How does light intensity affect the rate of photosynthesis

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Guillaume Wright 10C2                                                             GCSE Science Coursework 

C/W      Collecting Oxygen gas during Photosynthesis - How does light intensity   29/4/01

affect the rate of photosynthesis

Aim :       To find out how light intensity will affect the rate of photosynthesis in a piece of Canadian Pondweed.

Prediction :       I predict that as I increase the distance between the light source and the Canadian Pondweed (reducing the light intensity), the volume of oxygen produced within the time limit (the measure of the rate of photosynthesis) will decrease. This will happen because as the light source is moved further away, less light rays will reach the surface of the Pondweed, shown by this diagram;

                                            Light source              

                                                         

                                Light rays                                  Pondweed                 Light rays       Light source

                 Pondweed

      This will mean that less light (particles or waves) will go through the clear Cuticle and upper Epidermis and reach the palisade layer. This means that less light will reach each cell on average, and as light is needed for photosynthesis to make the water and carbon dioxide react together (making it a limiting factor) and so less light means less water and carbon dioxide is reacted together in the time, therefore there will be a decreased rate of photosynthesis, and less oxygen produced will be produced as a by-product of the photosynthesis reaction.

       I know that photosynthesis will take place in the plant as there is all the factors needed for photosynthesis available and present (water, carbon dioxide, chlorophyll, and light) to the plant and therefore according to the photosynthesis equation, photosynthesis will occur and the products will be made. Therefore I know that oxygen will be produced for collection, during my experiment, because oxygen is stated in the equation as one of the products made by the plant, in the word and symbol equations; 6H O + 6CO  (sunlight and chlorophyll)                   C H  O  + 6O  which means; water + carbon dioxide (sunlight and chlorophyll)                          Glucose + oxygen (oxygen gas = O ).

       I also predict that the rate of photosynthesis will increase in direct proportion to the increasing light intensity up till a certain point where the rate will decrease and level out. The proportional increase will happen because of the reason that I have already mentioned above, and there will be a point where the light intensity will not affect the rate of photosynthesis because another factor will be limiting the rate of photosynthesis e.g. level of carbon dioxide or temperature.

Background:   Photosynthesis is the process used by plants so that they can use raw inorganic material i.e. water and carbon dioxide to make it’s own food/biomass (organic) which not only they use, but is also the ultimate source of energy for animals. A by-product of the photosynthesis reaction is that oxygen gas is given off and is used by both plants and animals in aerobic respiration. I have already shown the equation, and this shows how the plant harnesses the sun’s energy using it’s chlorophyll to react the water and light together for photosynthesis to produce glucose and oxygen. There are different factors that affect the rate of photosynthesis and these include factors directly involved in the photosynthesis reaction e.g. light, water, carbon dioxide etc, and also other things that limit how much photosynthesis takes place in the plant e.g. temperature, time etc. All these factors are called limiting factors, but only up to a certain

Guillaume Wright 10C2                                                             GCSE Science Coursework 

C/W      Collecting Oxygen gas during Photosynthesis - How does light intensity   29/4/01

affect the rate of photosynthesis

point where it no longer affect the rate of photosynthesis because something else is. For my investigation I am going to investigate how light intensity affects the rate of photosynthesis as it is one of the easily measurable and changeable factors that affect photosynthesis. For this experiment I have carried out preliminary work using the same method for this experiment, but collecting the amount of bubbles produced (easier) so that I could check that the method worked and that the piece of Pondweed was photosynthesising. From this experiment I have decided that oxygen collection using this method is inaccurate, because the bubbles may be different sizes so I have changed this part of the method, and I will use a delivery tube to collect the oxygen. I have also decided to let the plant settle down for two minutes after I have put the Sodium Hydrogen Carbonate in and put the light on, so that it can have time to start photosynthesising, so I can collect the maximum amount of oxygen that I can for each measurement to make them more reliable. These were the results that I gained from my preliminary experiment. I also decided that I should take more measurements, but still using the same scale (50mm gap), so that I could gain a wider variety of results for a more reliable outcome.

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      The graph from this experiment was almost perfect with a line of best fit that showed exactly what I was expecting, in that there was a direct relationship between light and the rate of photosynthesis until a certain point where it started to level off. This proved that my method was good and I had kept all the other variable under control and that this was a suitable plan to use for my main experiment.

Plan:   

Variables -   To keep my experiment a fair one, I will keep all the other factors that ...

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