This experiment is a blind experiment meaning that the person who is recording the heart rate does not know the concentration of the caffeine being used. This ensures that the experimenter pays exactly the same amount of attention to each experiment ruling out personal bias. Sometimes, if an experimenter knows that there is a low concentration of caffeine being used they may pay less attention as they are expecting the heart rate to be less. This may give an inaccurate result.
The person recording the heart rate is kept the same to ensure there is no human error. For example one persons eyesight may be better than another’s, or the reaction time of recording the heart rate with a pen may be quicker.
Each daphnia must only be used once as using many different concentrations on the same daphnia may cause an unfair result, as the caffeine from the previous experiment would not have worn off and giving the daphnia more caffeine may cause they heart rate to increasing hence giving an unfair result.
The experiment is repeated to see how reliable the results are and to ensure a fair test.
Ethics of using live animals in experiments
Many ethical issues arise when talking about using live animals in scientific research and experiments. Although using daphnia in an experiment where the only risk they appear to be under is the concentration on caffeine, there are many issues that need to be thought about.
The daphnia in the experiment may undergo unnecessary pain and stress. Because the daphnia has been taken out of its natural environment it may feel stress as it is in unusual surroundings. Then the daphnia are then taken out of this artificial environment using a pipette and place under a microscope in a very small amount of water may also make their stress levels rise.
Another issue that this experiment raises is the fact that the animal cannot consent to be taking part in the experiment, whereas if this experiment was focusing on how caffeine affects human heart rate, consent could be given. This experiment would not harm the human, but it may harm the daphnia.
There is a risk in any experiment of the animals dying. As the daphnia are so small there is a quite high risk of them dying accidentally for example, the over slip for the microscope dropping on the floor, with the daphnia on it and killing them.
If the daphnia were used multiple times in this experiment it would be unfair on the daphnia as the caffeine may be harming them and we would not know.
Once the daphnia have been used in the experiment it needs to be considered what will happen to them after this. It is best to place the, back into their natural environment, so they adjust back to this, but this may again cause them more stress as they are being moved from one place to another.
Concentration
The 0.5% of caffeine must be made up to do this the following equation is used:
Volume of stock solution to be diluted =
Final volume of diluted solution x concentration of diluted solution
Concentration of stock solution
10cm3 = 10cm3 x 0.5 molar
0.5%
8cm3 = 10cm3 x 0.4 molar
0.5%
6cm3 = 10cm3 x 0.3 molar
0.5%
4cm3 = 10cm3 x 0.2 molar
0.5%
2cm3 = 10cm3 x 0.1 molar
0.5%
0cm3 = 10cm3 x 0.0 molar
0.5
Apparatus:
- Culture of daphnia
- Cavity slides
- Dropping pipettes
- Distilled water
- 05% solution of caffeine
- Cotton wool
- Measuring cylinder
- 7 beakers
- Stopclock
- Filter paper
- Microscope
- Pen
- Paper
Method:
- Make up the caffeine solutions and place in beakers. Label each beaker
- Get a daphnia out of the pond culture with a pipette
- Place daphnia on a cavity slide and place under a microscope. This is a control
- If the daphnia move to much use cotton wool to restrict their movement
- Start stop clock
- Record heart beat every 2 minutes for 15 seconds
- Do this for 6 minutes
- To measure heart rate dot paper every time the heart beats and count the number of dots after
- Soak off water with filter paper
- Add caffeine solution to cavity slide
- As this is a blind experiment the person recording the heart rate should not know what concentration was put on the cavity slide
- Start stop clock immediately
- Place under a microscope
- If the daphnia move to much use cotton wool to restrict their movement
- Record heart beat every 2 minutes for 15 seconds
- Do this for 6 minutes
- To measure heart rate dot paper every time the heart beats and count the number of dots after
- Put daphnia in a beaker of water after used label this beaker “used daphnia”
- Repeat experiment with a new daphnia for every concentration and each time doing a control
Results
Trial 1
Trial 2
Conclusion
We can see from the mean score that caffeine does have a visible effect on the daphnia, but we can see difference in concentration did not that much of a significant result. All of the heart beats seemed change by about 4 beats per 15 seconds, the change may have been because they were stressed that the water had been taken away from them and a new solution was added that they were not used to. This would have affected their heart beat. There are some exceptions to in trial 1 when the 0% caffeine was added the control heart rate was 52 beats per 15 seconds and it went up to 68 beats per 15 seconds. This change in heart rate could not have been because of caffeine but an extraneous variable for example stress. This means that all of out results that we have found may be due to this extraneous variable and not due to caffeine being added.
Accuracy and Reliability
The daphnia’s heart rate was very difficult to record as it was beating so fast, this meant that the person who was recording the number of times the heart had beat must be very careful when recording the results. Some of the dots looked like one dot when they were actually two and some looked like two dots when they were actually one. This meant that some of the results might an exact representation of the results therefore they may not be reliable.
The daphnia were also moving around in the cavity slide this also meant that the heart rate was difficult to record; we had to restrict the daphnia’s movement so we could see them under the microscope, by using cotton wool, but this soaked up all the solution out of the cavity slide and may have caused discomfort for the daphnia raising ethical issues.
All the daphnia had very different heart rates even when in the control solution, if I were to do this experiment again I would use daphnia which had similar heart rates when in the control, so the results when added to caffeine could have been seem a lot easier.
The experiment also needed to be repeated more three times, because if there were a problem with one the daphnia’s heart, for example it was stressed and this increased the heart rate it would be impossible to tell which daphnia had a unreliable heart rate if we were to do only two experiments. Doing three or more experiments means that if one of the sets of results were invalid it means we can easily see this and analyse the results on the other two sets of results.
Evaluation
If I were to repeat this experiment I would use daphnia with the same speed heart beat for each test as there are many varying degrees of heart beat even when in the control. Looking at the results the daphnia with the fastest heart rate in the control all seemed to be placed in the lower concentration of caffeine. This meant our results look as though caffeine slows down the heart rate.