Photosynthesis required practical

Aim: To investigate how light intensity affects the rate of photosynthesis in pondweed by measuring the oxygen given off.

Variables

• Independent: Light intensity, changed by moving the lamp to different distances from the pondweed.
• Dependent: Rate of photosynthesis, measured as the number of oxygen bubbles or the volume of gas released per minute.
• Control: Temperature of the water around the plant; Concentration of carbon dioxide (same sodium hydrogencarbonate solution); Species, length and piece of pondweed; Lamp power and background light (dark room)

Equipment

• Pondweed such as Elodea or Cabomba
• Boiling tube or beaker and clamp stand
• Sodium hydrogencarbonate solution
• Lamp, ideally LED
• Metre ruler
• Stopwatch
• Paper clip to weigh down the weed
• Thermometer
• Optional: gas syringe or inverted measuring cylinder to collect gas

Method

1. Fill a boiling tube with sodium hydrogencarbonate solution to supply carbon dioxide, and clamp it in a stand.
2. Cut a fresh piece of pondweed about 10 cm long, attach a paper clip and place it in the tube cut end upwards.
3. Place the lamp 10 cm from the pondweed, measured with a metre ruler, in a darkened room.
4. Leave the pondweed for about five minutes to adjust to the light level.
5. Count the number of oxygen bubbles produced in one minute, or collect the gas and measure its volume.
6. Check the water temperature with a thermometer to make sure it has stayed the same.
7. Move the lamp to 20, 30, 40 and 50 cm, allowing time to adjust each time, and repeat the count.
8. Repeat the whole experiment three times and calculate a mean for each distance.

Safety

• Keep water away from the lamp and mains sockets, and do not touch the lamp while wet.
• Lamps get hot; allow them to cool before moving and use an LED where possible.
• Wash hands after handling pondweed.

Results and calculations

Record the number of bubbles or volume of gas per minute at each distance and calculate means. Plot rate against distance, or against light intensity using light intensity ∝ 1 ÷ distance² (the inverse square law), and describe how rate increases until another factor becomes limiting.

Common mistakes and improvements

• Heat from the lamp raising the water temperature; use an LED bulb or place a beaker of water between the lamp and the tube as a heat shield.
• Bubbles of different sizes make counting inaccurate; collect the gas and measure its volume in a gas syringe or measuring cylinder instead.
• Other light sources in the room affecting results; carry out the investigation in a darkened room.
• Not waiting for the plant to adjust after moving the lamp; leave a fixed acclimatisation time before each count.

Exam tips

• Explain the result: as light intensity increases, rate increases because more energy is available for photosynthesis, until carbon dioxide concentration or temperature becomes the limiting factor and the graph levels off.
• Use the inverse square law to calculate relative light intensity, for example halving the distance makes the light four times more intense.
• Explain why sodium hydrogencarbonate is added (it releases carbon dioxide so CO₂ is not limiting) and how temperature is controlled.
• Interpret limiting factor graphs and suggest why a commercial greenhouse might add heat, light or carbon dioxide.