Edexcel A (SNAB) A Level Biology:复习笔记4.2.3 Importance of Water & Inorganic Ions to Plants

• Plant cells perform a variety of different functions
• In order to perform these functions efficiently, the plant requires water and inorganic ions (minerals)
• They are absorbed through the root hairs on the root and travel up the stem in xylem vessels
• A plant will show certain symptoms (e.g. yellow leaves, stunted growth) when there is a deficiency in any one of these substances

Water

• Important component required for photosynthesis
• Provides a transport medium for minerals
• Maintains turgidity in plant cells though pressure in cell vacuoles
• Regulates temperature - to ensure that enzymes can function at their optimum rate

Magnesium ions

• Important requirement for the production of chlorophyll
• This provides the green colour of stems and leaves and is essential for photosynthesis

Nitrate ions

• Without nitrate ions, the plant would be unable to synthesise DNA, proteins and chlorophyll
  • Enzymes are important proteins for which nitrate ions are needed
• These molecules are essential for plant growth, as well as the production of fruit and seeds

Calcium ions

• These form important cell wall components
• Plants require calcium ions for proper growth

Diagram showing the importance of magnesium and nitrate ions for plants

• The following experiment could be done with any one of the mineral ions mentioned earlier
• For this example, the focus will be on investigating the effect of a calcium deficiency on plants

Apparatus

• Nutrient broths
• Test tubes
• Seedlings
• Aluminium foil
• Mass balance

Method

• Prepare three nutrient broths containing every mineral that a plant requires, but with different concentrations of calcium ions in each (high, medium and low)
  • Label three test tubes for each of the nutrient broths (three 'high', three 'medium' and three 'low' = nine in total)
• Take nine seedlings, ensuring that they are from the same plant and are the same age, and record the mass of each
• Place one seedling on top of each test tube, suspending the roots in the nutrient broth
• Cover the test tubes with aluminium foil to keep light away from the broth
• Place the test tubes near a source of light and leave them for a few days
• Remove each plant from the broth and carefully blot it dry before measuring the mass again
• Record the end mass and use that to calculate the mean change in mass of the plants for each of the different nutrient broths
• Make a note of any physical differences between the plants of the different groups

Variation of the practical

• Another variation of the experiment mentioned above, is to investigate the effect on plant growth when the mineral is completely lacking
• In this experiment you would have broths that contains all of the minerals, except the one that is being investigated
• There would also need to be two control broths
  • One would contain all the minerals
  • The other would contain no minerals