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Investigate the factors that affect the rate of photosynthesis on a green plant.

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Introduction For this piece of coursework I have been set a task to investigate the factors that affect the rate of photosynthesis on a green plant. I stress that it is a green plant because not all plants have the pigment chlorophyll, that's why not all plants are green. Photosynthesis is a chemical reaction occurring in the leaves of green plants. Using the energy from sunlight, it changes carbon dioxide and water into glucose and oxygen, where oxygen is the waste product. Glucose can be converted to sucrose and carried to other parts of the plant in phloem vessels. Glucose can also be converted into starch and stored. The starch can later be turned back into glucose and used in respiration. 6CO2 + 6H20 + Light energy==(chlorophyll)==> C6H12O6 + 6O2 For this experiment I will be using a Canadian pondweed called Elodea. Elodea will be good for this experiment because it has a tendency to release oxygen bubble from a cut end. Affecting factors As photosynthesis occurs there are many factors that can help and/or hinder the rate it occurs. The following factors can affect the rate of photosynthesis * Light intensity * Temperature * Carbon dioxide concentration * Water concentration * Chlorophyll concentration * Pollution * Mineral deficiency * Light colour/wavelength As light levels increase, photosynthesis increases until the light compensation point (LCP) occurs. The LCP is the light level where photosynthesis and respiration balance each other; where CO2 and O2 will move neither in nor out the leaf. Plants can survive at LCP only if stored reserves are available. With increasing light levels, photosynthesis increases and then levels off. This peak is called the light saturation point (LSP), after which there is no increase in photosynthesis with increasing light levels. Now another factor becomes the affecting factor. Energy bonds in CO2 and H20 are less than the bonds in O2 and C6H12O6 so extra energy needs to come from somewhere. ...read more.


Set up the apparatus as above but leaving out the pondweed. 2. Fill up the heat guard, beaker, boiling tube full of tap water. 3. Take one piece of elodea and cut to 8cm in length. 4. Pull 1-2cm of leaves off the top of the plant leaving a clear stem. 5. On the other end hook a paperclip around to hold the pondweed down when in the water. 6. Take the pondweed and submerge it in the water in the boiling tube. 7. Once in the water cut the tip of leafless end at an angle making sure it is under water at all times. 8. Put the plant at the set distance from light. 9. Set up the light guard around the whole experiment. 10. Leave the plant to acclimatize to its conditions for 2 minutes. 11. Take light intensity reading. 12. Once acclimatized blow into water with a straw for 10 seconds. 13. Now count the bubble for 10 minutes while recording the amount of bubbles produced every minute. 14. Repeat steps 8 to 12 for every distance twice. I set up two plants at the beginning of the experiment to see which one would photosynthesise better; I did this to make sure I will have a plant photosynthesising properly. Prediction I predict that if the intensity of light increases, so will the rate of photosynthesis. Furthermore, I hypothesise 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 slow 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. I also predict that if I double the distance between the lamp the plant the light intensity will half. Because of this I will predict further that half the light intensity will half the rate of photosynthesis. ...read more.


With low light intensity, the elodea receives some light energy from background light such as sunlight seeping through blinds or the light from the lamp of another student's experiment. To eliminate all background light, the experiment must be performed in a completely dark room. The method of the experiment could probably also be improved to obtain more reliable results. As already mentioned, the a capillary tube should be used in place of a test tube to accurately measure the volume of the oxygen produced. Due to the high rates of photosynthesis of the pondweed, readings should be taken within shorter time periods. I had originally chosen to count the number of bubbles in one minute but this produced miscounts in the readings. If during a repeated experiment, counting bubbles is still used, there is a smaller chance for human error when counting within a smaller time frame. To take my experiment further I could instead of making the distance the variable, I could make the light intensity the variable. Because when carrying out the experiment it wasn't exactly double the light intensity when you double the distance. So to get around this problem the plant could be moved away from the light until it is a certain light intensity. So for every reading I will set the light intensity to go up 50 lux every test done. I will do this buy increasing the distance between the plant and the lamp until it reaches the correct lux. This would make sure that the effecting factor was light intensity and not the distance. Even though the distance does affect light intensity and light intensity is supposed to be proportional to the distance this wasn't accurate enough to produce accurate results. This could lead to the question of whether or not other types of light, such as fluorescent lights or halogen lights, would have a different effect on the rate of photosynthesis. The only problem of this experiment is that in my school we don't have very accurate equipment and the light intensity fluctuates anywhere between 10-20 lux on the machine. 1 Ben Grinsted ...read more.

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