VARIABLES
PREDICTIONS
It is predicted that the heart rate of the Daphnia will slowly increase when differing amounts of caffeine solution is administered to it. If it is tested on humans or animals, it may be proven to be unethical. However, if a greater amount of the caffeine solution (1%) is added to Daphnia it may cause paralysis.
EQUIPMENT LIST
- Cotton
- Caffeine solution (0.1%, 0.2%,0.3%, 0.5%, )
- Daphnia
- Pond water
- Wide mouth pipette
- Tally counters/ stop watch
- Dropping pipettes
- Microscope
- Cavity slide
- Beakers
- Petri dishes
METHOD
A microscope with microscope light is set up under a low objective lens and one Daphnia is scooped up carefully from the glass beaker with a wide mouth pipette and placed on a cavity slide. A caffeine solution of 0.1% is dissolved with 3 drops of pond water altering the concentration of caffeine solution. A few strands of cotton wool are placed around the Daphnia to ensure it is held in place and doesn’t move while it’s been observed under the microscope. An extra drop or two of water is added to prevent the Daphnia from dying as the cotton wool tends to absorb pond water. The position of the cavity slide is adjusted until a clear image of Daphnia is observed. Each student from a group is assigned to observe the Daphnia under the microscope and count the number of heartbeats.
If unable to locate the heart, the movement of the legs can be used as a substitute. Another student in the group is required to record the time by using a stop watch or tally counter and this procedure is repeated a few times to obtain a 2nd and 3rd reading and the average is worked out. The ‘used’ Daphnia is returned into the pond water and a ‘fresh’ one is ‘taken out’. These steps are repeated for 5 times with different amounts of caffeine solution and each average value of the heart beats of Daphnia is calculated and the results recorded in a table with a graph for comparison.
RISK ASSESSMENT, SAFETY PRECAUTIONS AND ETHICAL CONSIDERATIONS
JUSTIFYING THE VALIDITY AND RELIABILITY OF THE METHOD CHOSEN
Students will practice counting the heart rate of Daphnia before the start of the practical to familiarise themselves with the technique. Only suitable apparatus and experimental procedures should be followed to ensure precise and valid results. Results should be repeated several times and the method used for the practical followed carefully to avoid errors. Only the most precise equipment must be used if available (tally counters).Repeats should be kept at a minimum and anomalous results investigated. Measurements taken need to be cross checked with a partner to ensure precision.
A TABLE TO SHOW THE EFFECT OF INCREASING % CAFFEINE SOLUTION ON DAPHNIA HEART RATE
DISCUSSION OF RESULTS
DATA ANALYSIS
It was predicted that the Daphnia’s heart rate will steadily increase as the different ranges of caffeine solution is applied to it. This is as a result of the Daphnia’s transparent body which made it easier to count the heart beats. From the graph, we can acknowledge that the results agree with the prediction. In animals, a certain dose of caffeine solution could prove fatal and may not produce accurate results as it would be difficult to monitor their heart rate and it will also be unethical.
Although animals were not available to experiment on due to ethics, when the highest dose of caffeine solution was administered on Daphnia, it caused their hearts to beat twice as fast and other groups of students carrying out the same practical noted the same pattern. The biological difference is that the effects of caffeine on humans it’s a gradual process and its side effects may not manifest themselves as quickly as you would notice on a Daphnia.
JUSTIFYING THE VALIDITY AND RELIABILITY OF THE RESULTS
The results are reliable and valid as a result of a trial run carried out before the start of the experiment which required monitoring the heart rate of the Daphnia. Also, suitable apparatus was used to carry out the practical and the experimental procedures were followed thoroughly. Each set of results were repeated 3 times and an average was calculated. To ensure precision, the measurements were cross-checked and this helped to improve familiarity with technique and practical skills so this experiment can be acknowledged as a fair test.
CRITICISMS OF DATA
Unfortunately, it was impossible to carry out more repeats due to the restrained use of caffeine solution as it would have been unethical to explore the effects 1% of caffeine would have on Daphnia.
POTENTIAL ANOMALOUS RESULTS
Looking at the results shown on the table, there is only one set of anomalous results which is the percentage of caffeine solution at 0.3%.This suggests an error as the results tend to fluctuate which proves that the three repeats may not be as close to the true value as it should be.
Apart from the anomalous result identified above, the recorded values for each repeat are close together which implies that the results show accuracy and precision. Furthermore, the Daphnia behaved as predicted.
POSSIBLE SOURCES OF ERRORS
Even though there are obvious anomalies, there is the possibility of random errors. One of the most common errors was lack of concentration when measuring or recording an investigation or set of results. Lack of time may cause difficulty in cross-checking measurements.
Also, the equipment used to measure the caffeine solution might affect the results produced if the minimum volume is a lot more than required. Systematic errors are due to personal bias or techniques used to carry out the practical.
LIMITATIONS
The main limitation was lack of time and also insufficient tally counters were made available per group which meant that the heart rate counts may have been miscalculated as stop watches were used instead. Also there was a restricted use of caffeine solution.
REDUCING ERRORS
Extra care should be taken whilst using the caffeine solution so measurements need to be taken at eye level to ensure accurate results and the practical will need to be repeated again using tally counters. Furthermore, clear marked trays should be available to dispose of Daphnia in an appropriate manner with clear marked trays for ‘fresh’ and ‘used’ Daphnia so they don’t get mixed up.
CONCLUSION
In conclusion, the overall results agree with the prediction that different doses of caffeine solution will increase Daphnia heart rate. This is because if the Daphnia’s transparent body which ensures that we see results a lot faster than any other living organism. Although alternate sources were not available as it would have been termed unethical, the results would have been a lot slower if performed on humans or animals.
IMPROVEMENTS
Sufficient time will enable the students to complete the experiment and also cross-check every measurement and calculations with the aid of tally counters to improve accuracy of results. An increased percentage of caffeine solution would make it possible to carry out more repeats.
FURTHERWORK
It would be useful to repeat the practical with 0.3% of caffeine solution and also carry out the same experiment on humans and compare how caffeine affects their heart rate and also see medical expertise before carrying out this experiment.
Resources
As Salter’s Nuffield Biology
(Tehttp://www.fws.gov/midwest/pwlc/student%20work/aquatic%20inverts%205th%20grade_clip_image003.gifxt book)
As Salter’s coursework exemplar