Light Intensity 1 D 2 This example essay topic

AIM- To identify the factors affecting the rate of photosynthesis and, choosing one factor, to ascertain the effects it has. Photosynthesis is an endothermic reaction that occurs in plants, by which plants use light energy to make glucose. It needs energy from the photons of light and it is their anabolic effect on the plant that gives the energy for the reaction to take place. During this process carbon dioxide combines with water to from glucose, and oxygen is released. The glucose made then has many uses in the plant: respiration, making ATP, active uptake... Sunlight and chlorophyll must be present for the reaction to take place, and the light is trapped in the chlorophyll: sunlight Carbon dioxide + Water Glucose + Oxygen.

Chlorophyll The amount of oxygen given off is an indication of the rate of photosynthesis. The more oxygen being given off, clearly the faster the rate of the reaction, and the more photosynthesis occurring / the faster the rate of photosynthesis. POSSIBLE VARIABLES- from background research and previous experiments I know the following variables / limiting factors to affect the rate of photosynthesis: . Light Intensity - the basic energy source.

Temperature- increases enzyme reactions until the point of denature... Water- a basic reagent- a lack of water also causes stomata to close inhibiting diffusion of CO 2 in and out of the leaf... Chlorophyll- this is what traps the light energy for the reaction. Carbon dioxide - the more CO 2 in the air, the more that can diffuse into the leaf to be a basic reagent for the photosynthesis reaction. Of these variables I have chosen to investigate light intensity because there are various reasons why other variables would not be suitable: . Temperature- this variable is not specific to increasing the rate of photosynthesis, but rather to general rates of reaction, as I have seen in previous experiments into reaction rates...

Water- this would be too difficult to control as lowering the water levels too much would kill the plant and ruin the investigation... Chlorophyll- again this variable would be too hard to control, as we could not get a whole range of results. Leaves come in variegated form, where parts either contain chlorophyll or they don't. There is no way with our basic equipment to ascertain precise chlorophyll levels in the plant leaves...

Carbon Dioxide- again with this variable there is either carbon dioxide present or not (adding soda lime). It would be very difficult to obtain or measure precise carbon dioxide levels in the air, or keep that environment from contamination of normal carbon dioxide levels... I Chose light intensity- as it is possible to vary this more (resulting in a range of results) by increasing distances between the plant and the lamp gradually to diminish light intensity. Also light is the key variable for photosynthesis- without it no photosynthesis would occur as there would be no energy source.

My aim therefore is to investigate the effect of light intensity on the rate of photosynthesis by varying the distance of a lamp from pondweed and measuring the volume of Oxygen given off. OUTPUT VARIABLE- the volume of oxygen given off. PREDICTION- I predict that as light intensity increases (distance from bulb decreases) so will the rate of photosynthesis increase. Light is a key factor of photosynthesis and without it plants cannot get enough energy to make glucose. Light intensity itself is directly proportional to the rate of photosynthesis as the more light energy a plant receives and traps in the chlorophyll, the more it can produce and so doubling energy in = doubling energy out. HOWEVER...

From scientific research I know that the relationship between light intensity and distance is: Light intensity = 1/ d 2 This shows that light intensity is inversely proportional to the distance squared because the light energy spreads out as it travels further away from its light source (ie as distance increases). This is because light energy travels along the circumference of an expanding circle. As the circle expands and distance becomes greater, this causes the light intensity to decrease as the same amount of light energy must be equally dispersed over a larger area / circumference. This is not a linear relationship because doubling the distance causes the spreading out light energy to reduce by more than a half as the circumference of a circle = 2 r and this is not a linear quality. Also the equation backs this up, as it is a quadratic quality.

Therefore by doubling the distance away from the plant I expect to quarter the volume of oxygen released as the light intensity will be quartered and so the rate of photosynthesis will be quartered (see above). I also predict that the control left in the dark will not produce any oxygen as there is no light available for photosynthesis to occur. TO MAKE MY EXPERIMENT A FAIR TEST: . Keep constant all other variables: . Keep a fixed volume of water in the surrounding beaker of each experiment (in excess). Add an excess (1 spatula) of sodium hydrogen carbonate to the water so that CO 2 levels are in excess and not limiting the rate of photosynthesis...

