Variables
Independent variable: Caffeine concentration (0.1%, 0.2%, 0.3%, 0.4%, 0.5%)
Reason: Independent variables are what the experimenter changes. Since I am investigating how concentration of caffeine affects heart rate of Daphnia, changing this variable can effectively test my hypothesis.
Dependent variable: Heart rate (time taken for 20 beats)
Reason: Dependent variables are what the experimenter measures. Measuring the heart rate helps me to test my hypothesis – whether increasing the concentration of caffeine increases heart rate of Daphnia.
Controlled variables: Temperature of water, temperature of caffeine, light intensity, amount of water, amount of liquid, and stress of Daphnia.
Reason: Controlled variables are factors other than that under study, but can also affect heart rate of Daphnia.
Control
Daphnia in 0% caffeine solution is a control. The Daphnia is in an environment with distilled water only, which ensures that any change in heart rate is due to absorbtion of caffeine only. Here water acts as a standard environment.
Accuracy and validity
- Measurements: suitable apparatuses are used to measure independent and dependent variables. It is also important to measure controlled variables, making sure they are constant. A thermometer can be used to measure temperature of water and caffeine before they are transferred onto cavity slides The caffeine solution’s temperature is kept constant at room temperature by keeping the microscope’s light off whenever I am not viewing the Daphnia and measuring its heart rate.
- Each experimentor repeated the measurement of heart rate for 3 times, so that any anamolous results can be spotted easily.
Safety precautions
When a cavity slide breaks, dispose it into a special bin safely.
Results
Presenting Data
I have decided to plot a line graph. Firstly, it can show the relationship between the 2 data: “caffeine concentration” and “time taken for 20 beats” which is not immediately obvious from the table above.
Secondly, both the dependant variables (time taken) and the independent variables (caffeine concentration) are continuous.
The dependant variables are put on the y-axis, and the independent variables are put on the x-axis.
Graph A: a line graph of time taken for 20 beats(s) against caffeine concentration(%)
Then, I plotted a second graph to show the relationship between caffeine concentration and heart rate. Heart rate is obtained by dividing 1 by time taken:
Graph B: A line graph of heart rate(1/T) against caffeine concentration(%)
Analysis
Graph A: As caffeine concentration increases, the time taken for 20 heart beats decreases. When concentration=0%, the range of results is largest. This maybe because of random errors occurred when counting the beats.
The line of best fit, which does not pass through concentration=0.2%, has a negative gradient. But as caffeine concentration increases, the gradient becomes less negative. I can observe that the line is steeper when concentration increases from 0% to 0.3%, and then less steep when concentration rises from 0.3% to 0.5%.
Graph B: A line of best fit is drawn after identifying the result at concentration=0.2% as anomalous. This line has a positive slope, showing that as caffeine concentration increases, heart rate of Daphnia increases. The gradient gradually decreases as concentration increases. The line starts to get less steep when caffeine concentration increases from 0.3% to 0.5%.
Conclusion
The higher the caffeine concentration, the higher the heart rate of Daphnia.
Explanation:
- The caffeine molecule is similar enough to adenine to fit into adenosine receptors, blocking the A1 adenosine receptor on the surface of heart muscles. Adenosine’s general effect in the brain is to inhibit neural activity. When caffeine binds onto adenosine receptors on the heart, it does not lower cell activity like adenosine does. As adenosine receptors are taken up by caffeine, adenosine cannot bind onto adenosine receptors anymore, hence activity speeds up and heart rate rises.
- Caffeine can also inhibit phosphodiesterase, an enzyme which degrades a stimulatory signal transmitted by excitatory neurotransmitters. This inhibition by caffeine would lead to higher activity of the excitatory neurotransmitter, resulting in higher heart rate.
Errors and improvement
- The counting of heart rate was done manually. When the beating gets very quick, we cannot react and count quickly enough. This leads to inaccurate heart rate.
- We started the experiment with water, and than the lowest concentration of caffeine(0.1%), and then gradually increased to a maximum(0.5%). We should do blind sampling by covering the concentration.
- The in-built light of the microscope has a heating effect on the caffeine and the Daphnia. Temperature control was difficult because a light source is necessary when viewing the Daphnia. We could prepare a heat sink by putting some ice water between 2 cavity slides. This heat sink can then be put under the slide with Daphnia to regulate temperature.
- Caffeine was not completely mixed with water, so the concentration obtained was not what was expected.
- Pipette was not cleaned and dried thoroughly, so there were impurities in the caffeine solution.
- I did not add exactly 5 drops of caffeine-water solution onto the cavity slide every time, so the amount of liquid surrounding the Daphnia was not constant.
Ethics
This experiment involved living organisms; hence it is important to consider ethical issues.
All living organisms should have the right to live in a natural system as freely as possible. Some Daphnia might die of heart failure when the concentration of caffeine is too high, or when they are in a stressed state for too long. We cannot obtain consent from the Daphnia before carrying out the experiment. But does this does not necessarily mean that we have absolute power over them.
