Required Practical 06: Photosynthesis

Question 1

Describe how you would investigate the effect of light intensity on the rate of photosynthesis in an aquatic plant. (8)

Answer 1

1. Set up test tube rack containing a boiling tube 10 cm away from the light source.
2. Fill boiling tube with sodium hydrogen carbonate solution.
3. Place piece of pondweed into boiling tube with the cut end uppermost and gently push it down with the glass rod.
4. Leave the boiling tube for 5 minutes.
5. Start the stop watch and record the number of bubbles produced in one minute.
6. Repeat the count twice more so that the mean number of bubbles per minute can be calculated.
7. Move the test tube rack 20 cm, then 30cm and 40cm, from the light source and repeat steps 4‒6.
8. Plot a graph of light intensity (x) against rate of photosynthesis (bubbles per minute, y)

Question 2

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

Answer 2

A lamp will emit more heat energy than an LED - therefore it may change the temperature of the experiment and affect the rate of photosynthesis.

Question 3

What is the relationship between the distance from a light source and light intensity?

Answer 3

Inverse square law:
Light intensity ∝ 1 ÷ distance²

Question 4

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

Answer 4

If the 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 5

Why is sodium hydrogen carbonate solution used during this experiment?

Answer 5

It releases CO2, which is used by the pondweed during photosynthesis.

Question 6

Why can the number of bubbles produced be used to calculate the rate of photosynthesis?

Answer 6

The number of bubbles produced in a given time is proportional to the rate of photosynthesis where oxygen bubbles are formed, in the reaction, as a product.