RP6: Light Intensity

Question 1

What is the aim of this experiment?

Answer 1

• Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed, measured by counting the number of O2 bubbles formed per minute.

Question 2

Describe how you would investigate the effect of light intensity on the rate of photosynthesis

Answer 2

1. Place a test tube rack containing a boiling tube 10 cm away from the light source, measured
   using the ruler.
2. Fill the boiling tube with a fixed volume of sodium hydrogen carbonate solution.
3. Place the cut pondweed into the boiling tube with the cut end at the top. Gently push the
   pondweed down with the glass rod.
4. Leave the boiling tube to rest for 5 minutes.
5. Start the stopwatch and count the number of bubbles produced in one minute.
6. For each light intensity/distance, repeat the count twice more and take a mean.
7. Record in a table as seen below.
8. Repeat steps 1-7 for 3 more distances (20, 30, 40 cm) of the boiling tube from the light source.
9. Plot a graph of the rate of photosynthesis (given by the no. of bubbles) against light intensity (using the inverse square law, light intensity = 1/distance2 between pondweed and light source).

Question 3

List some risks

Answer 3

• There is a potential allergy risk from the pondweed.
• Lamp may get hot.
• Be careful to keep water away from electrical power outlets and wiring

Question 4

List some sources of error

Answer 4

• Temperature may also be a factor affecting the rate of photosynthesis and this is not taken account of.
• Bubbles may form too quickly to be counted.

Question 5

Why is it best to use an LED light source rather than a lamp?

Answer 5

• A lamp will emit more heat energy than an LED
• Therefore it may change the temp of the experiment and affect the rate of photosynthesis

Question 6

How can the measurement of oxygen production be made more accurate?

Answer 6

• If the pondweed is placed under a filter funnel with an inverted measuring cylinder over the spout, the volume of oxygen produced can be determined

Question 7

Why is sodium hydrogen carbonate solution used during this experiment?

Answer 7

• It releases CO2 - used by pondweed during photosynthesis

Question 8

Why can the no. of bubbles be used to calculate the rate of photosynthesis?

Answer 8

• Oxygen bubbles are formed as a product during photosynthesis
• The no. of bubbles produced in a given time is proportional to the rate of photosynthesis