Coursework on Photosythesis

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Coursework on photosynthesis

“Investigating link between light intensity and photosynthesis”

Aleksandra Kozielska, IB


Plants produce oxygen. Photosynthesis is the main source of O2, therefore we would like to conduct an experiment on this vital and extremely important process.

Background information

The process of photosynthesis converts light energy into the chemical energy of sugars and other organic matters. This process consists of a series of chemical reactions that require carbon dioxide - CO2 and Water - H2O, and store chemical energy in the form of glucose. Light energy, especially sun, drives the reactions. Oxygen - O2, is a by-product of photosynthesis and is released into the atmosphere. Photosynthesis occurs in plants, algae and other species that contain chlorophyll within them, as chlorophyll is vital for photosynthesis. It allows plants to create energy from the sunlight.


Our aim is to conduct an experiment which would allow us to establish the link between light intensity and its effect on photosynthesis. We would like to find out, whether distance between source of light and the plant have an influence on strength of photosynthesis.


We will measure two variables - the independent one which is light and also the dependent one which is photosynthesis. We will be measuring the number of bubbles that the plant produces at certain period of time while exposed to certain amount of light. Afterwards, we hope to be able to distinguish the link between intensity of light and photosynthesis and, therefore, test the theory that the more light there is the more photosynthesis occurs.


Our prediction supports the theory mentioned above. The more intense light there is, i.e. the closer the lamp to the container with pondweed, the more photosynthesis will occur, until a certain point in which the temperature or carbon dioxide levels will be the limiting factors. It is said that photosynthesis strongly relies on light, which is one of the most important factors for this process to take place. Another hypothesis is that as long as the plant is exposed to this artificial light, the photosynthesis will keep on occurring without big differences even when we change the distance between the plant and the source of the light.


Test tube
Beaker-500ml (container)
15cm strip of Elodea (pondweed)
Bicarbonate of soda
Weighting scales

The method we used to conduct the experiment was quite complex. Firstly, as soon as all the equipment was prepared, we placed Elodea in the container full of water. Secondly, we covered it with the funnel and placed a test tube on the top of the funnel, making sure that the test tube was full of water, as well. It looked like in the attached picture below:

Before exposing the plant to the artificial light, we placed in the water 1.5g of bicarbonate of soda to ensure enough carbon was available for photosynthesis. Finally, we measured 10cm distance from the middle of the container to the light and turned on the lamp. Three people were observing the test tube and counting the bubbles that were coming up from the pondweed, within exactly 10 minutes. The fact that there were three people in the group makes the experiment more reliable since all of them confirmed the number of bubbles as accurate. The same procedure was repeated four times. Every time, we were changing the distance between container with elodea and the lamp, by 20 cm. So the lengths were consequently: 10cm, 30cm, 50cm and, lastly, we did not use the lamp at all, to investigate if photosynthesis occurs without any light.

To make certain that the experiment was as much accurate and valid as it could possibly be, we were filling the baker with plain water every time we changed the distance, adding new bicarbonate of soda to assure the temperature of the water was the same at each point of our measurements....
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