Hypothesis: As the caffeine concentration rises, so does the mean heart rate of daphnia.

AS and A Level Science

The Effect of Caffeine on the Heart Rate of Daphnia

Hypothesis:

As the caffeine concentration rises, so does the mean heart rate of daphnia.

Risk Assessment:

In this experiment there are the following risks both for the human that carries out the experiment and the daphnia:

The human that carries out the experiment should be cautious not to touch the lamp with wet hands since it uses electricity.
Also, be careful not to spill any of the pond water on the floor.
As for the daphnia, there is a chance for them to die as when they are exposed to light, they are also exposed to heat in which they aren’t used to.
Moreover, when they are given caffeine of a high concentration their heart may beat so fast that will cause their death.
Furthermore, there is the risk of the daphnia to dry up, which may kill it in the middle of the experiment.

Method:

These are the steps that we followed when doing the experiment:

Use a pipette to transfer the daphnia to a cavity slide containing pond water of known volume.
Add some cotton fibres to the cavity slide and move them with a fine paint brush in such a way that the daphnia can’t move.
Use a calculator to calculate the heart rate. Set its reading to zero and add 1 to the answer.
One student will use a stopwatch to count 30 seconds which is the time during which the measurement of the heart rate will take place.
Turn the microscope lamp on and locate the heart of the daphnia through the microscope.
Measure the beat rate.
Repeat steps 1-6 about 4 or 5 times and get an average. Then calculate the heart beats per minute (just by doubling the average).
Clean the cavity slide using a tissue and make sure there is no pond water left.
Use a pipette to transfer a known volume of caffeine solution of concentration 0.125% to the cavity slide.
Transfer the daphnia to the cavity slide using the pipette.
Measure the heart beats of daphnia per 30 seconds, following the same procedure as before.
Repeat steps 9-11 using caffeine solutions of concentrations 0.25% and 0.5%

Notes for the experiment:

Pond water is used as the daphnia are used to living there and we want to keep as many aspects from their normal life as we can, so that the results are more accurate. This is also the reason for which they should be kept in pond water.
The microscope lamp should only be on while counting the beat rate as it might alter the temperature and when the temperature is increased the heart rate may go up because of this and not because of the aspect we are investigating.
A cavity slide and ...

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Notes for the experiment:

Pond water is used as the daphnia are used to living there and we want to keep as many aspects from their normal life as we can, so that the results are more accurate. This is also the reason for which they should be kept in pond water.
The microscope lamp should only be on while counting the beat rate as it might alter the temperature and when the temperature is increased the heart rate may go up because of this and not because of the aspect we are investigating.
A cavity slide and NOT an ordinary flat microscope slide shall be used, as the cavity slide will help keep the daphnia in a small volume of either caffeine solution or pond water and not letting it escape.
Cotton is used to keep the daphnia still so that the heart rate can be easily counted.
This experiment involves a living organism so we need to be careful with it. So, we should leave it in conditions similar to those that it was used to live: 5oC up to 20oC (ice can also be used to lower the temperature). Furthermore, we shouldn’t give it caffeine solutions of high concentration immediately as they might kill it.
The independent variable for the experiment is the concentration of the caffeine that’s given to the daphnia. The dependent variable is the beat rate of the daphnia.
Just like every other experiment, this one also includes some limitations. These are the following:

the heat produced when the lamp is on cannot be handled and might be the reason for the heart rate to increase
the heart rate might increase because of the effort of the daphnia that is trying to escape from the cotton fibres

Results and Analysis:

This table summarises the results that we got by doing the experiment.

These results are better depicted by the following chart.

Conclusions:

The chart does not confirm what our hypothesis was: that the higher the concentration of the caffeine solution the more the beats per minute of the daphnia. This, though, might be a mistake since the caffeine solution may not have been exactly as we wanted it in each occasion, hence making the results inaccurate. Also, when trying to press the “=” button of the calculator faster, many of those additions were not calculated since the calculator can’t add as fast as we press the button.

Evaluation:

If I were given the opportunity to repeat the experiment these are the parts of it that I would change:

I would repeat the experiment with the caffeine solution more times, so that the results I would get will be more accurate.
The same person would calculate the beat rate for all the experiment, as some may be able to press the “=” button more quickly and others may not have been able to keep up with the fast rate.

Going Further:

To further analyse the effect of a drug, like caffeine, on the heart rate of a living organism, like the daphnia, these are some changes in the experiment that could have been done: