The effect of caffeine concentration on the heart rate of Daphnia.
The effect of caffeine concentration on the heart rate of Daphnia
Aim
To conduct an experiment into the relationship between the heart rate of a Daphnia and its exposure to various concentrations of caffeine.
Method
For this experiment we had:
Culture of Daphnia in various concentrations (%) of caffeine (0.0, 0.0025, 0.005, 0.025, 0.05, 0.25, 0.5)
Two cavity slides
A pipette
Stop watch
Microscope
Ice water
Firstly, a job was designated to each member of the group and each had a counter, a recorder and a controller. The controller always set up each slide to ensure that the other members of the group were unaware of the concentration of caffeine that the Daphnia were exposed to, thereby eradicating bias, and also controlled the time. The counter would count the number of heart beats in ten second intervals by stabbing at a piece of paper with a pen, in an 'S' shape. This allowed for easy counting and a control on human error. The recorder counted the dots on the paper to ensure that an extra heart beat was not added here and there in order to make the results match their prediction.
A microscope was then set up and the x10 lens was used to ensure that the Daphnia's heart could be seen clearly. The slide was then set up. Some ice water was then placed on one cavity and the second cavity slide placed on top of the first. This was to help with the regulation of temperature. Placing ice water below the Daphnia helped to counteract the effects of the microscope light in heating up the slide thereby helping to validify our results. One Daphnia (from one of the concentrations) was then placed on the slide in the cavity and positioned under the microscope. If the Daphnia moved around too much some of the caffeine solution was taken off the slide to limit the amount of liquid it had to swim in. The heart beats were then counted in ten second stints. This was felt to be a suitable measurement as this meant that the number of heart beats could be multiplied by six to obtain the heart beat per minute. It also allowed for the fact that the hand and the eye of the counter could not accurately count the heart beats for more time as it was too fast in many cases. A way to improve accuracy in the counting of heart beats would be to film it or use a Stroboscope if one were available. Once a particular Daphnia had had three readings taken from it a change was made to another Daphnia and the ice water replaced. Three readings per Daphnia were taken and five Daphnia per concentration were used. This served two purposes. One, the Daphnia did not succumb to any effects that the heat from the light may have caused and it that it did not suffer any effects of being taken out of the liquid for too long. Secondly, replicates were taken to increase reliability, the more replicates taken, the more reliable the experiment was. Also the more replicates taken the more chance there was to have of spotting a potential inaccuracy or anomaly in the results.
N.B. The Independent variable is the concentration of the caffeine and the dependent variable is the heart rate.
A Table to show the Relationship between the Caffeine Concentration and the Average Heart Rate of a Daphnia
Concentration of caffeine / %
Number of heart beats of Daphnia per 10 secsonds
Mean number heart rate per a minute
0.0
56
47
44
56
57
50
65
55
312
59
52
49
46
51
46
46
0.0025
...
This is a preview of the whole essay
N.B. The Independent variable is the concentration of the caffeine and the dependent variable is the heart rate.
A Table to show the Relationship between the Caffeine Concentration and the Average Heart Rate of a Daphnia
Concentration of caffeine / %
Number of heart beats of Daphnia per 10 secsonds
Mean number heart rate per a minute
0.0
56
47
44
56
57
50
65
55
312
59
52
49
46
51
46
46
0.0025
60
58
51
55
59
58
56
56
352
58
62
69
68
55
58
56
0.005
60
56
56
57
55
53
45
48
321
43
61
61
53
51
53
51
0.05
52
51
54
47
49
50
50
52
318
55
53
55
54
58
56
58
0.25
59
57
57
59
61
59
54
51
338
50
57
53
58
60
56
53
0.5
47
46
47
46
50
57
47
48
289
46
Results
I have chosen to represent my results in a graphical form as well as a table.
I have chosen to show the relationship of caffeine concentration and the heart rate of a Daphnia in the form of a line graph due to the fact that both the independent variable and the dependent variable are continuous, but the relationship within the data is not immediately obvious from the table.
