Collected Data.
Table 1.Data collected during the 2 minutes when the plant had access to additional light at a controlled distance. The bubble end and start location were measured by eye and rounded to the nearest 0.05cm.
Processed Data
Light intensity is calculated by this formula:
The intensity of light falling on a given object from a constant source is inversely proportional to the square of the distance between them.
In mathematical notation:
where I is the light intensity and R the distance in metres.
The constant in this proportionality is the energy used by the light that is 4800 J (obtained by: 40 Watt × 120 seconds = 4800 J).
Therefore the light intensity is calculated by:
The unit used for light intensity is kJm-2.
The speed, in cm s-1 is calculated by calculating the change in displacement and dividing it by the time (120 seconds).
Table 2. These are the processed results of the raw data.
Graph 1. This is the graph of light intensity plotted against the speed of the bubble, a good indicator on the rate of increase of photosynthesis. A best-fit equation was formulated and a log relationship was chosen because that is the one that arise in a light intensity against rate of reaction graph. However the equation in this situation only gives us a rough estimation of what the speed will be only on an exact same kind of lab.
It can be seen, even with only the calculated values, that after about 100 kJm-2 the rate of increase stabilizes , meaning that it is very near to its optimum value and that another factor than light must be changed for a maximized optimum.
In Blackman and Wilson’s “Physiological and ecological studies in the analysis of plant environment” they found that "the net assimilation rate during the season of active growth is linearly related to the logarithm of the light intensity". This is why a logarithm best-fit equation was produced, which smoothly fits the graph. In addition, a logarithmic relationship was to be expected as the way the light intensity varied with distance meant that it increased in an exponential way.
Conclusion
The results of this lab do show a relationship between light intensity and output of excess oxygen theoretically due to photosynthesis in the elodea plant. In the case of the used elodea, in a room of stable temperature and other matching conditions of the lab have an optimum excess of oxygen create the potometer complex’s bubble to move at a speed of about 0.0035 cm s-1. In other words, the rate of increase of the reaction of photosynthesis quickly approaches the possible optimum in the shown conditions. The excess energy is left to convert into heat thus wasted, this waste of energy due to excess happened around 10cm distance of the elodea from the lamp. In addition, one must not interpret the light intensity of each different distance as the amount of energy received by the plant. The light intensity calculated means the energy per square meter emitted in the three-dimensional sphere of emitted light, the energy received by the elodea is only a tiny fraction if this as it is only a small patch in this imaginary three-dimensional sphere. This leads us to be able to say that elodea uses a very small amount of energy to photosynthesize, meaning that a lot of energy was wasted in the process and the physical concept of energy efficiency tells us that a large amount of energy was wasted for the photosynthesis of one plant. Additionally, the results we have obtained also lead to the observation that light is a limiting factor as depriving the elodea from light had a negative impact on photosynthesis rate.
Finally, light being an extremely fluctuating factor (day and night cycles), much more than carbon and hydrogen availability we can also assume that light is one of the most important limiting factors of photosynthesis in wild plants. Secondly we can explore the complex shape that elodea has evolved and adapted into and its need to photosynthesize is a possible explanation for its high number of leaves, among many other explanations as to why little but numerous leaves where more appropriate than large ones for this pondweed.
Evaluation
The most important assumption in this lab is that the speed of the bubble is a good estimation of the rate of increase, however quite a few complications can question this assumption.
Firstly a plant also requires oxygen to survive and therefore some oxygen excreted was taken back in by the elodea. This means that the speed of the bubble only represented the excess of oxygen which was not taken in. Therefore, these do not represent an accurate rate of increase of photosynthesis, explaining why some numbers seem to be very close, as most of the oxygen was probably being taken back in. Another possible cause of problems is the fact that there was no wait between each step, meaning that the plant’s photosynthesis system did not have time to adapt completely to the new situation of increased or decreased light/energy input.
In addition, the plant was only supplied once in CO2 in a closed environment (large syringe) greatly reducing the possibility of CO2 diffusing back in. Therefore if this supply in CO2 diminished overtime then this experiment would have actually been about measuring the rate of photosynthesis and the availability of CO2, especially since we started measuring the bubble speed with the light at its closest and incrementally moved it away, meaning that CO2 was being consumed and its availability reduced as time passed. Despite this being unlikely, we could have randomized the location of the light distance and not just increase gradually. Then if CO2 was what we had actually been measuring, a plot of this graph would reveal meaningless data and no relationship with the distance. Then if CO2 is found to have an effect one would be obliged to repeat the tedious task of replacing the water each time with the same concentration as the initial time.
Something else to change is the distances measured in this experiment. As our way to estimate light intensity was by this equation:
This meaning that we had a large range of light intensity with no bubble speed measurement (From 120 to 480 kJm-2). Therefore one should have introduced many more values between 10 and 20 cm, for example 10-12-14-16-18-20cm then 30-40-50-60cm. Then the validity of the log relationship would either be further confirmed or simply discarded.
Also estimating light intensity so meant that a high uncertainty existed and one should have used the readily available lux meters instead. They have two advantages, firstly is that they are not subject to human raw observation but more importantly use the lux SI light unit, a much more concise way of measuring light intensity(energy per square meter cannot be converted into lumen but provides a good estimation nonetheless).
Another issue which might seem trivial but actually has some impact is the orientation of the elodea. As air naturally rises in water there is a high probability that air accumulated at the top of the syringe and did not cause the bubble(hence the measurements) to change and reflect the real rate of reaction. To solve this one would simply have to turn the syringe upside down with the cut end of the leaf upwards (where the gas easily exits).
Finally it would be a good idea to test the gas for its content in O2 as this is one of the best indicators for the rate of photosynthesis. This is a bit simpler than it may seem as but using the physical and chemical proprieties of gases one can estimate quite accurately the percentage by adding pyrogallol (absorbs oxygen and therefore changes the volume of a gas) and observing the results. The same is true for testing CO2 presence with potassium hydroxide. Additionally, both of these elements are relatively easy to be obtained.
Bibliography
BLACKMAN, G. E. and WILSON, G. L. "Physiological and ecological studies in the analysis of plant environment. VI." BLACKMAN, G. E. and G. L. WILSON. Physiological and ecological studies in the analysis of plant environment. VI. 1951. 63-94.
Fourier . 6. Affect of Light on Photosynthesis Rate. 18 March 2009 <http://www.fourier-sys.com/pdfs/new_experiments/nova_biology/photosynthesis.pdf>.
SMITH, EMIL L. "LIMITING FACTORS IN PHOTOSYNTHESIS: LIGHT AND CARBON DIOXIDE." From the Laboraiory of Biophysics, Columbia University, New York (1938): 15.
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