Required practical activity 5: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.

Question 1

Hazards of practical

Answer 1

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 2

Aim of practical

Answer 2

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 3

Equipment

Answer 3

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 4

First step of practical

Answer 4

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.

Question 5

second steps of the practical;

Answer 5

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

Question 6

third steps of practical

Answer 6

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 7

Sources of error

Answer 7

Temperature may also be a factor affecting the rate of photosynthesis and this is not taken account
of. (USE A THERMOMETER

Bubbles may form too quickly to be counted. Use a

Question 8

What else can you change to see the rate of photosynthesis

Answer 8

different concentrations of sodium hydrogen carbonate

different temp

or different amounts of chlorophyll