HL 1 Biology

9.2a Plant Transport Systems

AA-CC: 122, 125-7; DBQ p.127

<<Root Systems and Mineral Ion Absorption>>

  • Formation of a root: earliest stage in development of an embryo plant
  • seedling root develops: branches
  • hairs: developed in roots
  • branches and hair growth increase surface area of root system = increase capacity for absorption
  • Active transport: establishes concentration gradients in roots
    • osmosis: water absorbed
    • solute concentration inside root cells is greater than water in soil
    • protein pumps present in plasma membranes of root cells
  • Mineral ions: absorbed by active transport: when in contact with the correct pump protein
  • diffusion mass flow:  carry ions through soil
  • hyphae: thread like fungus growing out into the soil which absorbs mineral ions
  • mutualistic relationship: relationship of both the fungus and plant benefiting from the minerals obtained


  • transpiration: the loss of water vapour from the leaves and stems of plants
  • prevents other small molecules to pass through (carbon dioxide and oxygen)
  • stomata:  pores through the epidermis with waxy coating which absorbs carbon dioxide for photosynthesis
  • Problem for plants: gas exchange without water loss being impossible
  • guard cells: cells which control the aperture of the stoma, adjusting from fully opening to closing in order to  minimize water loss
  • Opening and closing of the stoma affected by external stimuli: light (to open), low carbon dioxide concentrations in air spaces inside leaf, shortage of water

<<Abiotic Factors Affecting Transpiration>>

  • Rate of transpiration: amount of water vapor plants lose from leaves and stems per unit time
  • Biotic factors- things plants can control which effect the rate:
    • size of plant
    • thickness of waxy cuticle
    • how widely spaced stomata are
    • if the stomata are opened or closed
  • Abiotic Factors:
    • light – open stoma and increases rate & in the dark, closes
    • humidity – lower humidity outside leaf = higher rate
    • wind – still air = reduce rate
    • temperature – high temperature = increase rate

<<Water Transport in Xylem>>

  • structure of xylem vessels: help transport water inside plants efficiently
  • pressure inside xylem vessels – usually lower than atmospheric pressure
  • adhesion: causes water to be drawn through cell wall from nearest available supply to replace the water lost by evaporation
  • transpiration pull: a pulling force generated by low pressure that transmits through the water in the xylem vessels down the stem to the ends of the xylem in the roots
  • cohesion: pulling of water upwards in the xylem vessels
  • cavitation: column of liquid breaking since liquids unable to resist the low pressures in xylem vessels


The Renner Experiment (p.127)

1) The clamping of the stem had impacted the rate of water uptake to decrease from 17cm3h-1 to 0cm. 

2) The effect of cutting off the top of the shoot on the rate of water uptake is that the rate of water uptake has reduced from 10cmh-1 to close to 4cmh-1.

3) The rate of water uptake caused by the vacuum pump can be measured to be 10cm3h-1 and 5cm3h-1 for the rate of water uptake before the shoot top was cut off. Thus, the difference between the rate of water uptake cause d by the vacuum pump and the rate caused by the leaves immediately before the shoot top was cup off: 10cm-5cm = 5cm3h-1.

4) The experiment showed how the pressures generated in the xylem by the leaves of the shoot to have an impact on the water uptake. As there were more pressure present in the xylem, it resulted to a higher rate of water uptake than shown in the vacuum. It is shown in the diagram as well in which the shoot has shown a higher water uptake rate than the vacuum pump.

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