Photosynthesis: counting oxygen bubbles
Shine a lamp on pondweed and count the oxygen bubbles. Change the light distance, the light colour, the carbon dioxide and the temperature to find what limits photosynthesis.
Core concept
Plants use light energy to make glucose from carbon dioxide and water, giving off oxygen; light, carbon dioxide and temperature can each limit the rate
Topic: Plants
Green plants make their own food by photosynthesis: How green plants make their own food from light, water and carbon dioxide. All key words: carbon dioxide: A gas we breathe out. Plants take it in to make their food. All key words + water → glucose + oxygen: A gas in the air that living things need to breathe and that fire needs to burn. All key words, using light energy: What is needed to make things move, heat up, light up or grow. All key words trapped by chlorophyll: The green colouring in leaves that catches sunlight for photosynthesis. All key words. Water plants such as Hydrilla give off the oxygen as bubbles, so counting bubbles each minute shows how fast photosynthesis is happening. The rate depends on light intensity, the amount of carbon dioxide, and temperature, because the reactions are controlled by enzymes. Whichever of these is in shortest supply limits the rate: it is the limiting factor.
What you will learn
- State the word equation for photosynthesis.
- Measure the rate of photosynthesis by counting oxygen bubbles.
- Explain how light, carbon dioxide and temperature affect the rate, and identify a limiting factor.
Before you start
- Know that plants need light to grow.
Materials
- Hydrilla (pondweed) sprig
- Sodium hydrogencarbonate (baking soda) to supply carbon dioxide
Equipment
- Lamp, beaker, funnel and test tube, ruler, stopwatch, thermometer, colour filters
Procedure
- With some sodium hydrogencarbonate (carbon dioxide) and water at 30 °C, count the bubbles with the lamp 10 cm, 20 cm and 40 cm away.
- With the lamp 10 cm away, compare no extra carbon dioxide with lots.
- Heat the water to 50 °C and count again.
- Put a green filter on the lamp and count again.
How this simulation is simplified
- Light intensity falls with the square of the distance from the lamp; all bubbles are pure oxygen and equal in size.
- The rate follows a simplified limiting-factor model: maximum 40 bubbles per minute.