3 Transport in plants

• Why do 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).
• 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
• 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
• Structure and function of the 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.
• Structure and function of the 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.
• Structure and function of the 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 that affect 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
• How to measure transpiration rate


  Potometer. 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


  The tendency of water to move by osmosis, from high water potential to low. Pure distilled water has the highest water potential of 0. This is the basis by which water moves to the areas it is needed within plants.
• Apoplastic pathway


  A method of osmosis through the root hair cells, where water moves through the cell walls and intercellular spaces. This pathway can only be used until water reaches the Casparian strip.
• Symplastic pathway


  A method of osmosis through the root hair cells, where water moves through the cytoplasm via plasmodesmata. To begin this pathway, water must be actively transported into cells.
• Cohesion-tension theory


  Water molecules form hydrogen bonds with each other, causing them to 'stick' together (cohesion). The surface tension of the water also creates this sticking effect. Therefore as water is lost through transpiration, more can be drawn up the stem from the roots.
• Adaptations of xerophytes


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


  • 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 (requiring energy). 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.