1) Effect of Caffeine on Daphnia Heart Rate
Independent Variable: Caffeine concentration (0.1%, 0.2%, 0.3%, 0.4%, 0.5%)
Dependent Variable: Heart rate of daphnia (beats per minute)
Control Variables:
• Temperature
• Volume of solutions
• Stress of daphnia
• Size of daphnia
• Time of acclimatisation
Method:
1. Transfer 1 daphnia onto cavity slide
Place cotton wool onto cavity slide to restrict the movement of the daphnia
2. Place a few drops of distilled water onto the daphnia using a pipette
3. Leave daphnia to acclimatise for 3 mins
4. Place slide onto the stage of a light microscope and adjust the focus until you can see the beating
heart of the daphnia
5. Count the number of beats in 20 seconds using a stopwatch, and multiply by 3 to calculate beats per
minute
6. Repeat the experiment, using different daphnia each time, to find a mean
7. Repeat using caffeine solutions of varying concentration in place of distilled water
Control Test: Use distilled water instead of caffeine solution
Data:
• Calculate a mean heart rate for each caffeine concentration
Theory:
• Caffeine is a stimulant, which increases heart rate by increasing the release of neurotransmitters
• Daphnia are used because they have translucent bodies, so the heartbeat can be observed
Conclusion:
• As the concentration of caffeine increases, the mean heart rate also increases.
Ethical Issues:
• Supporting the use of invertebrates:
o Invertebrates are simpler organisms than vertebrates; they have less sophisticated nervous
systems, so they could feel less pain
• Against the use of invertebrates:
o They cannot give consent and they may be subjected to painful procedures
o It is unethical to intentionally cause distress or suffering to a living organism
Validity:
• Counting of heart rate can be inaccurate
• Too high concentration of caffeine can kill daphnia
• Difficult to keep daphnia the same size
• When left under microscope, the lamp can increase the temperature of the daphnia, increasing heart
rate
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