PAG 5: Photosynthesis

Question 1

Cabomba - how to measure rate of photosynthesis?

Answer 1

When Cabomba photosynthesises, it gives off oxygen bubbles. The rate of photosynthesis can be monitored by counting the bubbles.

Question 2

3 factors that affect rate of photosynthesis

Answer 2

The rate of photosynthesis is affected by light intensity, temperature and carbon dioxide concentration. You will choose one of these variables to investigate.

[Different species of pondweed could be tested.]

Question 3

How vary light intensity

Answer 3

The Cabomba can be provided with light using a lamp,

The light intensity can be varied by changing the distance between the lamp and the Cabomba.

Question 4

How vary temperature

Answer 4

Temperature can be varied by changing the temperature of the water the Cabomba is in.

Question 5

How vary carbon dioxide

Answer 5

The Cabomba can be provided with carbon dioxide by placing the plant in a hydrogen carbonate solution

Carbon dioxide concentration can be altered by changing the concentration of the hydrogen carbonate solution

Question 6

What must happen if temperature is being controlled

Answer 6

If the temperature is being controlled, a large beaker of water placed between the Cabomba and the lamp can be used as a heat shield to stop the plant being warmed by the lamp.

Question 7

Safety

Answer 7

Wash hands after contact with Cabomba and pond water
Lamp may get hot so take care when near it
Keep electrical equipment dry and do not handle any electrical equipment if hands are wet
Dispose of the pondweed responsibly

Question 8

Equipment and materials

Answer 8

• Cabomba
• scissors
• boiling tube of water
• test tube rack
• large beaker of water
• lamp
• metre rule
• stopwatch
• thermometer

Question 9

Method (8) - changing light

Answer 9

1 Cut a piece of Cabomba 8–10 cm long. Place it in a boiling tube of water with the cut end uppermost.
2 Use the thermometer to measure the temperature of the water in the boiling tube.
3 Place the lamp 15 cm from the boiling tube. Place the large beaker of water between the lamp and the boiling tube.
4 Wait until there is a steady flow of bubbles from the cut end of the Cabomba. Count the number of bubbles observed in 2 minutes. Record this in your results table.
5 Measure the temperature of the water in the boiling tube again to make sure it has not changed.
6 Repeat steps 1–5 four more times but in step 3 increase the distance between the lamp and boiling tube by 2 cm each time.
7 Repeat the whole investigation (steps 1–6) two more times and record the results.
8 Calculate the mean number of bubbles for each distance, leaving out any anomalous values from your calculations. Record these in your table. Use these to calculate the rates of bubble production per minute.