The Effect of Light Intensity on The Rate of Photosynthesis in Elodea (pondweed)

In this experiment, we were testing the rate of photosynthesis in elodea. For a plant to photosynthesize, it needs carbon dioxide and water and sunlight, a factor of photosynthesis. In order for us to measure the rate of photosynthesis, we needed to measure the products that were made, glucose and oxygen. As oxygen was a visible product, we counted the oxygen bubbles made when we moved the source of light.

Hypothesis: Plants need light to photosynthesize. In this experiment, the light intensity is changed by changing the distance of the source of light closer or further away from the elodea, so theoretically, when the intensity becomes stronger due to the light source being closer to the Elodea, the rate of photosynthesis should increase and therefore the number of oxygen bubbles will increase too. From further away, the light intensity will be less, and therefore light will be a limiting factor, however when we increase the light intensity by moving the source of light closer towards the Elodea, which then means that light cannot be the limiting factor of photosynthesis. As light is no longer the limiting factor of photosynthesis, it will then photosynthesize at a high rate until another variable becomes a limiting factor.

Apparatus:

• Elodea
• Lamp
• Beaker
• Test tube
• Water
• Meter ruler
• Stopwatch
• Funnel

Method: Firstly, you fill the beaker with water and place the Elodea inside. Place the funnel on top of the elodea overturned, and place the test tube on top of the funnel. Set up your meter ruler and lamp 50 cm away from the Elodea. Record the data accumulated over five minutes and count the bubbles that rise to the surface. Repeat this with 40cm, 30cm, 20cm, and 10cm. Record the data. So that the experiment is fair to repeat the experiment three times to gather a mean from your data if you have time.

Variables:

Independent Variable:

• Light intensity

Dependent Variable:

• • Rate of photosynthesis
  • Number of oxygen bubbles produced

Controlled Variable:

• • • The time when the oxygen bubbles are counted must be the same (5 min)
    • The distance between the elodea and the lamp
    • The temperature of the water
    • The mass of Elodea
    • The volume of water

Risk Assessment: How It Can Be Avoided?

The Elodea could fall on the floor and the water could spill;

Keep the water and Elodea away from the edges of the table and make sure that it is away from anything it could damage, e.g. books or any electrical outlets.

Glass Equipment Could Break;

Use plastic utensils or handle all equipment safely and with caution;

I believe that my hypothesis was valid based on my results, excluding the 20cm measurement. I believe that the higher the light intensity is, increases the rate of photosynthesis as it is would no longer be a limiting factor of photosynthesis and therefore the plant would photosynthesize at a higher rate, which is shown in my results. My investigation has proved evidence to support my hypothesis. Evaluation: Overall, there was an increasing trend, however at 20cm there was an anomaly that occurred as it did not follow the pattern that the other results displayed. However, this could have been due to a number of factors, the plant may have been tired or there could have been another limiting factor, e.g. temperature or carbon dioxide. One of the variables that we could not control was the size of the oxygen bubbles or the age of the plant which may have influenced our experiment. I do not believe that our measurements were accurate due to influential factors that may have affected the results. There were external light intensities that also may have influenced the results. To improve my investigation, I would

• • • Make sure that no other light sources could affect the experiment, to close all the blinds and
    • Measure the oxygen produced using a capillary tube or another device which could measure more accurately how much oxygen is produced.
    • To avoid the lamp being crooked and not providing the full beam intensity the lamp could be fixated other than to move it closer to the Elodea

.

• • As light could also be sourced from the other experiments, have lamps facing opposite directions when doing experiments on a desk, or alternatively conduct the experiment in separate rooms
  • To get rid of any other anomalies, the experiment could be repeated three times.
  • Different pieces of Elodea should be used in each experiment as the Elodea’s photosynthesizing rate decreases over time.