Plant Transport

• Xylem vessels


  Function: transport water and minerals and provide support
• Xylem vessels


  • Dead hollow cells with no cell organelles and no cytoplasm to reduce resistance to water flow
  • No end walls to allow water to flow easily in a continuous water column with minimum resistance
  • Lignin in walls to make them waterproof to prevent leakage of water
  • Thick lignified walls for support and to prevent xylem vessels from collapse
  • Pits (non lignified parts) to allow lateral movement of water to nearby vessels

  
• Phloem


  Function: Transport sucrose and amino acids from source to sink
• Phloem transport of sucrose and amino acids

  1. Translocation of sucrose and amino acids dissolved in sap solution

  2. Through phloem (by diffusion and active transport) which allows bidirectional movement

  3. From source to sink
• Leaves act as a source of sucrose


  When there are few limiting factors such as high light intensity, more photosynthesis produces more glucose which is condensed into sucrose and translocated through phloem to the root acting as a sink
• Leaves act as a sink


  When there are limiting factors such as less leaves, less/no photosynthesis occurs so the root becomes the source of sucrose which is translocated through phloem upwards to the small growing leaves acting as a sink

  
• Water movement from soil to leaf

  1. Water moves by osmosis from soil into root hair cells where water potential is lower

  2. Water moves down the water potential gradient across the cortex by osmosis

  3. Water enters the xylem vessels through pits

  
• Turgor pressure


  Vacuoles filled with water by osmosis down water potential gradient press outwards on the cytoplasm, cell membrane and cell wall to support the root

  
• Factors affecting transpiration rate

  • Increase in temperature increases evaporation and water/concentration gradient, increasing transpiration rate
  • High wind speed moves water vapour molecules away from stomata maintaining a steeper concentration gradient, increasing transpiration rate
  • High light intensity causes stomata to open wider allowing more water vapour to diffuse out, increasing transpiration rate
  • Dry air has less water vapour particles so more water can diffuse out of the leaf, increasing transpiration rate
  • Humid air has more water vapour particles so less water can diffuse out of the leaf, decreasing transpiration rate
• Plants need water for maintaining turgidity, dissolving minerals, enzymatic activity/metabolic reactions, hydrolysis, as a solvent, and as a raw material for photosynthesis. Transpiration also helps cool down the plant.
• Wilting occurs when the amount of water lost is more than the amount of water gained/absorbed, or when the plant is placed in a high salt solution, or when there is a high rate of transpiration causing cells to lose water and become flaccid, losing turgor pressure.
• Leaf structure for transpiration

  • Spongy mesophyll surrounded by a layer of water that evaporates in the air spaces
  • Guard cells open stomata to allow diffusion of water vapour out of the leaf down its concentration gradient
  • Xylem vessels transport water and supply the mesophyll cells in leaf with water by osmosis replacing lost water

  
• Increase in number of leaves

  Increases the number of stomata and surface area exposed to environmental factors, increasing the rate of evaporation and transpiration, and increasing the rate of water uptake
• Investigating effect of light intensity on rate of water uptake


  1. Independent variable: 5 different light intensities by changing distance between light source and plant

  2. Controlled variables: same light source, plant species, number and size of leaves, temperature, humidity

  3. Dependent variable: measure distance travelled by air bubble every 3 mins over 15 mins at each light intensity

  4. Repeat 3 times to exclude anomalies
• Temperature is kept constant as it is a variable that affects the rate of water uptake, to have a fair test and valid comparison.
• Water uptake is not equal to water lost because some water is used in photosynthesis, as a solvent to transport minerals, remains in cells for turgidity, used as a medium for enzyme activity, and used in hydrolysis.

  
• Hydrophytes (floating plants)


  • Leaves: broad with large surface area to absorb more light, large air spaces for buoyancy and to absorb more light, stomata on upper surface for faster gas exchange with air
  • Roots: not attached to sea bed to avoid being cut by water current
  • Thin cuticle as no need to reduce water loss

  
• Xerophytes (desert plants)


  • Leaves: needle-like structure to reduce surface area exposed, fewer stomata to reduce water loss, thick waxy cuticle to reduce water loss, rolled leaves to avoid exposure to dry air and wind
  • Roots: close to surface to absorb rainfall, deep roots reaching water table, more root hairs for larger surface area
  • Stem: succulent with water storage, green to carry photosynthesis, upright to reduce surface area exposed
• Mechanisms for plants to cope with water shortage involve reducing water loss, such as wilting where leaves collapse and stomata close to reduce surface area exposed, and leaf fall in severe conditions where the plant removes chlorophyll for storage before letting the leaves fall.