Keep the water at a constant temperature for each experiment- 24 degrees C- and if it heats up from the lamp add more cold water. This will not affect my experiment as the water needed only needs to be of a certain level, and it will be in excess... Also a transparent screen can be placed between the lamp and beaker to prevent heat radiation... Use the same fresh elodea for each experiment to ensure the same leaf structures and basic photosynthetic rates... The same lamp should also be used in each experiment as the wavelength and intensity of the bulb should be kept constant...

Use the same length of elodea for each experiment. Cut the end of the elodea fresh with a razor blade to make sure that optimum photosynthetic rates are acquired... Keep a control in the dark to monitor all conditions. No photosynthesis should occur and no oxygen should be collected...

Give each experiment the same time to photosynthesise... Always keep the funnel containing the elodea right in the middle of the beaker so that it is always an equal and fair distance from the beaker edge. This way it will always be the same extra distance from the light source, and no unfair heating or light will be in place to mar my results. TO MAKE MY EXPERIMENT SAFE: I think this is a fairly safe experiment although: . When working with water and electricity be extremely careful to keep surface and hands dry so as not to cause an electric shock...

When cutting the elodea be very careful with the razor blade and make sure not to cut yourself... Be very careful when dealing with glassware. EQUIPMENT NEEDED-. 1 Lamp (60 Watt bulb).

1 Beaker (500 ml). 1 funnel. 1 measuring cylinder- 10 cm 2. This is to hold the elodea and measure the exact amount of oxygen given off...

1 22 cm sprig of elodea... Stop clock - to time investigations... Thermometer- to monitor water temperature... Blue tack- to hold measuring cylinder in place in beaker... Transparent screen- to prevent heat from lamp radiating the water...

Razor blade to cut a fresh edge on the elodea... Mains electricity socket. Water in excess. I spatula of sodium hydrogen carbonate (to add CO 2 to the water... Ceramic tile OVERVIEW OF METHOD AND PRELIMINARY WORK: I conducted a preliminary experiment by placing some elodea in an inverted funnel in a beaker of water. Over the funnel I placed an inverted measuring cylinder.

I then placed a lamp 2 cm away and, switching on, left it for 10 minutes to photosynthesise. I repeated this for 4, 6, 8 and 10 cm from the lamp. I counted the volume of oxygen given off. There were however some basic problems with this method: . Firstly I did not have much time, and so the 10 minutes I gave the plant to photosynthesise each time was not sufficient to create a worthwhile volume reading for the oxygen given off, and so my results were void. For the real method I shall count bubbles, and although this method is not terribly accurate, overall I will get a more accurate pattern off results...

Also I could not use the screen (as intended in my fair test outline), as this was not available. Instead I just had to be more careful with the temperature of the water (making sure that it did not overheat, and adding cool water whenever it started to heat up). In my preliminary work I placed the weed to near to the bottom of the funnel and observed bubbles escaping round the side of the funnel which marred my results. In my real experiment I shall place the weed directly in the measuring cylinder, and further up to avoid oxygen loss, and therefore resulting in more accurate results...

I shall try to obtain a range of at least 5 results (as in preliminary work- 2, 4, 6, 8 and 10 cm between the beaker and the lamp.) to get an accurate and substantial representation and pattern of results. In my preliminary work I also tried putting the lamp 50 cm away, yet no bubbles were observed. Therefore our results must be at much smaller intervals as fore-mentioned... I shall try to repeat each experiment twice so that any inaccurate results will be noticed, and so that I get more accurate results (by taking averages from a larger amount of data). For most experiments a control is needed, to which we can compare our results. In this case, we will leave one weed in the dark, and attempt to exclude all light, so we can observe what would happen in terms of photosynthesis and oxygen produced if the plant received no light at all.

