Investigation on How the Amount of Light Affects Photosynthesis.

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Murtaza Rana

Investigation on How the Amount of Light Affects Photosynthesis


In this investigation I hope to find out how the amount of light affects photosynthesis.


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 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. I am saying this because in photosynthesis the carbon dioxide and water convert, using light energy, to form glucose and oxygen. Chlorophyll, which is the green pigment present in the chloroplasts, stores the carbon dioxide and the light energy allows the process to happen Before I start my plan I would like to indicate that all green plants make food by the process of photosynthesis. This can be summarised by the following equation:

                    Carbon Dioxide + Water                   Glucose + Oxygen

6CO2 (g) + 6H2O (l)                              C6H12O6 (aq) + 6O2 (g)

The carbon dioxide comes from the respiration of plant cells and also forms the air and water around them. Chlorophyll, the green pigment present in the chloroplasts, is the catalyst for the process which absorbs the light. There are many factors, which affect the rate of photosynthesis, including light intensity, temperature and carbon dioxide concentration.

  • Carbon dioxide concentration – This can affect the rate of photosynthesis, since if there is too little CO2, it can become the limiting factor, thus slowing down the capability 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.
  • Water availability – water is also required in the photosynthesis reaction, and when it is lacking, the plants stomata close to prevent further water loss. This closing of the stomata cells also leads to little carbon dioxide being able to diffuse through. Clearly, in a water plant, like the pondweed, as long as the plant is fully submerged in water at all times, this will not be a problem.
  • Temperature – 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.
  • Light intensity- Increasing the light intensity of the experiment will mean that there will be more oxygen bubbles produced.

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 and not to let them affect the integrity of my investigation into the effect of light intensity. As I am going to investigate on how the amount of light affects photosynthesis I am going to use a lamp and move it away from a beaker. To do this I am going to use a plant called Elodea (a Canadian pondweed) which produces bubbles of gas when it is photosynthesising. This gas is mainly oxygen. I am going to fill a beaker with tap water and add a pinch of sodium hydrogen Carbonate. Then I am going to cut a leafy shoot of pondweed approximately 5cm long, slide a paper clip onto the shoot just behind the tip and place the pondweed into the beaker. After that I am going to switch the lamp on and bring it close to the beaker about 10cm back. I will leave the apparatus for about 3 minutes and count the oxygen bubbles which come out from the pondweed. If the experiment works I am going to do repeats so that the results are accurate. I am then going to repeat the experiment but this time I am going to move the lamp back 20cm and then so on. To make this a fair test I am going to use the same pondweed over and over again, add the same amount of hydrogen carbonate and I am going to use the same amount of water. Also, to make it a fair test I am going to leave the experiment for the same amount of time. The experiment will work only if the light from the light bulb is absorbed through the pondweed. While doing this experiment I am going to have to be careful not to get any sodium hydrogen carbonate in my eyes and be careful not to put the beaker full of water near the switch of the light bulb.

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I predict that changing the intensity of the light will affect the process of photosynthesis. The reason to why I have said light intensity affects 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 ...

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