transport in plants

• Adaptations of xylem tissue:

  • transports water and mineral ions around the plant
  • provides structural support
  • elongated, hollow tubes
  • lack organelles
  • walls are thickened with lignin for support
• adaptations of phloem tissue:

  • transports sugars and amino acids
  • mostly made up of sieve tube elements and companion cells
• adaptations of sieve tube elements:

  • They are connected end-to-end to form sieve tubes.
  • They have sieve plates with pores at their ends to allow flow of sugars and amino acids.
  • They lack nuclei and most organelles.
  • They have only a thin layer of cytoplasm
• adaptations of companion cells:

  • They are connected to sieve tube elements through pores (plasmodesmata).
  • The cytoplasm contains a large nucleus, many mitochondria to release energy for the active transport of substances through the sieve tube elements, and many ribosomes for protein synthesis.
• Distribution of xylem and phloem tissues:

  In the roots:

  • Xylem forms central cylinder surrounded by phloem.
  • This provides support as the root grows through soil.

  In the stems:

  • Xylem and phloem are in the outer region.
  • This forms 'scaffolding' to resist bending.

  In the leaves:

  • Xylem and phloem form a network of veins.
  • This provides support for thin leaves.

  
• Factors affecting the transpiration rate:

  • Light intensity - At high light intensities, stomata open for maximum CO2 absorption for photosynthesis, increasing the transpiration rate.
  • Temperature - At high temperatures, evaporation of water molecules is faster due to higher kinetic energy, increasing the transpiration rate.
  • Humidity - Low humidity increases the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.
  • Wind speed - High wind speeds increase the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.
• steps in using a potometer:

  1. Cut the shoot underwater at slant to increase the surface area for water uptake.
  2. Assemble the potometer with the shoot submerged in water.
  3. Keep the capillary tube end of the potometer submerged throughout the experiment.
  4. Check that the apparatus is airtight.
  5. Dry the leaves, and give the shoot time to acclimatise.
  6. Shut the tap, form an air bubble and record its position.
  7. Measure the distance the air bubble moves and the time taken.
  8. Change one variable at a time and keep everything else constant.
• how is rate of transpiration calcualted?

  volume of water uptake / time taken