The aim of this investigation is to find out whether the change in light intensity affects the rate of photosynthesis in pondweed

The variables that will be examined in this investigation are the Independent, the Dependent and the controlled variable these are: Independent which is going to change throughout the investigation and they are the light intensity which will be increasing and the distance between the pondweed and the lamp, the Dependent which is going to be measured is the amount of oxygen bubbles that appear due to the pondweed photosynthesising and the controlled variable which will not change as the name suggests and this includes the same wattage bulb, the same volume water in beaker and test tube and the same length of pondweed.

The equipment that was used in this investigation was: a Beaker, a Test Tube, Pondweed, a Lamp, a Metre Rule, a Thermometer, a Stop watch and water.

The Method that the investigation was carried out was:

• First we filled the beaker with 200ml of water and placed the 10 cm of pondweed into the test tube and put some water into the test tube.
• Next we placed the lamp and beaker at 40cm apart from each other for the first part of the practical.
•  We then turned on the lamp and started the stopwatch and waited 2 minutes for the process of photosynthesis to begin.
• We then stopped the stopwatch and started to count the amount of oxygen bubbles that rose to the top of the test tube.
• We then repeated this process again at the measurement of 35cm then 30cm and so on until we got to 5cm and then stopped and repeated the whole thing twice more so that we could have a range of results.

Diagram

Analysis

Looking back at the results shows that there isn’t really a pattern in them because they are all going up and then down or down and the up and its al changing. But if we take out a few results then we can try and realise from my prediction that the rate of photosynthesis is increasing while the light intensity increases. If the light intensity doubles the rate of photosynthesis does not usually double it increases but does not double. During the day the stomata on the leaves of a plant open they do this because the leaf needs to photosynthesise and it can do this because the guard cells around the stomata help it to open by having a thin elastic outer wall and having a thick inelastic inner wall and it swells up due to water intake using osmosis when the water goes from a low concentration to a high concentration and when it swells up it stretches the outer wall and it bends the inner wall and the stomata opens. The xylem and phloem tubes in a plant can reach anywhere in a plant and are part of the transpiration stream and are called vascular bundles the xylem carries water and minerals to the leaves of the plant for photosynthesis and the phloem carries the sugars that were produced due to photosynthesis which is translocation. The leaves need chlorophyll to absorb the light energy that is needed for all this to happen. During the reaction of water and carbon dioxide to glucose and oxygen the water molecules break down into the hydrogen and oxygen needed for the glucose and the carbon dioxide helps make the rest by breaking down as well. The limiting factor of this investigations can be explained like this: Without enough light a plant cannot photosynthesise very fast, even if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster. Sometimes photosynthesis is limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesise if it has run out of carbon dioxide. Temperature can be a limiting factor too. If it gets too cold the rate of photosynthesis will slow right down; equally, plants cease to be able to photosynthesise if it gets too hot. We also tried to use ICT to try and get rid of the anomalous results and the results can be seen at the bottom of this paragraph.

Conclusion

Looking at my results I can see that as the light intensity increases the rate of photosynthesis also increases this proves that my prediction was true.

Evaluation

My investigation went well because looking back at my results it shows that my prediction was true and this means that my investigation was done correctly as well. The reason it went well is because I was able to repeat the experiment and I was given the chance to do a preliminary coursework or a first draft to improve on. There was a couple of anomalous results which were not fitting with the pattern and looked out of place and this could be due to anything not being exact in the experiment. The reason could be to do with outside light not being completely blacked out the inside lights not being turned off or it could be the fact that we continued the experiment on for two days. To improve this we could black out the windows and turn off the inside lights we could have used the same plant over the two days or could have done the experiment in one day. The ICT experiment gave out no anomalous results and was a complete success and was completely accurate so it was better than the normal experiment.