased air movement increases transpiration
  • Waxy cuticle prevents transpiration
• Measurement of transpiration rate:

  • Potometer used
  • Plant cutting is placed in a water-filled tube that contains an air bubble
  • Rate of transpiration is calculated by measuring the movement of the air bubble over time
• Water potential as a mechanism of movement in plants:

  • Water potential= the tendency of water to move by osmosis, from high water potential to low
  • Pure distilled water has the highest water potential of 0
  • Basis by which water moves to the areas it is needed within plants
• Apoplastic pathway:

  • Method of osmosis through the root hair cells, where water moves through the cell walls and intercellular spaces
  • Can only be used until water reaches the Casparian strip
• Symplastic pathway:

  • Method of osmosis through the root hair cells, where water moves through the cytoplasm via plasmodesmata
  • Water must be actively transported into cells to begin this pathway
• Cohesion-tension theory:

  • Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion)
  • Surface tension of the water also creates this sticking effect
  • As water is lost through transpiration, more can be drawn up the stem from the roots
• Adaptations of xerophytes for dry conditions:

  • Small/rolled leaves
  • Densely packed mesophyll
  • Thick waxy cuticle
  • Stomata often closed
  • Hairs to trap moist air
• Adaptations of hydrophytes for wet conditions:

  • Thin or absent waxy cuticle
  • Stomata often open
  • Wide, flat leaves
  • Air spaces for buoyancy
• Mechanism of translocation:

  • Sucrose produced in leaves loaded into sieve tubes via active transport
  • Lowers water potential, causing water to move in from xylem
  • Assimilates move along the sieve tube towards areas of lower hydrostatic pressure (sink)
  • Sucrose diffuses into surrounding cells where it is needed