Hypothesis:
If the concentration of caffeine is increased, the daphnia’s heart rate will increase. I based this on the fact that caffeine increases the number of stimulatory neurotransmitters that are released which increases blood pressure.
Method:
To measure the heart rate of the daphnia I will first use a pipette to collect some daphnia from the water and place them into a dimple tray and then to a dimple slide and put a cover slide on top to stop the water evaporating too quickly. Then place the slide under a microscope and focus on the heart, being careful not to put the light too close to stop the daphnia drying out. After this count the daphnia’s heart rate for 5 seconds and multiply the result by 12 to get the heart rate for a minute. Get another person to count the time using a stopwatch and record the results in a table. Repeat this adding a drop of caffeine (0, 1, 2, 3 drops) each time, but wait for a minute before counting the heart rate to make sure the caffeine has taken effect. Repeat this with 5 different daphnia to make the results of the experiment more reliable. The control variable in this experiment is the time over which the heart rate is measured so this must stay the same to make sure the results are reliable. The independent variable is the amount of caffeine drops. This also needs to be controlled carefully to make the results fair. When carrying out this experiment you should be careful not to break any of the glass slides or the microscope and do not let it get too hot. You should also take into consideration ethical issues, for example return the daphnia back to the beaker of water straight after examination because although they do not appear to suffer in the same way as humans, they are still living things.