photosynthesis

• Exam Q: a student investigated photosynthesis using pondweed. method:

  • switch on lamp
  • after 20 mins record vol of 02 collected in measuring cylinder suggest 2 ways the method could be improved so the results would be more valid
  • repeat and calculate a mean
  • control the water temperature
  • control co2 concentration
• What is the first step in the practical investigation?

  • Take a boiling tube
  • Place it 10 cm from an LED light source
• Why is an LED light used in the experiment?

  It doesn't release much heat
• What should be done if a normal light bulb is used?

  Place a beaker of water between the light and boiling tube
• What does sodium hydrogen carbonate solution release?

  Carbon dioxide
• What is the purpose of adding pond weed to the boiling tube?

  To observe oxygen production during photosynthesis
• How long should the pond weed acclimatize in the boiling tube?

  Five minutes
• What gas is produced by the pond weed during photosynthesis?

  Oxygen
• How is the rate of photosynthesis measured in this experiment?

  By counting the number of bubbles produced
• How many times should the bubble count be repeated?

  Two more times
• What is the next step after calculating the mean number of bubbles?

  Repeat the experiment at different distances
• What distances should the experiment be repeated at?

  20 cm, 30 cm, and 40 cm
• What are the two main problems with counting bubbles?

  • Bubbles can be too fast to count accurately
  • Bubbles are not always the same size
• How can the problems with counting bubbles be solved?

  Measure the volume of oxygen produced
• What equipment is used to measure the volume of oxygen produced?

  A measuring cylinder
• What happens when the mean number of bubbles is plotted against distance?

  It shows a decrease in bubbles with distance
• What is the relationship between distance and the number of bubbles produced?

  If distance doubles, bubbles fall by a factor of four
• What is the scientific term for the observed relationship in the experiment?

  The inverse square law
• Why does light intensity affect the rate of photosynthesis?

  Light is needed for photosynthesis
• What should higher tier students be able to explain regarding the results?

  • How results are affected by the inverse square law
• What is the effect of doubling the distance from the light source?

  It causes the light intensity to fall by four times
• How does the inverse square law relate to photosynthesis?

  It explains the decrease in oxygen bubbles