Investigate the Effect of Caffeine on the Heart Rate of Daphnia

Last Updated: 20 Apr 2022
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Aim: In this experiment we are trying to find out how varying the concentration of caffeine can affect the heart rate of Daphnia shrimp.

Prediction: I predict that caffeine will increase the heart rate of the shrimp.


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I believe the results will comply with my prediction. Caffeine is a compound found in especially tea and coffee plants and acts as a natural pesticide.

Caffeine is a central nervous system stimulant meaning it increases the amount of neurotransmitters released meaning coffee is a stimulant that affects the central nervous system by increasing its activity. In this practical the caffeine should have a noticeable effect on the heart rate of Daphnia as Daphnia absorbs chemicals present in the solution around it without selection. I also predict that there would be an increase in heart rate once placed in decaffeinated coffee as decaffeinated coffee still has caffeine content but significantly lower per milligram than caffeinated coffee.

However I will expect the increase of heart rate of the culture placed in caffeinated coffee to be significantly higher than the culture placed in decaffeinated coffee. Daphnia: The reason Daphnia is used as the organism in this experiment is due to its transparent body which makes the heart visible. Daphnia is better than Brine shrimp because brine shrimp is not transparent meaning heart rate has to be measured by counting leg movement. Variables: The independent variable that we change. In this case it is the caffeine content.

We would be experimenting with water, decaffeinated coffee and caffeinated coffee to observe and compare its effect on the heart rate of Daphnia. The control for this experiment is a solution with no caffeine which allows us to see if the change in heart rate is due to the caffeine or other factors. To ensure that this experiment is a fair test only the concentration of the caffeine will be changed. Caffeine is the independent variable. I tested each particular concentration of caffeine 10 times.

I decided this prior to the experiment because there was bound to results that are out of pattern so repeating this many times would eliminate systematic errors resulting in more accurate and reliable data. The dependant variable is the heart rate of the shrimp will be measured over 10 seconds Ccontrol variables: These are variables that should be kept the same. As a result the effect other factors other than coffee content are limited. This is needed as the objective is to investigate the effects of caffeine content. The control variables ensure valid data.

Volume of solution The time heartbeat is measured within Temperature: Daphnia may be very sensitive to the heat therefore the kinetic energy given to them by heat could have effects on the heart rate. Same culture of Daphnia: To minimise genetic differences. The size of the shrimp can change the degree of the effect of caffeine. The larger shrimp will have a lower heart rate. I will ensure this as best as possible by keeping the Daphnia in the same light intensity by shining a lamp directly at it.

Preliminary experiment

I did a preliminary experiment to familiarise myself with the methods so no time is wasted during the actual experiment. It helped me identify the limitations in the experiment and how to conduct the experiment appropriately. For example I noted not to add too much of each solution on the microscope as there will be a loss of surface tension and it will spill. I also learnt to not use the slide covers as they kill the Daphnia Analysis The mean heart rate of Daphnia shrimp placed in solution B is 26. 5% higher than the heart rate of the Daphnia placed in the control solution.

Furthermore it is 1.9 % less than Solution A ( less difference than I predicted. From these results we can infer that Solution B is the solution containing decaffeinated coffee. This is because the mean heart rate is lower than caffeinated coffee and higher than the control solution. This is because decaffeinated coffee contains caffeine but significantly lower per milligram than caffeinated solution. However this Solution A, caffeinate coffee caused the daphnia heart rate to increase the most. This proves the hypothesis because the greater the caffeine content, the more neurotransmitters are released.

The increase in this neuronal stimulation is interpreted as an emergency in organisms causing adrenaline to be released therefore heart rate increases. There still appears to be fluctuations that are probably caused by systematic errors. The data from shrimp placed in solution A (caffeinated coffee) has the greatest variation in results at 168 compared to solution B, 108. Evaluation Reliability: My results appear fairly reliable because there is an obvious pattern occurs and it supports my prediction. The experiment was carried out as a blind trial whereby the solutions except for the control are unknown.

This helps eliminate bias whereby I don’t choose some results and ignore others to support my prediction making the results more reliable. Validity Systematic errors could have caused this for example I did not give time for the Daphnia to absorb the solution before counting the heart rate. These factors could have also caused the fluctuations in the results so accuracy is questionable. The variation in caffeinated and decaffeinated coffee could have been caused by systematic errors. For example, the Daphnia could have already been dead.

There was a significant drop between the eighth and ninth test of solution A ( around 54. 7%) I did not expect this because caffeine is a stimulant drug. This suggest that the shrimp have different tolerance towards caffeine and some may have been dying. Furthermore the heart rate of some shrimp could be distinctly different from the other shrimps as they were trying to escape the cotton fibres which requires more energy thus hear rate increases. This suggests that the culture of Daphnia used in the experiment could have contained shrimp that are different age and size.

Although I tested each concentration 10 times, there is still a lot of variability of the results. Water, on the other hand has the lowest variability of only 60. The variability in the other contents are at 168 (caffeinated coffee) and 108 ( decaffeinated coffee) This could perhaps be the effect of other chemicals in caffeine which Daphnia are responsive to. Accuracy Certain methods of collecting data were not accurate therefore affecting the validity. For example, a random error such as not tapping the pen in the rhythm of the heartbeat affected the number of heart beats per minute.

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Investigate the Effect of Caffeine on the Heart Rate of Daphnia. (2016, Aug 26). Retrieved from

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