Investigating Photosynthesis (WJEC GCSE Biology: Combined Science)

• It is possible to investigate the effect of environmental factors on photosynthesis, e.g.
  • A water bath can be used to alter environmental temperature
  • A lamp with different bulbs, or at different distances, can be used to alter light intensity
  • Sodium hydroxide can be used to assess the effects of removing environmental carbon dioxide

Using sodium hydroxide to absorb carbon dioxide

• Sodium hydroxide absorbs carbon dioxide from the air, so it can be used to demonstrate that carbon dioxide is required for photosynthesis:
  1. Destarch two plants by placing them in the dark for a prolonged period of time
     • The plants will use up their starch reserves as they are unable to photosynthesise
     • This removes any starch that may be present in the leaves before the start of the investigation
  2. Place one plant in a bell jar which contains a beaker of sodium hydroxide
  3. Place the other plant in a bell jar which contains a beaker of water
     • The water will not absorb carbon dioxide from the surrounding air, so acts as an experimental control
  4. Place both plants in bright light for several hours
  5. Test both plants for starch using iodine solution, as described below
• We would expect to see the following results:
  • The leaf from the plant placed near sodium hydroxide will test negative for starch; the lack of carbon dioxide means that photosynthesis cannot occur
  • The leaf from the plant placed near water will give a positive starch test result as it has all the requirements for photosynthesis

Using sodium hydroxide to absorb carbon dioxide diagram

Sodium hydroxide can be used to absorb carbon dioxide (condition A) and compared to an experimental control (condition B)

Testing a leaf for the presence of starch

• It is possible to test whether or not a plant has been photosynthesising by checking for the presence of photosynthetic products, e.g. glucose
• Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances or stored as starch
• Starch is stored inside chloroplasts so testing a leaf for starch is a reliable indicator that photosynthesis has occurred

• Leaves can be tested for starch using the following procedure:
  1. Destarch two plants by placing them in the dark for a prolonged period of time
     • This removes any starch that may be present in the leaves before the start of the investigation
  2. Place the leaf in boiling water
     • This kills the leaf, stopping any chemical reactions occurring in the cells
  3. Place the leaf in hot ethanol for 5-10 minutes
     • This removes the chlorophyll so a colour change can be seen more clearly
     • Note that ethanol is highly flammable so any Bunsen burner used in Step 1 must be switched off before starting Step 2
     • A water bath can be used here instead
  4. Rinse the leaf in water
     • This softens the leaf and removes any ethanol
  5. Spread the leaf on a white tile and add several drops of iodine solution
     • Iodine solution is used to test for the presence of starch
• The expected results would be as follows:
  • In areas where photosynthesis has been taking place the leaf will turn blue-black; this is a positive starch test result
  • In areas where photosynthesis has not been taking place the leaf will not turn blue-black, but will remain the brown-yellow colour of iodine; this is a negative starch test result

Testing a leaf for starch diagram

Leaves can be tested for starch by boiling in water, then in ethanol, then using iodine solution to indicate where starch is present

Using sensors & dataloggers

• Electronic devices such as gas sensors and data loggers can be used to measure the uptake and/or release of the gases involved in photosynthesis; this can give a measure of the rate of photosynthesis:
  1. Place a plant into a sealed container, such as a bell jar
     • The sealed container prevents gases from entering or leaving
  2. Use a gas sensor connected to a data logger to measure the concentration of oxygen and/or carbon dioxide inside the container
     • The sensor detects atmospheric gas concentration
     • The data logger records the concentration over time
  3. Take continuous measurements over a set period of time, e.g. 1 hour

• The changes in concentration of the different gases over a set time give a measure of the rate of photosynthesis
  • Carbon dioxide is used in photosynthesis, so the concentration of carbon dioxide will decrease over time
  • Oxygen is produced, so the concentration of oxygen will increase over time

Sensor & data logger diagram

A sensor attached to a data logger can be used to measure changes in oxygen and carbon dioxide levels over time