I have used the range within the results to place error bars on the chart, as the values I measured are spread about the mean, and as you can see this indicates the variation of results. I could also have used the standard deviation or standard error to quantify the variation. The variation in these results is high, meaning a greater 'degree of error'. This indicates any trends apparent in these results hold a low significance. Neither can we say that there is causation involved. The results obtained do not reliably tell us if the concentration of caffeine causes the heart rate to increase.
From looking at the graph and my results I am going to take the results from the 0.0025% concentration and the 0.5% concentration as anomalous. This is due to the fact that in the 0.0025% concentration is high and inconsistent with the other results and it was also observed that a variety of sizes of Daphnia were used leading to an inconstancy. In the 0.5% concentration, the result decreases significantly; this may be due to the fact that not many replicates were made due to the speed at which the Daphnia died, and the Daphnia we did manage to measure were already suffering from heart failure. Once we have taken these results as anomalous we can see a slight trend. Between 0.0% and 0.005% the heart rate increased slightly but not hugely. Between 0.005% and 0.05% the heart rate decreased slightly but not significantly. However the heart rate between 0.05% and 0.25% did increase significantly.
A way to improve my results would be to carry on doing replicates, as the more replicates undertaken the more reliable my results will be.
Conclusion
Due to the large variation in my results there is a large degree of error therefore trends are not very significant. Therefore I conclude that it is not obvious from my results that caffeine has an effect on the heart rate of Daphnia.
Discussion
The chemical formula for caffeine is C8H10N4O2 and is known medically as trimethylxanthine. Caffeine also belongs to a class of compounds called methylxanthines. When in its pure form, caffeine is a white crystalline powder with a very bitter taste. Medically, caffeine is useful as a cardiac stimulant and also as a mild diuretic. Recreationally, it is used to provide a "boost of energy" or a feeling of heightened alertness. Among its many actions, caffeine operates using the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain. On a spectrum, caffeine's effects are more mild than amphetamines, cocaine and heroin, but it is manipulating the same channels.
Adenosine is one of these chemicals. Adenosine is a relatively simple, nitrogen-containing compound used widely by the body. It forms the core of adenosine triphosphate, or ATP, the energy-storage molecule that powers most of the biochemical reactions inside cells. In the brain, adenosine secretion by cells such as neurons often reflects how busy the cells are. Areas that are active generate adenosine; areas that aren't active tend not to. As adenosine is created in the brain, it binds to adenosine receptors. The binding of adenosine causes drowsiness by slowing down nerve cell activity. In the brain, adenosine binding also causes blood vessels to dilate to allow more oxygen perfusion to the body during sleep.
To a nerve cell, caffeine looks like adenosine. Caffeine therefore binds to the adenosine receptors. In fact, it is caffeine's blockade of the A1 adenosine receptor in the heart that causes the heart to pound after a significant caffeine dose. This is due to the fact that it does not slow down the cell's activity like adenosine does. Therefore the cell cannot "see" adenosine any more because caffeine is taking up all the receptors adenosine binds to. Instead of slowing down because of the adenosine level, the cells activities speed up.
This leads to an increase of neuron-firing in the brain. The pituitary gland detects the activity and thinks an urgent situation is occurring; it therefore releases hormones that tell the adrenal glands to produce adrenaline. Adrenaline is the "fight or flight" hormone, and it has a number of effects on your body including raising blood pressure by constricting the flow of blood to blood vessels on the surface, increasing blood flow to muscles, and increasing heart rate.
Caffeine and similar compounds also inhibit a class of enzymes known as cyclic nucleotide phosphodiesterases. These enzymes are, in part, responsible for degrading a stimulatory signal produced when excitatory neurotransmitters activate different neurons in the central nervous system. Thus, when they are inhibited by caffeine, the stimulatory signal remains active for a longer period of time resulting in a greater sense of alertness but also a higher heart rate, blood pressure and respiratory rate.
