Transport in plants

• Xylem


  Tissue in plants responsible for the transport of water and mineral ions
• Phloem


  Tissue in plants responsible for translocation of organic solutes, e.g. sucrose, and amino acids
• Arrangement of vascular tissue

  • Xylem arranged centrally into a star shape with phloem outside it
  • Helps to anchor plant into the soil, resisting pulling forces
• Vessel


  Dead cells that conduct water in flowering plants
• Tracheid


  Dead cells that conduct water in flowering plants, ferns and conifers
• Parenchyma


  Packing tissue in xylem
• Fibres

  Provide support in xylem
• Water movement across the root

  1. Apoplast pathway

  2. Symplast pathway

  3. Vacuolar pathway
• Endodermis


  • Single layer of cells surrounding the vascular tissue in the root
  • Cell walls impregnated with suberin forming an impermeable Casparian strip
  • Drives water from the apoplast pathway into the cytoplasm
  • Helps to regulate the movement of water, ions and hormones into and out of the xylem
• Casparian strip in endodermis

  • Raises the water potential of endodermal cells by forcing water into them
  • Lowers the water potential of fluid in the xylem, forcing water into the xylem by osmosis (root pressure)
• Mineral uptake by roots

  1. Actively transported into root hair cells against concentration gradient

  2. Can also pass along apoplast pathway in solution

  3. Enter cytoplasm via active transport at Casparian strip

  4. Then pass via diffusion or active transport into the xylem
• Water is diverted when it comes to the endodermis because of the Casparian strip
• Respiratory inhibitors like cyanide would reduce mineral uptake
• Transpiration


  Passive process that pulls water up the xylem
• Cohesion-tension theory


  • Explains how water moves up the xylem
  • Relies on adhesive forces between water molecules and xylem, and cohesive forces between water molecules
  • Root pressure and capillarity are also involved but alone would not be sufficient
• Movement of water from roots to leaves

  1. Transpiration pull creates as water evaporates from leaf air space through stomata

  2. Water drawn from adjacent cells by osmosis

  3. Water molecules pulled up to replace those lost due to cohesive forces
• Factors affecting rate of transpiration


  • Temperature
  • Humidity
  • Air movement
  • Light intensity
• Potometer


  Measures the rate of water uptake, which approximates the transpiration rate
• When setting up a potometer, it is important to cut the stem and fit it underwater to prevent air bubbles
• Leaves should be blotted dry as any water on the surface could create a humid layer
• Using a potometer

  1. Introduce an air bubble at the end of the capillary tube

  2. Measure the distance it travels in a set period

  3. Volume can be calculated from the diameter of the capillary tube
• Repeats should be carried out when using a potometer
• To calculate volume taken in by a potometer, use the formula: volume = 3.14 x r^2 x d, where r is the radius of the tube and d is the distance the bubble travels
• Precautions are needed when setting up a potometer to collect valid data
• Setting up a potometer

  1. Cat the stem and fit it to the potometer underwater

  2. Seal all joints with Vaseline to prevent air entry

  3. Dry the leaves as any water on the leaf surface could create a humid layer
• Using a potometer


  1. Introduce an air bubble at the end of the capillary tube

  2. Measure the distance the air bubble travels in a set period

  3. Calculate the volume using the diameter of the capillary tube

  4. Carry out repeats
• Calculating volume using a potometer

  Volume = 3.14 x r^2 x d (where r is the radius of the capillary tube and d is the distance the air bubble travels)
• Potometer


  • Used to measure the rate of water uptake by a plant
  • Consists of a reservoir, capillary tube filled with water, and a leafy shoot
• Using a potometer to measure water uptake is inevitable
• Mesophytes


  • Live in temperate regions with adequate water supply
  • Conserve water by closing stomata, shedding leaves, becoming dormant, or producing seeds that can overwinter
• Xerophytes


  • Adapted to living in dry environments by reducing water loss
  • Adaptations include sunken stomata, hairs around stomata, rolled leaves, thick cuticle
• Hydrophytes


  • Grow partially or fully submerged in water
  • Adaptations include stomata on upper leaf surface, large air spaces, poorly developed xylem, little or no cuticle, no need for support tissue
• Phloem


  A living tissue consisting of sieve tubes, companion cells, and phloem parenchyma
• Sieve tubes


  • Walls perforated with pores to produce longitudinal tubes that contain cytoplasm but no nucleus
  • End walls become perforated by pores, forming the end plates
• Companion cells

  • Have dense cytoplasm with nucleus and many mitochondria
  • Connected to sieve tubes by plasmodesmata
• Phloem parenchyma acts as a packaging tissue
• Radioactive labelling of carbon dioxide

  Used to show that products of photosynthesis are transported in the phloem
• Ringing experiments

  Cutting the outer ring of a stem to remove the phloem while leaving the xylem behind causes a bulge above the ring, suggesting that sugar moves down the stem in the phloem
• Aphid feeding experiments


  Analysis of the liquid extruding from the aphid's stylet showed it contained sucrose, providing evidence that sucrose is transported in the phloem
• Mass flow hypothesis


  The most accepted theory for how organic solutes are transported in the phloem