Oxygen – low levels of oxygen in the atmosphere can suppress and slow down the seeds germination process. As the seed begins to germinate, its metabolic activity is at first anaerobic but when the seed coat swells and bursts, aerobic respiration begins.
Water – most seeds contain approximately 5 - 20% water and therefore they must take up water to resume metabolic activity. The enzymes inside the seed become activated when water is absorbed and these enzymes then begin to digest the stored food inside the seed.
Acid rain – is formed by the burning of fossil fuels. However, this burning process does not directly deposit acid into the atmosphere but instead releases large amounts of substances we call acid precursors. The substances are gases and are either sulphur oxides or nitrogen oxides. Sulphur dioxide is emitted through the combustion of fossil fuels containing sulphur as an impurity. Coal combustion is by far the major source of sulphur dioxide emitted into the atmosphere. During combustion, sulphur is oxidized to form sulphur dioxide. Sulphur dioxide rises into the atmosphere and is oxidized once again into sulphur trioxide, which then reacts with atmospheric water droplets to form sulphuric acid. Sulphur dioxide emission is the most common contributor to acid rain, responsible for about 70% of the total. The greatest source of sulphur dioxide is electrical utility plants, which pump approximately 15 million tons of sulphur dioxide into the atmosphere each year, out of the total 22 million tons generated annually by human activities. Other contributors of sulphur dioxide include industrial processes and automobiles and other motor vehicles.
Nitrogen oxides are also formed through the burning of fossil fuels. In contrast to sulphur, nitrogen is not an impurity but rather a part of the organic material making up fossil fuels. Fossil fuel combustion releases nitrogen into the atmosphere, usually in the form of nitric oxide. Nitric oxide is oxidized by certain atmospheric molecules, such as ozone or hydrogen dioxide, to form nitrogen dioxide. Nitrogen dioxide reacts with OH in the atmosphere to form nitric acid. Nitrogen oxides account for approximately 30% of all acid rain. Major sources of nitrogen oxide emissions are automobiles and fossil fuel burning power stations.
Nitric acid and sulphuric acid eventually fall back to the Earth's surface as acid deposition. This precipitation can be wet (rain, snow, or fog) or dry (gases or acidic salts).
Method
In my trial experiment I tested cress seeds under four very different conditions to see exactly which conditions were vital to a seeds germination process. This trial experiment helped me plan out my method for my final experiment as well because I then knew exactly which variables would have to be kept constant.
I tested twenty seeds in a cold environment with 10cm³ of water and plenty of oxygen and light. I tested twenty cress seeds in a warm environment with no oxygen or light limitations but a limited amount of water. I did this by merely soaking the seeds in water before placing them inside the Petri-dish. I tested another twenty seeds in a warm environment with 10cm³ of water and enough oxygen but I covered the dish in tin foil so that the seeds had no access to light. I also tested twenty cress seeds in a warm environment with plenty of light and oxygen and 10cm³ of water. These trials were so essential to do because otherwise in my final experiment it may not even have been the acid rain affecting the seeds germination process but a crucial condition that was not present but needed to be.
The seeds that were kept in the cold were experiment 1, seeds with little water were experiment 2, seeds with no light were experiment 3 and the seeds with water, warmth and light were experiment 4.
Experiment 1: These seeds, after being left a few days, were only in the early stages of germination. The seeds had secreted a colourless jelly in order to protect the seeds form drying out. Only 9/20 of the seeds had begun to establish roots and these roots were about 1.2mm long on average.
Experiment 2: These cress seeds did not germinate because they had dried out. The seeds had not produced any form of jelly and so had simply just dried out.
Experiment 3: These cress seeds germinated well and grew to an approximate length of 60mm (length of root included), and eleven of the shoots and developed little leaves called cotyledon. 20/20 of the seeds germinated. However, the shoot and leaves were distinctively yellow in colour, the reason for this is because throughout the germination process the seeds had been kept in the dark and so had not yet developed and chlorophyll.
Experiment 4: These seeds germinated the best out of the four. All 20 of the seeds germinated and grew to an approximate length of 70mm and 16 of the shoots had developed a cotyledon. The shoots were also healthy and very green in colour.