Therefore, just like us, a Daphnia's heart will beat faster when it receives a dose of caffeine, and this gives us a biological reason for our results.
However too much caffeine can be harmful as we saw in our results. Too much caffeine in the system can cause too much stress and strain on the heart, resulting in the exhaustion of the heart, leading to heart failure.
Evaluation
Within this experiment there were many possible errors that could have occurred which lead to the anomalous results. The biggest source of errors was probably in the 'human error' category. A number of errors may have been made due to the inaccuracy of human senses. The stop watch only counted in seconds, therefore within the space of a second the Daphnia's heart may have beat five times, and if the counting was not stopped exactly on the ten second mark, or started at nought for that matter, there could be a significant anomaly in the results. The eye is very inaccurate, especially when it has to count such a fast heart beat. If the person counting blinked or could not effectively keep their dot-making in time with the heart beat then this may have led to more anomalies. In this experiment an attempt was made to rule out any error which may have been created by bias by undertaking a 'blind test'. This was done by using different people at different stages of the experiment to perform the various tasks. The person counting was not aware of the concentration that the Daphnia had been exposed to so would not over-estimate or underestimate the heart rate according to predictions.
Another category of error is that of 'systematic error'. There may have been a systematic error which was not obvious, such as with the stop watch and the dotting of the paper. However, as mentioned previously these errors where not obvious in this experiment. The sizes of Daphnia were kept the same as much as possible. However, due to their size this was quite hard, so there may have been larger and smaller Daphnia used. Another aspect of this category is that the higher the concentration of caffeine, the less soluble the oxygen in the liquid becomes which could have lead to an increase in the heart rate.
The last category of error is 'random error'. Inaccuracies in the results may have been down to biological variations. As already mentioned, an attempt was made to keep the size of Daphnia the same, but depending on size and biological variations, the Daphnia may have had different heart rates at rest, as can be seen from the results of the 0.0% caffeine concentration. The amount of liquid on each slide will also have affected the heart rate of Daphnia. If there was only a small amount of liquid the ratio of oxygen to carbon dioxide would have rapidly decreased, thereby increasing the Daphnia's heart rate as it tried to get as much oxygen as possible. If the slide became overheated due to the light beneath it, this too could have had an adverse affect on the heart rate of the Daphnia, as it became increasingly dehydrated.
However, there may also have been random error on the part of the experimenters. There may have been lapses in concentration leading to inaccurate dot-making and timing as well as a lack of thorough cleaning of utensils. If the pipette, for example was not cleaned thoroughly, there may have been some liquid of a different concentration which may have subsequently affected the next Daphnia to be picked up by it. If the previous slide had not been cleaned properly, the amount of water left could also have effected the concentration of the next liquid it came into contact with.
Apart from sources of error, other considerations must be taken when conducting an experiment. Firstly safety. There were not many safety issues which arose from this experiment. The experimenters were aware of what to do in the instance of a slide breaking exposing them to sharp glass. They were to dispose of it cleanly and safely in a designated bin. If the liquid being used had been hazardous they would, of course, have taken the necessary procedure to protect themselves; for instance wearing protective clothing. One safety issue that may arise if a stroboscope were to be used would be the effect on people who may suffer from epilepsy. The second consideration is that of ethics. Living organisms were used in this experiment, and inevitably some suffered as a direct cause of the experiment. Many exposed to the higher concentrations died. This meant there was a need to weigh up the costs and benefits of using these organisms for this particular experiment. The cost of the experiment was the poisoning of the Daphnia and as a result their death. On the other side of the argument, the next generation of doctors and scientists may be being trained through this experiment. As a result of this weighing up, it was decided that it was ethically justified to use these Daphnia in this experiment, especially as they were bred as fish food and would have died anyway; and they were a valid method of measuring heart rate.
Anjali Lockett 1 05/05/2007
