6. Plant Nutrition

• Photosynthesis

  • Green plants make the carbohydrate glucose from the raw materials carbon dioxide and water
  • At the same time oxygen is made and released as a waste product
  • The reaction requires energy which is obtained by the pigment chlorophyll trapping light from the Sun
  • So photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light
• word eq

  
• how plants get the materials they need

  
• Chlorophyll

  • Chlorophyll is a green pigment that is found in chloroplasts within plant cells
  • It reflects green light, giving plants their characteristic green colour
  • Chlorophyll absorbs light energy; its role is to transfer energy from light into energy in chemicals, for the synthesis of carbohydrates, such as glucose
  • Photosynthesis will not occur in the absence of chlorophyll
• chlorophyll

  
• Use & Storage of Carbohydrates

  1. Converted into starch molecules which act as an effective energy store

  2. Converted into cellulose to build cell walls

  3. Glucose can be used in respiration to provide energy

  4. Converted to sucrose for transport in the phloem

  5. As nectar to attract insects for pollination

  6. Converted to lipids for an energy source in seeds

  7. Converted to amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots
• the fate of glucose

  
• Photosynthesis


  Produces carbohydrates
• Plants contain many other types of biological molecule such as proteins, lipids and nucleic acid (DNA)
• As plants do not eat, they need to make these substances themselves
• Carbohydrates


  Contain the elements carbon, hydrogen and oxygen
• Proteins

  Contain nitrogen as well (and certain amino acids contain other elements too)
• Other chemicals in plants

  Contain different elements as well, for example chlorophyll contains magnesium and nitrogen
• Without a source of these elements


  Plants cannot photosynthesise or grow properly
• Plants obtain these elements

  In the form of mineral ions actively absorbed from the soil by root hair cells
• Mineral


  A term used to describe any naturally occurring inorganic substance
• Mineral deficiencies in plants

  
• mineral deficiencies table

  
• Plants make glucose in photosynthesis, but leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances and transported or stored as starch
• Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising
• Procedure to test leaves for starch


  1. Drop leaf in boiling water to kill cells and break down cell membranes

  2. Leave leaf for 5-10 minutes in hot ethanol in a boiling tube to remove chlorophyll

  3. Dip leaf in boiling water to soften it

  4. Spread leaf out on white tile and cover with iodine solution
• In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf
• Testing whether chlorophyll is needed for photosynthesis

  1. Use a variegated leaf (partially green and partially white)

  2. The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain blue-black

  3. The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored
• Care must be taken when carrying out this practical as ethanol is extremely flammable, so at that stage of the experiment the Bunsen burner should be turned off
• The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame
• Testing a variegated leaf for starch

  
• Investigating the Need for Light

  1. Destarve plant by placing in dark cupboard for 24 hours

  2. Partially cover leaf with aluminium foil

  3. Place plant in sunlight for a day

  4. Remove leaf and test for starch using iodine
• Destarving the plant ensures any starch already present in the leaves will be used up and will not affect the results of the experiment
• Results of the experiment

  • Area covered with aluminium foil remains orange-brown (did not receive sunlight and could not photosynthesise)
  • Area exposed to sunlight turns blue-black
• This proves that light is necessary for photosynthesis and the production of starch
• Investigating the Need for Carbon Dioxide

  1. Destarch two plants by placing in the dark for a prolonged period of time

  2. Place one plant in a bell jar which contains a beaker of sodium hydroxide (which will absorb carbon dioxide from the surrounding air)

  3. Place the other plant in a bell jar which contains a beaker of water (control experiment), which will not absorb carbon dioxide from the surrounding air

  4. Place both plants in bright light for several hours

  5. Test both plants for starch using iodine
• Leaf from plant placed near sodium hydroxide


  Remain orange-brown as it could not photosynthesise due to lack of carbon dioxide
• Leaf from plant placed near water

  Turn blue-black as it had all necessary requirements for photosynthesis
• An example setup for an experiment to test whether carbon dioxide is necessary for photosynthesis in plants.

  
• Plants usually used

  • Elodea or Cabomba types of pondweed
• Photosynthesis


  Oxygen gas produced is released
• Oxygen released


  Can be seen as bubbles leaving the cut end of the pondweed
• Measuring rate of photosynthesis

  Number of bubbles produced over a minute can be counted
• More bubbles produced per minute

  Faster the rate of photosynthesis
• More accurate version of experiment

  Collect the oxygen released in a test tube inverted over the top of the pondweed over a longer period of time and then measure the volume of oxygen collected