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Grades 7–10 Intermediate About 25 min Version 1 Awaiting review Premium

Photosynthesis: counting oxygen bubbles

Green plants make their own food by photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll. 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

  1. 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.
  2. With the lamp 10 cm away, compare no extra carbon dioxide with lots.
  3. Heat the water to 50 °C and count again.
  4. 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.