After this trial experiment it is quite clear that the cress seeds in my final experiment will require plenty of water so that the seeds don’t dry out, plenty of oxygen, plenty of light to keep the seeds healthy in colour and warmth to help the seeds germinate more quickly.
Independent variable: The only factor in my experiment that I am going to change is the levels of concentration of acid rain. These levels will range right from 0% - 100%. I will need to change the concentration without changing the volume of liquid so I will need to firstly set the desired volume of liquid for each different concentration. The volume of liquid will be set at 9cm³ in each Petri-dish. The concentration can be then changed by adding more water each time but less of the acid rain mixture each time as well, so as to maintain the volume of 9cm³. The range of
concentrations are as follows;
Here I cover a range of ten different
combinations and so ten different concentrations
of acid rain will need to be tested. The cress
seeds will be tested at the same time in
separate Petri-dish so no previous
precautions need to be made.
Dependent variable: I will measure the length of the root and shoot of the cress and I will also record exactly how many of the seeds have germinated. The rot and shoot of the cress will be measured in millimetres so that it is as accurate a reading as possible. By measuring in as small a unit as millimetres my results will be as precise and reliable as I can get them. I will also measure each seed in every dish individually and then take an average, rather then measuring the longest or nearest one.
I will also need to decide what will and will not count as germination. If the seed coat has split and the slightest bit of root is growing out, then I shall count that as germination.
Control variable: I will need to keep a number of important factors constant for my results to be very accurate and reliable.
The levels of light will need to be kept constant so that the seeds all receive the same treatment and my results will be as precise as possible. The levels of light can be kept constant by putting the Petri-dishes under a lamp or near a window. It is important that the dishes are kept separate as well and not piled up on top of one another, this way the seeds will have access to equal amounts of light.
The volume of liquid on the cotton wool will also need to be kept constant at 10cm³ in each dish. If less liquid is put into one dish compared to another then it may take longer for the seed in that dish to germinate because water is taken in until the seed coat bursts, revealing the root. If less liquid is the Petri-dish then it may take longer for the seed to swell and burst and in turn my final results will be affected, rendering them unreliable. If too much liquid is put into one of the dishes then the seeds might be flooded which would lead to a lack of oxygen inside the Petri-dish. If this was to happen then the cress seeds germination would be severely effected and therefore obvious anomalous results could occur.
Levels of oxygen will need to be kept constant as well in addition to this so that the seed can easily germinate. This will be easy to do because the dishes are well ventilated, however, if something was to block or cover the dish then it would take longer for the seeds in that dish to germinate. This is because oxygen is needed in a similar way to water; the seed takes in the oxygen until the seed coat bursts, letting out the root. If the seed is deprived of oxygen then it will take a lot longer for the seed to germinate.
The duration of which the seeds are left for should be the same for each dish. If a certain set of seeds are left for a lot longer then some anomalous results could appear because the certain set of seeds had been given more time to germinate and grow.
The number of cress seeds should be the same in each dish. It wouldn’t directly affect my final results but it will be a lot more easy and fair to compare the seeds in each dish and not a selected number of seeds. With the hundred seeds I have been given and the ten different concentrations I have chosen to test, I shall require to put ten seeds in each dish.
Temperature ideally should be monitored but it will be very difficult to keep the temperature absolutely constant. Anything from a drafty door to an open window could change the temperature of the room. The temperature preferably should be kept at approximately 21˚C (room temperature). Temperature is important because the enzymes inside the seed need warmth for kinetic energy. These enzymes work at an optimum temperature of around room temperature which is the temperature at which they have the most kinetic energy. When these enzymes have a lot of kinetic energy they move around more quickly, reacting with more substrates in a given time than they would if they had less kinetic energy. As a result of this the reaction takes place a lot more quickly.
Safety precautions: because of the fact that acid is involved in the experiment certain safety precautions need to be taken into consideration. Safety goggles should really be worn just incase some acid is flicked somehow into your eye or if you get it on your fingers then directly rub your eyes. You will need to stand up as well because that way you won’t be able to spill a strong acid on to your lap.
Apparatus:
Results:
Above: The relationship between the concentration of the acid rain mixture and the number of cress seeds that germinated.
*R= root
S= shoot
Above: The relationship between the concentration of the acid rain mixture and the length of which the cress seeds grew to.
