Activity 8a: To examine the effect of light intensity on the rate of photosynthesis

Chapter 8

Step 1

Take some pondweed and place it in a beaker of water containing a teaspoon of sodium biocarbonate dissolved in it to keep the CO2 concentration constant. Place the whole apparatus in a water bath to keep the temperature constant

Step 2

Cover the pondweed with an upturned filter funnel and invert a test tube filled with water over the nozzle of the funnel

Step 3

Move the lamp to a set distance (60cm) from the pondweed and allow it to sit for five minutes to allow the plant time to adjust to the conditions

Step 4

Measure the rate of photosynthesis by counting the number of bubbles rising from the pondweed per minute at this light intensity. Repeat the count three times and record the average

Step 5

Love the lamp to 50cm from the pondweed and leave for five minutes to allow the plant to adjust. Count the number of bubbles that rise in a minute three times and record the average

Step 6

Repeat this at 40, 30 and 20cm

Step 7

Draw a graph of the rate of photosynthesis (as bubbles per minute) against the light intently (as 1/distance from the lamp). To use whole numbers, multiply your answer by 100

Result

As the light intensity increases, so does the rate of photosynthesis. Then, a limiting factor comes into effect. In this case it can be CO2 concentration or temperature