The aim of my experiment was to determine whether or not the intensity of light would affect the rate of photosynthesis in a plant. To do this, I placed a piece of Canadian pondweed in varying light intensities, and observed the amount of oxygen being given off. I used Canadian pondweed because of its unusual quality of giving off bubbles of gas from a cut end, when placed in water. Photosynthesis occurs only in the presence of light, and takes place in the chloroplasts of green plant cells. Photosynthesis can be defined as the production of simple sugars from carbon dioxide and water causing the release of sugar and oxygen.
The chemical equation for photosynthesis can be expressed as: (light) 6CO2 + 6H2O i?? C6H12O6 + 6O2 (in the presence of chlorophyll) The fact that all plants need light in order to photosynthesise has been proven many times in experiments, and so it is possible to say that without light, the plant would die. The reason that light intensity does affect the rate of photosynthesis is because as light, and therefore energy, falls on the chloroplasts in a leaf, it is trapped by the chlorophyll, which then makes the energy available for chemical reactions in the plant.
Thus, as the amount of sunlight, or in this case light from a bulb, falls on the plant, more energy is absorbed, so more energy is available for the chemical reactions, and so more photosynthesis takes place in a given time. There are many factors, which affect the rate of photosynthesis, including light intensity, temperature and carbon dioxide concentration. The maximum rate of photosynthesis will be constrained by a limiting factor. This factor will prevent the rate of photosynthesis from rising above a certain level, even if the other conditions needed for photosynthesis are improved.
It will therefore be necessary to control these factors throughout the experiment so as not to let them affect the integrity of my investigation into the effect of light intensity. Predictions I predicted that as the intensity of light increased, so would the rate of photosynthesis. Furthermore, I hypothesised that if the light intensity increases, the rate of photosynthesis will increase at a proportional rate until a certain level is reached, and the rate of increase 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, in this case probably temperature. Preliminary work Initially, to ascertain a suitable range of distances at which to record results for my experiment, I did a preliminary investigation in which I recorded the number of bubbles of oxygen given off in a given time at various light intensities. To alter the light intensity, I placed a lamp at various distances from the plant.
I also therefore needed a way of accurately measuring the light intensity, and I did this using a photometer. I recorded the lux reading (unit of light intensity) at each distance. I got the following results: Results of preliminary experiment Distance Light intensity No. Bubbles (cms) (lux) 45 55 12 40 80 12 35 110 13 30 149 14 25 208 16 20 310 18 15 590 20 10 945 21 5 1015 21 Although this is a very quick, simple and efficient way of obtaining an idea of the trends for the graph, and the boundaries for the measurements, this experiment was not in itself in my opinion accurate enough to be the basis of my main experiment.
This lack of accuracy was mainly due to the fact that by simply counting the bubbles, I was relying on each bubble being exactly the same size, which they clearly were not. The preliminary experiment will, however, give me a best fit curve to which I can compare my main graph, and also points at either end of my results at which it is clear to see light intensity has little or no effect. Here, it was in fact at a light intensity of around 950 when it seems that another factor such as temperature or carbon dioxide concentration has become a limiting factor.
In my main experiment therefore, it will not be necessary to take readings above this point. It also shows that while my outer limits are justified, it would be better to take more readings between the distances of 10 and 20 centimetres, as the distance between the points is large at this point, and so I have decided to take readings at the following distances: 5, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40 and 45, cm’s. Method Input variables – light intensity is to be varied by increasing and decreasing the distance from the light source to the plant.
Output variables – volume of oxygen produced (rate of photosynthesis) is to be measured by finding the volume of oxygen produced in a minute, and thus finding the rate of photosynthesis Control variables -Light wavelength (colour) – light energy is absorbed by the pigment, chlorophyll, in the leaf. Chlorophyll easily absorbs blue light, in the 400-450 nm range, and also easily absorbs red light, in the 650-700 nm range. However it does not easily absorb green or yellow light, rather it reflects them, decreasing the amount of light absorbed, and therefore the rate of photosynthesis.
This can easily be controlled, simply by using the same lamp throughout the experiment. Carbon dioxide concentration – This can affect the rate of photosynthesis, since if there is too little CO2, it can become the limiting factor, thus impeding the viability of the experiment. In this case, as long as the experiment is done over a short period of time, the amount of carbon dioxide used up by the plant will not be sufficient enough to cause the carbon dioxide concentration to become the limiting factor.