Obviously we will not be able to count bubbles as they are released in the dark, but we will be able to observe whether after the 10 minutes any oxygen was given off at all. I would predict that it would not be as plants do not photosynthesise in the dark. Any gas that is given off is likely to be carbon dioxide, as plants also respire all the time. We could then use this information to find out how much of the bubbles from our other results were in fact oxygen, or carbon dioxide from respiration. We will then vary the amount of light the plant receives, at set intervals (as mentioned above), and compare this data to the control. PLAN OF RESULTS TABLE Distance between lamp and Elodea (cm) Number of oxygen bubbles produced Temperature of the water (oC) result 1 result 2 average no lamp- in dark 2 4 6 8 10 METHOD 1.

Cut 22 cm of elodea on the white tile using a razor blade and taking care not to cut yourself. 2. Set up apparatus as shown below: 3. Place one spatula of sodium hydrogen carbonate into the water so that CO 2 is in abundance and is not the limiting factor. 4. Place in the dark and leave for 10 minutes (record time using the stop watch) 5.

After 10 minutes remove plant from the dark and see whether any Oxygen has been given off (i.e. whether any gas bubbles have displaced the water at the top of the measuring cylinder.) 6. Repeat the experiment, only this time place the beaker in the dark room but with a light 2, 4, 6, 8, and then 10 cm away. 7. Throughout the experiment always monitor the temperature of the water using the thermometer, and if it starts to heat up, add cool water so that your results are not marred. 8. Record all results and repeat experiments twice so that maximum accuracy can be achieved.

OBTAINING EVIDENCE I carried out my experiment fairly and safely, following the guidelines I set. I repeated each experiment to get more data and so more accurate results, however time did not allow for me to repeat each experiment twice. Although this was the case, my two sets of results still seem to coincide and so I think that they are sufficiently accurate. Results table 1- no of oxygen bubbles produced compared to distance. Distance between lamp and Elodea (cm) Number of oxygen bubbles produced Temperature of the water (oC) result 1 result 2 average no lamp- in dark 2 4 6 8 10 ANALYSIS results table 2- no of oxygen bubbles produced compared to 1/ distance squared 1/Distance between lamp and Elodea squared (cm-2) Number of oxygen bubbles produced Temperature of the water (oC) result 1 result 2 no lamp- in dark 2 4 6 8 10 As we can see from graph 1, the number of bubbles of oxygen produced (i.e. volume of oxygen) is inversely proportional to the distance between the beaker and the lamp.

This is as I predicted and so I have achieved the results I wished for. The graph clearly shows that as distance between the beaker and the lamp increases, the no of bubbles given off decreases. In fact we see from graph 1 that the no of bubbles quarters by doubling the distance from the lamp: distance of 2 cm: 129.00 bubbles distance of 4 cm: 33.25 bubbles distance of 8 cm: 9.50 bubbles We see that these figures are very near 1/4 the no of bubbles when double the distance. In the evaluation I shall explain why I think they are not exact. The reason that the oxygen given off quarters as the distance doubles is because light energy spreads out as it travels further away from its light source (i.e. as distance increases). Light energy travels along the circumference of an expanding circle.

This is not a linear relationship because doubling the distance causes the spreading out light energy to reduce by more than a half as the circumference of a circle = 2 pr and this is not a linear quality. If light intensity is quartered as distance doubles: light intensity a 1/ d 2, this would explain why the amount of oxygen given off is also quartered as distance doubles. This is because light intensity is directly proportional to rate of photosynthesis (doubling energy in = doubling energy out). This in turn is directly proportional to volume of oxygen released, as doubling the rate will also double the bi-product (oxygen) produced. By looking at graph 2 we do indeed see that the volume of oxygen (no of bubbles released) is directly proportional to 1/distance 2 from lamp. As you double the 1/d 2, you double the bubbles given off: 0.040 cm-2: 20.9 bubbles 0.080 cm-2: 42 bubbles This is very accurate - only 0.2 of a bubble out.

