CO2 are the raw materials. There is hardly ever a shortage of supply of water in a plant but only 0.03% of the air around is CO2 so it’s actually pretty scarce as far as plants are concerned.
Chlorophyll is like an enzyme in that it works best when it’s warm – about 25-37°C. But if it exceeds that limit the chlorophyll enzymes work slower and at 45°C they die off, photosynthesis can no longer take place.
Aim
My aim is to prove that the rate of photosynthesis of the pondweed increases with a rise in light intensity.
Equipment List
∙ A test tube/boiling tube
∙ A 60watt lamp
∙ Pondweed – Elodea
∙ Hydro carbonate
∙ Water
∙ Water shield/heat shield
∙ A paper clip
∙ A ruler
∙ A spatula
Method
Firstly we set up the test tube, the water shield, and the lamp. The lamp should be pointing at the test tube with the water shield between the two to prevent the lights heat getting to the pondweed. In order to stop the pondweed from floating on the surface of the surface of the water and hydro carbonate, we must attach a paper clip to the bottom of it. And it is also important to cut the bottom of the pondweed at a 45° angle in order to maximize the surface area in which to absorb Carbon Dioxide. Vary the light intensity for photosynthesis by adjusting the distance of the lamp from the plant. Once the distance is set switch the lamp on activating a stopwatch at the same time. Measure the rate of photosynthesis by recording the number of bubbles given off in a set time. Depending on each group conducting the experiment, one can decide for how long to count the bubbles coming up from the plant for the duration of the experiment.
Accuracy
The test carried out must be done as accurately as possible, if not the graph will not show a positive correlation and as result nothing will be proven. Here are the ways to achieve the maximum accuracy in this experiment:
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The use of a water barrier efficiently in order to block out all heat.
- The same plant should be used throughout the investigation.
- The light intensity should be constant.
- Distances must be measured accurately.
- The table lamp is the only light source
- The carbon dioxide concentration in the water containing the plant should remain constant during the investigation.
Variables
The only variables throughout the investigation should be the light intensity by moving the lamp or changing the voltage supply.
Safety
There is no specific safety for this experiment except electrical hazards and no safety equipment is necessary.
Prediction
I predict that the rate of gas produced (O2 in the form of bubbles) increases with an increase in light intensity and therefore more photosynthesis is being produced.
Results
For each distance three set-ups were made, this greatly increases the accuracy of the graph and makes it easier to find a positive correlation. The mean value of the 3-4 results are used for plotting graphs.
The results above are fairly accurate, however there are ways to improve the accuracy here:
In order to prove my prediction I must, (using the table of results), plot a Bubble/Time curved graph, and a Bubble/Light Intensity curved graph. These are the two essential graphs as they would be sufficient to prove my prediction, but to look at the investigation and explain it in different aspects I could also do Light Intensity/Distance, and Bubbles/Distance curved graphs.
- (Graphs are on graph paper at the end of the investigation)
It is good to start with a high light intensity, e.g. at a distance of 10-20 cm between the plant and the light source, and then increase the distance by 10-20 cm each time until the rate of photosynthesis becomes undetectable.
The source errors are very important to maintaining a high accuracy. However, the fluctuation in room temperature is not as important as the other source errors as water is much less expansive than air. Therefore the system is very stable to small fluctuations in room temperature during the experiment.
I think that the source error that describes the consistency of the hydrogen carbonate is the one that probably affected my groups results the greatest. We were fairly careful with the other source errors but ideally we should have diluted the sodium hydrogen carbonate solution to roughly 5% to provide an abundant supply of it.
Other Investigations
The apparatus in the investigation can be used to study other factors on photosynthesis, such as:
- Light Quality – by covering the table lamp with cellophane of different colours one can investigate which colours from the colour spectrum effect photosynthesis the most or least.
- Carbon Dioxide Concentration – This can be varied by using sodium hydrogen carbonate solution of different concentrations (e.g. 0-5%.)
- Temperature – by connecting the measuring cylinder to a pipette, the apparatus can be put into a water bath. The following temperature range can be studied: 20-50ºC
Limiting Factors
The rate of photosynthesis is affected by three factors:
The Amount of Light (and the Wavelength)
The chlorophyll uses light energy to perform photosynthesis. It can only do it as fast as the light energy is arriving. Chlorophyll actually only absorbs the red and blue ends of the visible light spectrum, but not the green light in the middle, which is reflected back. This is why the plant looks green.
The Amount of Carbon Dioxide
CO2 and water are the raw materials. Water is never really in short supply in a plant but only 0.003% of the air around is CO2 so it’s actually pretty scarce as far as plants are concerned.
The Temperature
Chlorophyll is like an enzyme in that it works best when it’s warm but not too hot. The rate of photosynthesis depends on how active the chlorophyll enzymes are, (at 45°C the enzymes die and photosynthesis can no longer take place.)
At any given time one or the other of the above three factors will be the limiting factor which is keeping photosynthesis down at the rate it is.