Analysis/conclusion
My first graph plotted for the relationship between the concentration of the acid rain mixture and the number of seeds that germinated shows the points that I plotted form a straight line, running from the top left of the graph to the bottom right. This straight line shows that as the concentration of acid is increased, less cress seeds germinated. The graph also proves my prediction was correct because the straight tells me, as I had previously predicted, that the relationship between the concentration of acid rain and the number of seeds that germinated is directly proportional.
The graph supports my prediction because all of the seeds germinated at 0% concentration and none germinated at 100% concentration. The nine other points in between these two points are at spaced out stages between nought and ten seeds. An example to help prove this particular relationship is directly proportional would be that at 40% on my graph, five out of the ten seeds germinated but then when the concentration of the acid is doubled to 80%, the number of seeds that germinated is almost halved to just two seeds. Then at 0% concentration all ten of the cress seeds germinated, at 50% concentration five seeds germinated and finally at 100% concentration none of the cress seeds germinated. This particular relationship is directly proportional because, for instance at 40% the acid rain mixture denatures a certain number of enzymes in each cress seed. However, when the concentration is doubled so that it is twice as strong, double the numbers of enzymes to before are denatured and the seed’s germination is slowed down. The seed’s germination is slowed down to a speed twice as slow as the seed’s germination process when the concentration was 40%.
The graph I obtained from my final results resembles that of the previous graph I had included in my prediction.
My second graph also agrees with my prediction because the relationship between the concentration of the acid rain mixture and the complete length of the seed’s growth is not directly proportional. My graph’s line of best fit is a steep curve, running from the left to the right. The steep curve starts off at 9.31mm and runs down to 0mm. The curve is so steep because, as I already stated in my prediction, the slightest change in pH will denature some of the enzymes inside the seeds, the optimum pH for the cress seeds is pH7 and so anything that differs from that will denature some of the enzymes. As soon as these enzymes start to become denatured the length of the cress will be affected. After 10% concentration the average length of the seed’s growth dramatically drops, this may be due to the fact that too many enzymes had been denatured and so the growth of the cress was seriously stunted. The cress only grew well at 0% and 10% concentrations of the acid rain mixture because the acid was so dilute, or not even present. It is very clear that the pH did affect the cress seed’s germination because as soon as some of the acid was present in the liquid, the cress did not grow very well. At 20% the cress length drops for 5.62mm to 1.19mm, this suggests that the pH dropped and twice as many enzymes were denatured. The graph is not directly proportional because the cress seeds were either affected badly or affected barely, there weren’t really any middle stages unlike on my first graph. By comparing the two graphs it is quite clear the differences between directly proportional and not directly proportional. The graph I obtained from my final results looks very similar to the graph I included in my prediction about this relationship.
I can conclude that my prediction was correct and the acid rain did denature enzymes, slowing the growth of the cress down considerably.
Evaluation
I encountered only a few problems during my experiment which could have affected my final readings.
Firstly when I was measuring out the water and the acid rain mixture with a syringe it proved to be a little difficult and so the volumes in each Petri-dish may not have been exactly equal. There may have also been a little of the acid rain mixture or water in the beaker I used to mix the two liquids in or maybe some that didn’t quite come out, this would have altered the volumes as well. If the volumes were not equal then this could have affected my final results, causing them to be unreliable. If too much liquid was in the Petri-dish then the levels of oxygen could have been seriously disrupted and the cress seeds could have been lacking in oxygen supplies. But then again if the volume happened to be less than 10cm³ then it may have delayed the cress seed’s germination process. To germinate the seeds must take in water and so if the water levels were low then there may have not been enough water for the seeds to germinate at the normal rate.
Secondly, I used a small beaker to mix together the water and acid rain mixture before pouring it into the Petri-dish. However, I did not wash this beaker out and so there may have well have been some acid still left in there when I came to mix up another concentration. This left over acid may have changed the pH of the mixture slightly causing the mixture to denature more enzymes than it was expected to do so. Again this may have caused some slightly anomalous results in my final readings.
Lastly, I found that when I needed to measure the seeds it proved to be very difficult. The cress’ roots were all curled up and sometimes when I tried to uncurl then they simply broke off. This problem most probably affected my final results because some of my measurements were not very accurate.
