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Factors Affecting the Rate of Photosynthesis

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Factors Affecting the Rate of Photosynthesis Tim Hodgson 11M Introduction: All living green plants photosynthesise. In photosynthesis, carbon dioxide, water and light are absorbed and used to form glucose for the plant and oxygen as a by-product. As with all reactions, the rate can be changed by altering a number of factors. In photosynthesis, the most substantial variables which affect the rate are temperature, carbon dioxide concentration and light intensity. This is the premise of this experiment - investigating the limiting factors in photosynthesis. Due to restrictions in the environment, it would not be viable to alter the carbon dioxide concentration in the air in the classroom and it would be near impossible to achieve a suitable degree of accuracy if temperature was the independent variable. Therefore, light intensity had to be chosen as the factor to be investigated. The plant on which the theory and prediction will be tested will be Canadian pondweed. ...read more.


The most relevant graph of those observed is as follows: CO2 conc. 0.075% Volume of 0.05% Oxygen 0.025% Light Intensity Temp: 25oc Where the curve levels out, another limiting factor is preventing the increase in rate. Using this graph I can make a prediction of what I expect the results to be. Prediction: The process of photosynthesis is as follows: light energy is used to split water molecules into hydrogen and oxygen. The oxygen, the product which we shall be using as an indicator for the rate, is then expelled through the leaf. Therefore, if light intensity is increased, light energy is increased and the product, oxygen, is also increased. Therefore, my first prediction is that as light intensity increases, the no. of bubbles of oxygen produced per minute will also increase. If the light intensity is doubled, this will mean that there will be twice as much light energy absorbed. ...read more.


The Canadian pondweed was then placed in the water, as shown in the diagram, and the stopwatch was started. The number of bubbles produced in one minute was observed and recorded. This process was then repeated with the lamp at distances of 20, 30, 40, 50 and 60cm from the beaker. Then the whole process was repeated in order to obtain an average result. Diagram: Results: Volume of water (cm3) Distance from lamp (cm) Light Intensity (mV) No. bubbles produced per minute Average 1 2 3 400 10.0 322 161 160 164 162 400 13.0 313 133 131 137 134 400 20.0 255 99 93 84 92 400 30.0 192 68 60 60 63 400 40.0 143 45 42 39 42 400 50.0 107 33 35 34 34 400 60.0 83 30 26 25 27 400 70.0 68 22 23 21 22 400 80.0 55 19 18 19 19 400 90.0 46 16 17 16 16 max L.I. = 500 mV room L.I. = 7 mV temp = 22oc ...read more.

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Here's what a teacher thought of this essay

3 star(s)

Overall, this is a fairly competent piece of work which describes an investigation into the effect of light intensity on the rate of photosynthesis. The conclusion and evaluation have been omitted but the introduction, method and results sections are sufficiently detailed to warrant a C grade at GCSE. To gain the higher grades, a number of improvements would have to be made. These include:

[1] A more detailed introduction based on previous work carried out by other scientists on limiting factors. There is no shortage of this online.

[2] A clear hypothesis is required to form the basis of the investigation.

[3] A more detailed discussion of variables and the way in which they were controlled. Key variables such as temperature and carbon dioxide were given rather scant mention.

Overall, 3 stars.

Marked by teacher Ross Robertson 24/06/2013

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