The aim of the experiment is to determine what effect light intensity has upon the rate of photosynthesis of Canadian Pondweed (Elodea).

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Claire Shaftoe 10JTe

Biology Assessed Practical

Introduction- Photosynthesis is the chemical process, which takes place in every green plant to produce food in the form of glucose.  Plants use the suns energy to join together water and carbon molecules to make the glucose, which is sent around the plant to be used in respiration.

6CO   + 6H  O → C  H  O + 12O  (in the presence of light energy and chlorophyll)

Aim- The aim of the experiment is to determine what effect light intensity has upon the rate of photosynthesis of Canadian Pondweed (Elodea).

Prediction- I think that as the light intensity increases, the rate of photosynthesis will as well. This is because plants need light energy to photosynthesise, to join water and carbon molecules together to form glucose. If more light energy is available to the plant, then it will be able to photosynthesise at a faster rate. The light provides energy for the reaction to take place, and so if a plant has more light then it should have more energy to use in the reactions, and the rate of reaction should become faster. After a certain point, the rate of reaction will stop increasing and become constant. This will either be because another factor (such as Carbon Dioxide or temperature) has become limiting, or in the shortest supply, and preventing the rate of photosynthesis from becoming any faster, because even if the plant had the greatest light intensity possible, it would not be able to photosynthesise quicker if there wasn’t enough CO2 for it to, or if it wasn’t warm enough. However, if neither of the factors are limiting, then the reason for the rate of PNS becoming constant would be because the plant simply cannot photosynthesise at a faster rate.

Method- I will prepare the apparatus as in my diagram, but leave out the pondweed, funnel, test tube, water and sodium bicarbonate. The beaker should be as close to the plastic screen as possible. Then I will fill the beaker with 200ml of water and 1.5g of sodium bicarbonate. Once the sodium bicarbonate has been stirred thoroughly in the water I will place the elodea in the upside down funnel, so that the oxygen produced will be channelled upwards. I will have to make sure that there are no air bubbles in the funnel once it is in place. When that is ready I will place the water filled test tube upside down on top of the funnel, so that the ‘spout’ is inside it. After that I will measure the distance that the side of the beaker is from the light bulb, starting at the first distance that I plan to investigate. When the equipment is set up, as I need it, I will turn on the lamp and start the stopwatch. I will count the number of oxygen bubbles emitted by the plant, as a result of photosynthesis, in a minute. Then I will record my results in a table, and turn the lamp off for a minute before repeating the test. I will repeat each test 3 times, and take an average. This is so that I can collect more accurate results and recognise and repeat any anomalous results.

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The distances from the lamp to the experiment that I plan to use are 5cm, 10cm, 15cm, 20cm, 25cm and 30cm. I will use these because it is a fairly wide range, but the gaps between the distances aren’t so big that I won’t be able to get any really concise and conclusive results.

        Only one variable is allowed to be altered, in the experiment. Light intensity is the variable being tested, and by moving the lamp closer or further away I will be changing the intensity of the light. The variables, or factors that must be kept the ...

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