CORE PRACTICAL 1: INVESTIGATE THE EFFECT OF CAFFEINE ON HEART RATE IN DAPHNIA

Question 1

AIM

Answer 1

To investigate the effect of caffeine on heart rate in Daphnia

Question 2

Independent Variable

Answer 2

Caffeine concentration (M)

Question 3

Dependent Variable

Answer 3

Heart rate of Daphnia (beats per minute)

Question 4

Control Variables

Answer 4

1. Temperature – measure temperature with thermometer. Carry out in same place
2. Volume of solutions – use same number of drops on Daphnia each time
3. Stress of Daphnia – try to minimise stress of Daphnia
4. Size of Daphnia – try to pick Daphnia of around the same size for repeats
5. Age and Species of Daphnia
6. Time to acclimatise – leave Daphnia in caffeine solution for same amount of time to acclimatise (e.g. 5 minutes)

Question 5

Ethical Considerations

Answer 5

1. Returning the Daphnia to their natural habitat after use
2. Storing the Daphnia in conditions that replicate their natural environment
3. Working at a good pace to minimise time Daphnia are under any possible stress
4. Turning off the microscope lamp when not in use, as Daphnia are poikilotherms (cold-blooded)
5. Not using an excessive amount/concentration of caffeine

Question 6

Why Use Daphnia?

Answer 6

Daphnia (otherwise known as water fleas) are very common and so there is no real threat to the species’ existence or its dependent species (via food webs). There is also no threat to Daphnia reproduction because they reproduce asexually as genetic clones – hence no loss of genetic variation. Daphnia possess a less developed nervous system compared to humans, so they have a reduced awareness of pain. Finally, Daphnia are transparent and so the heart is visible, which avoids the need for dissection.

Question 7

Describe how to carry out the investigation:

Answer 7

1. Remove 1 Daphnia with a pipette and place it in cotton wool to suspend movement. Then place it on a cavity slide under a microscope
2. Determine the base heart rate without caffeine
3. Dab around the Daphnia with a tissue to remove the pond water and replace with drops of caffeine solution (e.g. 0.1M)
4. Leave the Daphnia for 5 minutes to acclimatise and then observe & count (using a counter) the heart rate under the microscope for 30 seconds (multiply number by 2 to calculate beats per minute)
5. Repeat this for measurements across 5 different caffeine concentrations (e.g. 0.2M, 0.3M, 0.4M and 0.5M) made with dilutions
6. Repeats can then be carried out with two other Daphnia

Question 8

Conclusion

Answer 8

An increase in caffeine concentration results in a faster heart rate (more beats per minute, bpm).

Question 9

Results & Calculations

Answer 9

Make sure to present your data in an appropriate table and graph. Work out any mean values from your repeats. The standard deviation can be displayed on the graph using error bars

Question 10

RANDOM ERRORS THAT COULD OCCUR

Answer 10

1. Left for too long under microscope, temp increased (due to lamp) = increased heart rate – turn off microscope lamp when not in use next time
2. Different Daphnia showed different results due to genetic variation – carry out more repeats with different Daphnia to get an ‘average genes’ mean value, which is more accurate
3. Daphnia moving around can make it hard to count heart beats – place Daphnia in cotton wool next time to limit movement
4. Water from Daphnia extraction may have diluted the caffeine concentration – use a micro-sieve to filter out all pond water

Question 11

SYSTEMATIC ERRORS THAT COULD OCCUR

Answer 11

1. Too high concentration of caffeine kills Daphnia – use lower caffeine concentrations
2. Heart rate counted only for 30 seconds so may have missed a few heart beats when calculating beats per minute – record heart rate for full 60 seconds