I will place each daphnia in the solution for 5 minutes before measuring their heart rate again to give them time to take the caffeine into their body and et it have an effect on their heart rate. I will use daphnia with a similar sized body so that they absorb similar amounts of caffeine into their bodies.
As the heart rates of the daphnia and the difference in heart rate sis different I will calculate the percentage change in heart rate to make my results more accurate.
The independent variable in this experiment will be the concentration of the caffeine solutions as I will alter these to see the different effects. The dependant variable will be the heart rate of the daphnia as this is what I will be measuring to see the effects. The controlled variables will be the time the daphnia are left in the solution, the temperature, the size of daphnia. I will leave each the daphnia in the solution for 5.0 minutes by measuring on the stop clock and then measuring the heart rate for 60 seconds also using the stop clock to measure the time. The temperature will be quite consistent as I will do them all in the same room within an hour. I will also use daphnia of very similar size to make sure that the caffeine absorbed into their bodies is very similar so that the results are more reliable and valid.
I didn’t follow this plan as I did the experiment with my whole class. Instead, we placed a few daphnia in each concentration and were assigned a caffeine concentration to do. The daphnia were left for 5 minutes then placed on cavity slides with enough solution around them to stay healthy. We then put a very thin layer of cotton wool fibres on top of them to secure them in place.
We recorded the heart rate of the daphnia for 20 seconds using a counter to count the heart rate as it was quite fast. We measured the heart rate for 20 seconds, three times to ensure my results are consistent and reliable. When the daphnia are placed under the microscope the heart is clearly seen through their transparent bodies so it is easy to measure.
We used different daphnia for the different concentrations of caffeine solution and also measured the heart rate of daphnia in pond water as a control.
I needed:
- 20 daphnia
- Caffeine solutions of 0.1% 0.2%, 0.3%, 0.4% and 0.5%
- Pond water
- A microscope
- A pen and paper
- A stop watch
- A pipette
- Cotton wool
- A cavity slide
Results
Analysis
My results show that, in general, the daphnia’s heart rate did increase when in a caffeine solution and supported my original hypothesis. The heart rate increased by 59 beats per minute between the pond water and the 0.4% caffeine solution.
I think that the results for the concentrations 0.1% and 0.5% are anomalies because the rates are much lower than the others, and my results aren’t very valid. The caffeine concentration 0.5% is defiantly an anomaly, unless the daphnia’s original heart rate was much lower then the others, or the higher level of caffeine was beginning to kill the daphnia so the heart rate was lower. The people measuring the heart rate for the daphnia in the 0.5% solution could have been recording the beating of the leg instead of the heart or the daphnia’s heart may have been harder to see and they could have only seen half the beats that were actually happening. The daphnia used could also have been larger than the other daphnia tested so the effect of caffeine on its body isn’t as great as on the smaller daphnia. The heart beat is increasing steadily to 213.9 beats per minute and then it suddenly drops to 80 for 0.5% when the trend shows it should still be increasing. The daphnia used in the concentration of 0.1% could have had a lower initial heart rate than the others, or have been bigger than the other daphnia and not be affected by the caffeine as much as the others and this is why the heart rate is slightly lower than the others around it.
The results I have are not very reliable and accurate as the heart rate of each daphnia before the experiment would have been different and we had to assume they were the same as we did it as a class. If I had measured the heart rate before and then during the experiment I would have been able to work out the percentage change which would have made my results more accurate.
The apparatus I used was accurate and effective except I had to record each heart beat with a dot on some paper as there were not enough counters for the whole class to use. This meant that some people’s results were not as accurate as they could have been if they miscounted the dots. I think it should have been made very clear to everyone in my class what the heart was so that everyone counted the same thing instead of the legs beating. This would have hopefully made my results more accurate and valid.
I think that to improve the accuracy of my results drastically, I should have done the whole experiment myself and recorded the heart rate before and during the experiment to obtain a percentage change in heart rate. Also if I didn’t it all myself, I would know that my apparatus is reliable and that what I am counting is reliable each time. This would take much more time, but I think it would make my results more valid.
Evaluation
I used different concentrations of caffeine solutions to measure the heart rate of daphnia when in those concentrations. I found that for most of my results the heart rate increased when in a higher concentration of caffeine, but I had two anomalies. I think the results that I have, excluding the anomalies, are valid, but they would be more accurate and reliable if the percentage change in the heart rate was recorded instead of the new heart rate.