The fact that these two factors double together would make sense because light intensity and the amount of bubbles given off are both quartered by doubling the distance. This would imply that if they are inversely proportional to d, then they are both proportional to 1/d 2, and this is in fact true (see above). My prediction was therefore correct, and by analysing my results I think that I have sound enough evidence on which to base my conclusions above. EVALUATION The method used was a simple and effective way to investigate the effect of light intensity on the rate of photosynthesis. Although my results were not 100% accurate (as pointed out in the analysis), they were mainly correct, as shown by the smooth curve and straight line of the graphs, and as they clearly followed set patterns, I think that they are sound enough on which to base firm conclusions. My method was not highly sophisticated, yet by carrying out my experiment with great care, repeating my results and observing the patterns portrayed, I can say that my results are reasonably reliable.

I had no anomalous results, although obviously there were a couple of points that deviated slightly from the curve / line of the graph. There are a number of explanations for these slight deviations: . Although I managed the temperature quite well, it did fluctuate a bit, and this may have raised the rate of photosynthesis, and the oxygen produced. We can actually see that the result for 6 cm distance actually had a highish temperature, and also a slightly higher that expected result. To combat this in the future I should attempt to regulate the temperature by a more satisfactory method.

Perhaps I could heat the water slightly to start with, and as it gets hotter than the initial temperature, I could reduce my other heat input... Secondly, the pondweed did not photosynthesise at a constant rate. The bubbles were given off erratically, and therefore my results to not reflect 100% accurately what happened. To prevent this in the future, I could allow the plant to adjust to the set intensity of light for longer before I began to record the number of bubbles produced... Also the method of counting bubbles was not entirely satisfactory - even though my results were good and fairly reliable- as all the bubbles were of different sizes and so this was not a very fair portrayal. A great improvement for the future would be to leave the experiments running for a much longer time, Perhaps a whole day, to get a better idea of the volumes of oxygen given off.

Also instead of counting bubbles I should stick to my original method from my preliminary work of recording the exact volumes of oxygen with the measuring cylinder. Unfortunately this method was not suitable for the time that I had, as volumes were not high enough to record accurately. The entire experiment also may not have given an accurate reflection of the rate of photosynthesis. This could have happened for the following reasons... Unfortunately I did not have time to repeat each experiment twice, but only carried each one out twice. This may have affected all results, because there was only a small range of data to compare, and if one result was significantly wrong, I only had one other result to compare it to.

However I did not seem to have any great errors / anomalies and so I still think that my results are reliable overall. In the future I shall however repeat the experiments one or two more times in order to gain more data and so highly accurate and reliable results... Some of the oxygen bubbles produced may also have escaped out of the measuring cylinder, or dissolved into the water. Perhaps they were even used for respiration by micro-organisms living on the pondweed.

The oxygen lost in this way, however may have been a highly insignificant volume, and would have been very similar for all tests as they were carried out at the same time... Some of the gas given off may have been carbon dioxide from the plant's respiration, but again, this was unlikely to mar my results, as they would all have been affected at the same rate. Also most of this gas would have been used up in photosynthesis, so the volumes would have been minimal... As previously mentioned, when observing the bubbles I noticed that they were all of different sizes. It was hard to judge which I should consider for observation, as some were of negligible size.

I decided therefore to count all the bubbles I could, both large and small, even though this may also have resulted in some error. To combat this in the future I could collect the oxygen produced in a gas syringe, or inverted measuring cylinder, to measure the volume, which would be much more accurate than counting bubbles. Having said all of this, I believe that the evidence collected, supported by my evidence from research and previous enquiries, was sufficient on which to base firm conclusions. However, for further confirmation, and also more insight into the topic as a whole, I could extend the enquiry by doing the following things: . I could vary one other or all of the other variables mentioned in my plan... A sensible extra variable to investigate would be the colour, and therefore the wavelength, of the light, keeping the intensity of the light constant this time.

Taking into account that plants are green, and so this light will not be as effective for photosynthesis... I could also vary the wavelength of light, trying to coincide this factor with the one I already investigated (the greater the intensity of light, the greater the rate of photosynthesis)... I could repeat my experiments to get a wider range of data, leaving each one for a longer period... I could investigate different sorts of plants and see whether there is any difference in photosynthesis rate depending on their habitat / environment.