PAG 05: Photosynthesis

Question 1

Aim

Answer 1

Investigate the effect of light intensity on the rate of photosynthesis of pondweed (e.g: ​Elodea, Cabomba)

Question 2

Equipment

Answer 2

Equipment
● a boiling tube
● freshly cut 10 cm piece of pondweed
● a light source
● a ruler
● a test tube rack
● a stopwatch
● 0.2% solution sodium hydrogen carbonate
● a glass rod

Question 3

Method

Answer 3

Method

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 4

Sources of error

Answer 4

Temperature may also be a factor affecting the rate of photosynthesis that is not accounted for.

Bubbles may form too quickly to be counted.

Question 5

Controlled variables

Answer 5

● Carbon dioxide concentration
Controlled variables
● Species of pondweed
● Temperature
● Time allowed for gas formation

Question 6

Extension - other factors that can be tested include:

Answer 6

Other factors that can be tested include:
- Temperature: by immersing the set-up in a range of thermostatically-controlled water baths.
- Carbon dioxide concentration: changing the concentration of sodium hydrogen carbonate
solution used.
- Different species of pondweed could be tested.

Question 7

Potential hazards

Answer 7

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.