21. Investigation into the effect of light on the rate of photosynthesis

  • 00:25 What is the colour change of the indicator if photosynthesis is occurring (faster than respiration)?
  • 01:14 Why must the balls be restored to room temperature before use?
  • 01:22 What colour is the indicator solution at the start?
  • 01:30 What colour is the indicator solution after illumination?
  • 01:32 What does the colour change tell you about the rate of photosynthesis and respiration of Scenedesmus during illumination?
  • How could you use this method to find out how light intensity affects the rate of photosynthesis? 
  • For making algal balls 

  • 5cm3 Scenedesmus quadricauda culture

  • glass rod

  • 3cm3 sodium alginate solution (3%)

  • beakers

  • 10cm3 syringe without needle

  • distilled water

  • 200cm3 calcium chloride (2g/100cm3)

  • tea strainer

  • For running the experiment 

  • algal balls

  • glass vial + stopper

  • 10cm3 hydrogen carbonate indicator

  • colour chart for indicator (School Science Review 85 (312) 37–45) or colorimeter with 550nm filter

  • metre ruler

  • fluorescent lamp

  • timer

Hazard Risk Control measure

Calcium chloride solution -irritant to skin and eyes

Contact with eyes and skin

Eye: flood with tap water (10min) Skin: drench with water. Flood with tap water (10min)

Hydrogen carbonate indicator - irritant to eyes

Contact with eyes

Eye: flood with tap water (10min)

Light source becomes hot during use

Risk of burns

Do not touch light source when in use or shortly afterwards