Transport in plants

• Why do plants require a transport system?

  To ensure all cells receive a strong supply of the nutrients they require.
• What is especially important about a plant's transport system?

  It must be able to transport substances up their stem against gravity.
• What are the structural features of xylem and their functions?

  • Long, continuous columns made of dead tissue for water transport.
  • Contain pits for sideways water movement between vessels.
  • Thickened with a tough substance for structural support.
• What are the structural features of phloem and their functions?

  • Sieve tube elements transport sugars around the plant.
  • Companion cells are designed for active transport of sugars into tubes.
  • Plasmodesmata allow flow of substances between cytoplasm of different cells.
• Describe the structure and function of the vascular system in the roots.

  • Consists of xylem and phloem.
  • Xylem arranged in an X shape for resistance against force.
  • Surrounded by endodermis, providing a water supply.
• Describe the structure and function of the vascular system in the stem.

  • Consists of xylem and phloem.
  • Xylem on the inside for support and flexibility, phloem on the outside.
  • Layer of meristem cells produces new xylem and phloem tissue when required.
• Describe the structure and function of the vascular system in the leaves.

  • Consists of xylem and phloem, forming the midrib and veins.
  • Involved in transport and support.
• What is transpiration?

  The evaporation of water from the leaves of a plant.
• What is a consequence of gaseous exchange in plants?

  Transpiration occurs when the plant opens the stomata to exchange oxygen and CO2.
• What factors 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 can we measure transpiration rate?

  Using a potometer.
• How does a potometer work to measure transpiration rate?

  A plant cutting is placed in a water-filled tube that contains an air bubble, and the rate is calculated by measuring the movement of the air bubble over time.
• What is water potential in plants?

  The tendency of water to move by osmosis from high water potential to low.
• What is the water potential of pure distilled water?

  0.
• What is the significance of water potential in plants?

  It is the basis by which water moves to the areas it is needed within plants.
• What is meant by the apoplastic pathway?

  Water moves through the cell walls and intercellular spaces by mass flow until it reaches the Casparian strip.
• What is meant by the symplastic pathway?

  Water moves through the cytoplasm via plasmodesmata after being actively transported into cells.
• What is the cohesion-tension theory?

  Water molecules form hydrogen bonds with each other, causing them to stick together, allowing more water to be drawn up the stem from the roots as it is lost through transpiration.
• What are adaptations of xerophytes that allow them to live in dry conditions?

  • Small/rolled leaves
  • Densely packed mesophyll
  • Thick waxy cuticle
  • Stomata often closed
  • Hairs to trap moist air
• What are adaptations of hydrophytes that allow them to live in wet conditions?

  • Thin or absent waxy cuticle
  • Stomata often open
  • Wide, flat leaves
  • Air spaces for buoyancy
• Summarise the mechanism of translocation in plants.

  • Sucrose produced in leaves is 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.