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.
For glucose to be made in photosynthesis, water is split into hydrogen and oxygen molecules by the energy absorbed from the sun. The hydrogen then has to combine with the carbon dioxide to produce glucose. If this was left on its own, the hydrogen would eventually combine with the carbon dioxide but it would take a long time. That is why a catalyst is needed to quicken the process.
Enzymes are the catalyst used for the anabolic reaction. Enzymes work by colliding with the hydrogen and carbon dioxide. It is shaped to only accept hydrogen and carbon dioxide molecules. The lamp will provide more light energy, which the enzymes absorb to work faster. They work faster by colliding more frequently. Because of that, it produces more glucose quickly. As with all enzymes, it has an optimum temperature and after that, the enzymes denature. It cannot act as a catalyst anymore and the rate decreases.
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. We noticed that when the sodium hydrogen carbonate was added to the water many bubbles appeared. So it was necessary to decide which bubbles we should count. We decided to only count the larger bubbles which were formed from the cut end of the elodea. To alter the light intensity, I placed a lamp at various distances from the plant.
Results of preliminary experiment
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 reasonable idea of how the experiment should follow.
Method
The equipment needed in order to carry out the experiment-
Thermometer Metre ruler
Spatula Clamp
Lamp Sodium Hydrogen Carbonate
Stop clock water
Elodea Boling Tube
Paper clip
We cut a 10cm piece of elodea with the cut end kept up by anchoring it with a paperclip into a boiling tube with 10ml of water also with a spatula of sodium hydrogen carbonate to ensure a steady supply of carbon dioxide. Next we inserted a thermometer into the beaker, and record the temperature at the beginning and end of each experiment, merely as a precaution against a significant rise in temperature, which is not expected. We will attach the boiling tube to the clamp. We will then position the clamp and Set up a lamp at a set distance from the plant, ensuring that this distance is from the filament of the lamp to the actual pondweed, rather than the edge of the beaker. When bubbles are being produced at a steady rate we will then Start the stopwatch, and wait for 3 minutes, then record how many bubbles were produced in that time, at that distance.
Fair Test
We will keep the wattage of the bulb the same -60 watts, so that the results are fair and accurate. Also light energy is absorbed by the pigment, chlorophyll, in the leaf. 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.
The elodea will remain the same 10cm and also be constantly submerged in water so as not to affect the rate of photosynthesis. The temperature will also remain as constant as possible - Enzymes are used in the photosynthesis reactions of a plant. Therefore, temperature will increase the rate of photosynthesis, until a point at which the enzymes denature. I decided that the inaccuracy of maintaining a constant temperature would outweigh any advantages. I am therefore going to perform the experiment at room temperature, checking the temperature frequently.
Holly Squire Science 22nd