B4 - photosynthesis

• Light intensity practical:

  1. Light is placed at specific distance from pondweed
  2. Pondweed is left to photosynthesise for set amount of time - as it photosynthesise, oxygen released is collected in capillary tube
  3. At end of experiment, syringe is used to draw gas bubble in tube - ruler used to measure length of gas bubble - proportional to volume of oxygen produced
  4. Experiment is repeated twice with light source at same distance and mean volume of oxygen is calculated
  5. Experiment is repeated with light source at different distances

  • Any variables that can affect results are controlled
• • Distance and light intensity are inversely proportional
  • Light intensity = 1/distance^2
• • Sodium hydrogencarbonate is used because it releases carbon dioxide, which is needed for photosynthesis
• method:

  1. add 5g of sodium hydrogen carbonate to test tube
  2. place test tube in beaker of water to absorb heat
  3. place pond weed in test tube using forceps
  4. place test tube 10cm away from lamp
  5. wait 5 minutes to acclimatise
  6. count number of oxygen bubbles per minute
  7. repeat 3 times at same distance
  8. move plant to 20cm and repeat until 40cm away from lamp
• • to calculate volume of oxygen produced could be done by collecting gas produced in gas syringe
  • using coloured acetate filters could investigate effect of different wavelengths on light
• the effect of chlorophyll:

  1. drop leaf into boiling water to kill and preserve it
  2. leaf left for 10 minutes in hot ethanol in boiling tube to remove chlorophyll
  3. leaf dipped in boiling water to soften
  4. leaf spread out in Petri dish and covered in iodine solution
  5. areas that had chlorophyll stain blue black and areas without chlorophyll remain pale