4. Bioenergetics (required practical 6 - photosynthesis)

Question 1

What is the practical investigating?

Answer 1

The effect of light intensity on the rate of photosynthesis

Question 2

What are the steps for the practical? (8)

Answer 2

1. Take a boiling tube and place it 10cm away from an LED light source
2. Fill the boiling tube with sodium hydrogen carbonate solution
3. Put a piece of pond weed into the boiling tube with the cut end at the top
4. Leave this for 5 minutes to acclimatise to the conditions of the boiling tube
5. You should see bubbles of gas being produced from the cut end of the pondweed. This gas is oxygen, that is produced from photosynthesis
6. Start a stop-watch and count the number of bubbles produced in one minute
7. Repeat this 2 more times and calculate a mean
8. Do the whole experiment again from the start, increasing the distance to 20cm, 30cm and 40cm

Question 3

Why is an LED light used?

Answer 3

• these do not release very much heat
• and too much heat would change the temperature of the experiment

Question 4

Why is a sodium hydrogen carbonate solution used in this experiment?

Answer 4

Because it releases carbon dioxide, which is needed for photosynthesis

Question 5

What are the two problems with this practical?

Answer 5

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

Question 6

What is done if the volume of oxygen is wanted to be measured, not counting bubbles?

Answer 6

1. Take a boiling tube and place it 10cm away from an LED light source
2. Fill the boiling tube with sodium hydrogen carbonate solution
3. Put a piece of pond weed into the boiling tube with the cut end at the top
4. Place the pond weed under a funnel and catch the bubbles in a measuring cylinder filled with water
5. Use the measuring cylinder to measure the volume of oxygen produced

Question 7

What is the inverse-square law?

Answer 7

If we double the distance, the light intensity falls by four times

Question 8

If the light intensity falls by four times, why does the number of oxygen bubbles to fall by four times?

Answer 8

Because light is needed for photosynthesis