To improve my method for next time there are a few things I could develop to allow my results to be more accurate and reliable. I would possibly use a syringe with a smaller scale on it so that it would be much easier to measure out my liquids to mix up. I would wash out the beaker I would use to mix up the two liquids in after every different concentration and also dry it each time as well with perhaps some tissue. When mixing together the water and acid rain mixture next time, I would stir it with a glass rod to insure the liquids were mixed together evenly. And finally I would use some thin string to follow the shoot and root of the cress and then mark out where the cress started and finished, then simply measure the string on a ruler.
The data I received I believe is reliable. I conducted the experiment as accurately as I could with the apparatus I was given. If my method was improved with the suggestions I have already made then my final results of the experiment would be more reliable. My experiment would have been more reliable if perhaps I tested not just one but about three or more Petri-dishes of seeds at each different concentration. This way I could compare the number of dishes at each concentration and any anomalous results could be much more easily spotted. I could also test more seeds in each dish as well; this would help give me a better average overall. However, the experiment I carried out was on a small scale and further work should be carried out to aid my predictions.
On each of my graphs I have circled one anomalous result. It is very clear that the result I circled on each graph is anomalous because it is at 20% concentration on both graphs and out lies the rest of my results. This anomalous result could be the result of a number of different factors.
One or two of the seeds in the dish could have been already damaged and so didn’t germinate when perhaps they should of. This will have caused my average length for this concentration to be a lot less than it should have really been because one or two seeds didn’t even germinate, but if they were healthy would have.
My result for the average length at the 20% concentration could have been anomalous because the acid and water were not mixed together properly. Because I merely measured out the acid and water into a beaker then poured it on to the cotton wool may have this result unreliable. The fact that these two liquids were not mixed equally could have resulted in different patches on the cotton wool to be more concentrated than others causing more enzymes to be denatured in some seeds than others. This would have also caused the 20% concentration to be lower than it should have been, making it an anomalous result.
A few of the cress I tried to measure in this particular concentration broke off and so my measurements may not have been entirely accurate. My measurements could have been less than the cress roots actually were which would have resulted in my final reading for this concentration to be less than I should be.
I am confident in my conclusions because I believe my experiment was carried out as accurately as possible with the apparatus I was given. Both of my graphs show clear patterns with one obvious anomaly on each. After explaining how my one result was anomalous allows me now to feel confident that my experiment was as accurate as possible now that the one mistake I received has been acknowledged. I made no serious errors apart from not washing out my beaker used for mixing together the two liquids, so if my experiment was to be repeated with the improvements I feel confident the results would be very similar to the results I have now.
I somehow do not feel my results were very sufficient. I think that more than one set of seeds should be tested at each different concentration of the acid rain mixture. This would make anomalous results a lot easier to spot and would also give me a much better average of lengths. If more seeds were tested in each Petri-dish then a more sufficient average could have been reached.
If I was to carry out further work after completing this experiment to help prove and back up my prediction I could do a number of different things. I would repeat the experiment with more seeds in each Petri-dish to help give me a better average at the end. I would also test more dishes of seeds at the same concentration of the acid rain mixture, I would test about three more sets at each concentration.
I would carry out the experiment using a much more concentrated acid to see if different or similar results were obtained to my first experiment. I may possibly test more different pH values than eleven so that my graph could be much more accurate.
I would complete the experiment using different seeds to see if it had exactly the same affect. I would maybe use grass seeds because they do not take too long to grow. I would carry the experiment out outside in natural conditions but still keep a watchful eye over the seeds and control the conditions so that the grass seeds were not damaged in any other way, by any serious variation in conditions. By doing this I could see if it wasn’t just cress seeds that were vulnerable to acidic conditions.
Lastly, I would continue the experiment to see if the cress seeds that had barely germinated at the end of the two days still continued to grow, or if they simply died after a certain length of time. This would show whether or not the acid rain affected the seeds as they reached the later stages of germination or not. By carrying this out would show me whether acid rain allowed the cress to actually pass the germination process or if the acid rain just killed the cress after a certain length of time. I could also compare healthy cress to cress grown in a very dilute acid to examine if the life span of cress is also affected by acid rain.
All of the above would help improve my entire prediction and conclusion and would help develop my experiment.
