AQA A Level Biology复习笔记5.4.2 Practical Skill: Investigate the Effect of Minerals on Plant Growth

• Photosynthesis produces carbohydrates
  • Carbohydrates contain the chemical elements carbon, hydrogen and oxygen, which can be obtained from carbon dioxide and water

   

• However, plants contain many other types of biological molecules, such as proteins, phospholipids, nucleic acids (DNA and RNA) and ATP
• These molecules contain other chemical elements, for example:
  • Nitrogen is required to make the amino acids that form proteins, which are needed for cell growth
  • Phosphorus is required to make phospholipids for cell membranes, nucleic acids and ATP
  • Magnesium is required to make chlorophyll
  • Calcium is required to make calcium pectate for the middle lamella (the layer that provides support to plants by holding the cell walls of two plant cells together) and is important for membrane permeability

   

• Plants obtain these elements in the form of mineral ions (e.g. nitrate ions or phosphate ions) that they actively absorbed from the soil through root hair cells
• ‘Mineral’ is a term used to describe any naturally occurring inorganic substance
• Without a source of these minerals, plants cannot grow or function properly
• The effect of these minerals on plant growth can be investigated by growing plants under controlled conditions and selectively removing each of these minerals in turn so the plants cannot access them

Apparatus

• Bryophyllum plantlets
  • This plant reproduces asexually via budding, producing genetically identical 'daughter' plantlets
  • This removes the possibility that genetic differences are affecting plant growth

   

• Nutrient solutions:
  • Solution with all mineral ions (nitrate ions, phosphate ions, magnesium ions and calcium ions) present (this acts as a control to show that it is the lack of individual mineral ions that is affecting plant growth)
  • Solution with all mineral ions present except nitrate ions
  • Solution with all mineral ions present except phosphate ions
  • Solution with all mineral ions present except magnesium ions
  • Solution with all mineral ions present except calcium ions

   

• Measuring cylinder
  • To measure out equal volumes of nutrient solutions

   

• Test tubes
• Test tube rack
• Aluminium foil

Method

• Fill the tests tube with a set volume (e.g. 10 cm³) of each of the nutrient solutions
• Cover the top of each test tube with foil and create a small hole in the foil
• For each test tube, take a Bryophyllum plantlet and push the roots of the plantlet through the hole in the foil, ensuring that the roots are submerged in the nutrient solution
• Place the test tubes in the same location and ensure that light intensity and temperature are kept at the appropriate levels to ensure normal plant growth
  • This is to ensure that these abiotic factors are controlled and kept the same for each of the plantlets

   

• After a set period of time (e.g. 2 weeks) observe the plantlets and take qualitative and quantitative measures of their growth
  • Qualitative measures could include the colour of the plantlets and their leaves, or how wilted the plantlet has become
  • Quantitative measures could include the height or mass of the plantlets or the length and width of their leaves

   

Results

• The table below outlines some of the common effects on plant growth when there is a deficiency of each of the four minerals

An example of the possible effects of mineral deficiencies on plant growth