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The Effect of Caffeine on heart rate. I think that the amount of caffeine in the blood will determine the heart rate of the daphnia.

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Jerry George

The Effect of Caffeine on heart rate.


I think that the amount of caffeine in the blood will determine the
heart rate of the daphnia. It will be directly proportional to each
other i.e. the increase of the % of caffeine will also increase the
heart rate of the daphnia.

Equipment List

• Culture of Daphnia (water fleas)

• Cavity slides and cover slips

• Dropping pipettes

• Distilled water

• Cotton wool

• Standard glassware (beakers, measuring cylinders etc)

• Stop watch

• Paper towels or filter paper

• Microscope.


1. The beating heart of a water flea can be seen through its
translucent body,

by placing the flea in a few drops of water in a cavity slide. A cover

helps stop the water evaporating.

2. Place a few strands of cotton wool on a cavity slide; this will
help restrict

the movement of the water flea.

3. Using a pipette, transfer one large water flea to a cavity slide.
Remove the water from around the water flea using filter paper, then
add one or two drops of distilled water.

4. Use as much water as you can and do not use a cover slip.

5. Together these precautions will help maintain sufficient oxygen
supply to the flea. A cavity slide filled with iced water and placed
under the slide will act as a heat sink.

6. View the water flea under low power. Focus on its heart which can
be seen

through it translucent body.

7. Use a stop watch to record the number of heart beats per minute.

8. Tap a pencil on a piece of paper and count up the pencil marks at
the end of the time period.

9. Record the heart rate at intervals of two minutes over a 10 minute
period. It is a good idea to do a ‘blind’ study to avoid bias in the

10. In order to make the best assumptions of the results the reader of
the amount of heart beats there are and the concentration of the
solution should be uninformed.

11. Repeat the procedure using other water fleas from the culture
solution and fresh, clean slides and cover slips. Replace the water
with caffeine solution. Repeat the procedure using several different
concentrations of caffeine.


Concentration of caffeine  

Heart rate of Daphnia





Control 0%































When looking at the results that I have obtained I can see that when
adding more caffeine concentration to the water flea, the heart will
also increase and there fore the prediction that it will be directly
proportional is correct. For example when the control had only a heart
beat of only 160BPM the daphnia with 0.1% concentration of caffeine
had a heartbeat of 296. this is almost 1.8 times more heartbeats than
a regular one.

In every experiment, only the reliable and accurate results are
useful. The more of these we do and taking an average we will be able
to get a fairly good result and because we used the class results I
believe that this is going to be a highly reliable experiment. This
may not be the case when we get errors from places and so to get an
even better result we should look at the results and take out some of
the anomalous results.

An example of an anomalous result is that I had got a lower heart beat
rate when using a 0.1% concentration of caffeine and if my
prediction is to be right this cannot be. I then have to blame this on
systematic errors. As I have mentioned before we may have to see
different daphnia for different concentration this by itself will
produce a very high % error as we can only assume that the heart beat
for every daphnia will be the same but in reality it is not. This can
also be put down to just clumsy result taking. As we had left this one
till last it is clear that we could not really be bothered to be as
accurate as we had been with the other ones. Using daphnia is not same as using a human heart as the daphnia are easily stressed which mean they will have a higher heart beat rate compared to the human heart. Another example is that
the room may have been at different temperatures at different times
and therefore this is an error as we know that the temperature will
also make a difference to the heart beat.

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