Required Practical 6: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed

Question 1

Method to investigate the effect of light intensity on the rate of photosynthesis:

Answer 1

1. Take a boiling tube and place it 10cm away from an LED light source - LED light is used as they don’t release much heat and too much heat would change the temperature of the experiment
2. Fill the boiling tube with sodium hydrogen carbonate solution - sodium hydrogen carbonate solution releases CO2 which is needed for photosynthesis
3. Put a piece of pondweed into a boiling tube with the cut end at the top
4. Shine light from lamp onto beaker
5. Leave this for 5 minutes to acclimatise to the conditions in the boiling tube - should see bubble of gas produce from the cut end of the pondweed, this gas is O2 produced by photosynthesis
6. Start a stop-watch and count the number of bubbles produced in 1 minutes to measure rate of photosynthesis
7. Repeat this 2 more ties and calculate the the mean number of bubbles produced in one minute
8. Repeat entire experiment again changing the distance the boiling tube is from the LED light source in 10cm increments
9. Leave sufficient time at each new light intensity before taking measurements to allow pondweed to acclimatise
10. Use the same pondweed for all experiments
11. To control temperature place test tube in water bath

Question 2

What are the 2 main problems when investigating the effect of light intensity on the rate of photosynthesis?

Answer 2

• the number of bubbles can be too fast to count accurately
• bubbles are not always the same size - large bubble would count same as a small bubble

Question 3

How can you solve the problems in the method when investigating the effect of light intensity on the rate of photosynthesis?

Answer 3

• measure vol of oxygen produced instead of counting the bubbles
• place pond weed under funnel and catch bubble in a measuring cylinder
• use the measuring cylinder to measure the volume of oxygen produced

Question 4

How are the results of the experiment (investigating the effect of light intensity on the rate of photosynthesis) affected by inverse square law?

Answer 4

• if you double the distance from the light source, the light intensity will be 1/4 of the original
• as light is needed for photosynthesis therefore the number of bubbles will fall by four times
• This is because intensity = 1/d2 where d = distance