Transport in plants

• Structural factors affecting the rate of transpiration

  • Cuticle
  • Glossy leaf surface
  • Leaf size and shape
  • Hairy leaves
  • Stomata
  • Sunken stomata
• Cuticle


  • Thick cuticle (e.g. in cactus and aloe plant) increases the distance of diffusion of water vapor hence reduces the rate of transpiration
  • Thin cuticle increases the rate of transpiration to get rid of excess water
• Glossy/ waxy and shiny leaf surfaces

  Reflect light landing on the leaf surface, reducing internal heating of the leaf and stomatal opening, reducing the rate of transpiration
• Broad leaves


  Increase the surface area for water loss hence higher rate of transpiration
• Small and needle like leaves

  Reduce the surface exposed hence low rate of transpiration
• Hair on leaves


  Trap a layer of moist air on the surface of leaves reducing the concentration gradient hence reduces transpiration
• Few stomata


  Reduce the surface area reducing the rate of transpiration
• More stomata on the upper surface than the lower part


  Increases the rate of transpiration
• Reversed stomatal rhythm (opening during the night and closing during the day)

  Reduces the rate of transpiration
• Small stomatal apertures


  Reduce the rate of transpiration
• Sunken stomata (located on the inner part of the epidermis)

  Lowers the rate of transpiration as water vapor accumulates in the pits/depressions of the stomata reducing the concentration gradient
• Midday closure


  Some plants close their stomata on a hot dry sunny day to lower the rate of transpiration and protect the plants from wilting
• Environmental factors affecting the rate of transpiration

  • Temperature
  • Wind
  • Humidity
  • Light intensity
  • Atmospheric pressure
  • Availability of water
• Temperature


  High temperature increases internal temperature of the leaf which in turn increases the latent heat of vaporization leading to high rate of transpiration
• Wind


  On a windy day the rate of transpiration is high because wind blows away/ carries the water vapor on the leaf surface, increasing the concentration gradient between the inside of the leaf and the outside
• Humidity


  High humidity (a lot of vapor in the atmosphere) reduces the transpiration/Concentration gradient between the inside and outside of the leaf hence lowering the rate of transpiration and vice versa
• Light intensity


  High light intensity causes the stomata to open fully, increasing the rate of transpiration
• Atmospheric pressure


  Higher atmospheric pressure can hold a lot of water vapour in the atmosphere reducing the concentration gradient/concentration deficit hence reduces the rate of transpiration and vice versa
• Availability of water


  When less water is available, plants close their stomata thus reducing the rate of transpiration
• Experiment to investigate the rate of transpiration

  1. Arrange the apparatus as shown

  2. Cut the leafy shoot under water to prevent air bubbles

  3. Remove air bubbles by opening the tap

  4. Introduce one air bubble by exposing the end of the tube

  5. Control the movement of the bubble by opening the tap slowly

  6. The air bubble should be at the end of the capillary tube

  7. Record the movement of the bubble against time to calculate the rate of transpiration
• Potometer


  Apparatus used to investigate the rate of transpiration
• The process being investigated is transpiration
• Role of the oil layer

  Prevents evaporation of water from the surface
• Bright sunlight increases internal temperature/latent heat of vaporization hence faster rate of transpiration
• Fanning the shoot


  Faster drop in water level
• Removing all the leaves from the shoot

  No change in water level
• Placing the set-up in the dark

  Slower/very slow drop in water level
• A control setup in science uses the same conditions and the same equipment as the experimental setup; however, there are no variables tested in the control setup, as there are in the experimental setup
• The control for this experiment is another set up using a leafless twig
• Experiment to investigate the rate of transpiration from leaf surface

  1. Select one healthy leaf

  2. Pick two pieces of anhydrous cobalt (II) chloride paper and note their colour

  3. Place each piece on the two surfaces of the leaf

  4. Quickly cover them with dry glass slides holding them in position by use of bands

  5. Notice the time each of the two papers take to turn pink

  6. Hold a covered anhydrous cobalt (II) chloride paper to act as a control

  7. Record the time taken for the Anhydrous cobalt (II) chloride paper to turn pink in air, above the leaf surface and under the leaf surface

  
• The paper on the upper side of the plant takes longer to turn pink because there are fewer stomata on the upper side hence low rate of transpiration
• The paper on the lower side take a shorter time to turn pink because there are many stomata on the lower side hence faster rate of transpiration
• The control experiment is used to provide a contrast to the experiment
• The bare hands should not used to hold the paper because they would be moist hence affecting the Anhydrous cobalt (II) chloride paper
• Translocation


  The transport of manufactured food materials/ organic compounds from one part of the plant to the other
• Phloem


  The tissue where translocation takes place
• Food materials transported in the phloem

  • Sugar/ glucose
  • Amino acids
  • Vitamins
• Processes involved in translocation of manufactured food

  1. Diffusion

  2. Cytoplasmic streaming
• Parts of the plant where food materials are translocated to

  • Growing and developing regions (young shoots, leaves, flowers, fruits, roots)
  • Storage organs (tubers, corms, bulbs, rhizomes, seeds)
  • Secretory organs (nectar gland)
• Components of the phloem

  • Sieve tubes/ elements
  • Companion cells