I am going to repeat my experiment three times so that I will be able to draw to a reliable conclusion. In each experiment I am going to investigate 5 different distances between the lamp and the rest of the apparatus. My chosen distances are 5cm, 10cm, 20cm, 30cm and 40cm. I will count the number of bubbles given off at each distance for 1minute. Between the changes in distance I will allow a settling period of 3minutes for the pondweed to adjust itself to the change in light intensity.
I predict that the greater the intensity of light the faster the rate of photosynthesis, as there will be more available light for the chlorophyll to convert the carbon dioxide and water into glucose and oxygen. I predict that to begin with as light intensity increases, the rate of photosynthesis will increase at a proportional rate until a certain level is reached, and the rate of photosynthesis will then go down. Eventually, a level will be reached where an increase in light intensity will have no further effect on the rate of photosynthesis, as there will be another limiting factor e.g. temperature. If I was draw a curve of light intensity against the rate of photosynthesis, this is how I predict it would look:
I predict that I won’t be able to draw this curve with my results as I probably will not reach a high enough light intensity to level out my graph.
Preliminary
I completed a preliminary investigation to test my basic idea and equipment. I placed the pondweed 5cm away from the lamp. At first no bubbles were given off and so I decided to increase the temperature of the water from around 17°C to 25°C. The pondweed started to give off bubbles and I collected 17 in a minute. I then moved the lamp 10cm away from the equipment and collected 9 bubbles in a minute. After completing my preliminary investigations I decided that the water I use in my experiments should be at 25°C to encourage photosynthesis. Therefore all the water I use must be at 25°C to ensure a fair test. I didn’t experience any other problems with the equipment used in my preliminary investigation and so I decided I was ready to begin my experiments.
Practical – Obtaining my evidence
Aim: To discover what effect light intensity has on the rate of photosynthesis.
Apparatus: Canadian pondweed, funnel, supports, a test tube, a beaker, water at
25°C, a lamp, a ruler and a bung.
Method:
- Fill a beaker with 700cm³ of water at a temperature of 25°C.
- Cut the Canadian pondweed into a 3cm segment.
- Fill a test tube with water (also at 25°C) and put a bung into the end of it.
- Place a funnel on supports and lower it into the beaker with the pondweed inside it so that the water within the beaker also fills the funnel. Ensure that the stem of the weed faces up the funnel, as the majority of the bubbles will be released from here.
- Place the test tube of water over the funnel (shown below) ensuring that it is completely full of water to begin with.
- Move the equipment the designated distance from the lamp and switch the light on.
- Leave the equipment for a settling period of 3minutes for the weed to adjust to the light intensity.
- Record the number of bubbles given off in 1minute.
- Repeat this process for the 4 other distances between the equipment and the lamp.
- Repeat this experiment 3 times, recording all the results.
Results:
Conclusion: From my results I can conclude that the less light a plant receives the
less oxygen that it produces, thus the slower its rate of photosynthesis.
Therefore, light intensity does have an affect on the rate of
photosynthesis within plants.
Analysis
In my plan I predicted that the higher the intensity of the light the faster the rate of photosynthesis as chlorophyll absorbs sunlight and uses the energy in chemical reactions e.g. photosynthesis in which it produces glucose and oxygen. Thus the more energy (light) that it receives the faster the rate of photosynthesis. My results prove this theory and support my conclusion. I also predicted that at a certain distance, even if the light became brighter, the pondweed would not photosynthesis any faster. In my experiments I did not reach a high enough intensity of light to prove this theory 100% but my results suggest that this may well be so.
My graphs are in the form of best-fit curves rather than straight lines as there was a clear pattern within my results. This meant that the rate of photosynthesis increased as the light intensity increased. This is because photosynthesis is a reaction, which needs energy from light to work, so as the amount of energy available increased with the rise in light intensity, so did the amount of oxygen produced. My graph shows that the relationship between the light intensity and the rate of photosynthesis was non-linear, as all my graphs produced best-fit curves. The gradual decrease in the increase of the rate of photosynthesis is because of other factors limiting the reaction. These factors do not immediately limit the rate of photosynthesis but gradually until the rate of photosynthesis becomes constant.
Evaluation
As my graphs show I experienced anomalies in my results. I believe this is because there were many points where my accuracy left a little to be desired. Firstly, the distance between the lamp and the Canadian pondweed was not measured to a high degree of accuracy. Ideally the distance should have been measured from the filament of the light bulb to the centre of the plant but this was not possible. Another inaccuracy was in the time keeping as there was often a small difference in when I began timing the minute, e.g. after the first bubble had been released or just before.
Overall even though my experiment was open to some inaccuracies I believe it was accurate enough to support my predictions. To improve my results I could simply increase the length of time I measure the number of bubbles released to 5minutes although I would have to ensure that the water remained at the same temperature throughout. One way of doing this would be to place a Perspex block between the lamp and the plant, which would absorb most of the heat, while allowing the light energy to pass through. Another way of increasing the reliability of my results would be to take more readings.
I would like to extend my investigations into light intensity and photosynthesis by investigating which colour of the spectrum produces the highest rate of photosynthesis and what effect halogen and fluorescent lights have on the rate of photosynthesis.
Sources:
G.C.S.E. Biology – D.G. Mackean
Biology – Mary Jones & Geoff Jones