Photosythesis required practical

Introduction

During photosynthesis, plants use light energy to convert carbon dioxide and water into glucose and oxygen. The rate of photosynthesis is expected to increase with higher light intensity. Additionally, the amount of oxygen produced is proportional to the rate of photosynthesis—meaning that as light intensity increases, more oxygen is released. This can be easily measured in aquatic plants since the oxygen they produce forms visible bubbles that can be collected and counted.

Method

To measure the rate of photosynthesis using pondweed, follow these steps:

1. Use a measuring cylinder to add 40 cm³ of water to a boiling tube, and place the tube in a rack.

2. Cut a piece of pondweed approximately 10 cm long, and place it into the boiling tube, ensuring the top of the pondweed is fully submerged.

3. Set up a lamp 5 cm away from the boiling tube, and use a ruler to accurately measure the distance.

4. Place a clear piece of Perspex between the lamp and the tube to act as a heat shield, or alternatively, place the tube in a water bath maintained at 20°C to keep the temperature constant.

5. Allow the plant to acclimatise to the light for 1 minute.

6. For the next minute, count the number of bubbles of oxygen released by the plant.

7. Repeat the procedure, moving the lamp to distances of 10 cm, 15 cm, 20 cm, 25 cm, and 30 cm from the boiling tube.

8. The experiment should be conducted three times at each distance, and a mean average should be calculated for each distance.

Experimental setup

The inverse square law

Light intensity decreases in proportion to the square of the distance from the light source. This is known as the inverse square law. For example, the light intensity at 10 cm is four times lower than at 5 cm, and at 15 cm, it is nine times lower than at 5 cm. As a result, the number of bubbles produced should decrease accordingly as the distance from the light increases.

Improving accuracy of measurements