Daphnia Write-Up

Daphnia Write Up

In this experiment we investigated the effects of caffeine on the heart rate of living organisms. We chose to use daphnia in this experiment, a kind of small organism found in water, similar to a prawn or insect, which has completely transparent skin, making it easy to see the heart, and thus count the rate of its heart beat. My prediction was that the caffeine would increase the heat rate of the daphnia.

Procedure

The procedure was not too complicated, but there was a lot to keep in mind, and to be careful about. Since we were working with living organisms, we had to be extremely careful with there lives, and take care not to harm and stress them.

We set up a microscope, and got a ball of cotton wool ready.
We set up the different concentrations of caffeine. 0%, 0.125%, 0.25% and 0.5%. We couldn’t use tap water, only distilled water, as tap water contains chlorine which can harm the daphnia. So 0% contained no caffeine, only distilled water.
We took two glass slides, and put a drop of chilled water in between them: this was so that the heat of the microscopes bulb did not harm the daphnia.
We captured a daphnia from the main tank which contained many; we tried to select the largest one possible. We caught it using a pipette, carefully drawing it into the pipette without harming it.
After we had caught it, we put it into the first solution of 0% caffeine. We left it there for about 15 seconds, before collecting it with the pipette again and then placing it on the glass slide. We then placed a thin strand of cotton wool onto the daphnia; this caused it to get tangled up, and stop the daphnia from moving about too much.
We then observed the daphnia under the microscope, and found the heart. Once we had found the heart, we used a counter to count how many times the daphnia’s heart beat in 15 seconds. At the end of the investigation we multiplied this number by four to get the heart rate per minute.
We repeated steps 5 & 6 for each concentration, and finished of by putting the daphnia back in the main tank.

Unfortunately, our results show no correlation, probably due to the daphnia getting stressed. Here is a table of our results.

And here is a graph showing the correlation. However, the graph is useful at all since there was no correlation.

Ethical issues

Any experiment that involves living organisms may be controversial. However in this experiment the chances of killing the daphnia accidentally is very small; also it will not cause any permanent damage as the effects of caffeine are temporary. The experiment will only use one daphnia, so even if death ...

This is a preview of the whole essay

Unfortunately, our results show no correlation, probably due to the daphnia getting stressed. Here is a table of our results.

And here is a graph showing the correlation. However, the graph is useful at all since there was no correlation.

Ethical issues

Do not use so much cotton wool that the daphnia cannot get oxygen.
Do not use a cover slip, as it will crush the daphnia.
If possible widen the tip of the pipette so the daphnia does not get stuck.
Use distilled water so the daphnia does not suffocate, do not use tap water as it contains chlorine which is harmful to daphnia.
Do not use pure caffeine solution; the maximum should be 0.5%.
Turn off the microscope when not in use, otherwise the heat will harm and kill the daphnia.

Variables

Independent: The concentration of the caffeine solution.

Dependant: The heart rate of the daphnia

Variables which may affect the result:

The concentration of the caffeine solution.
How long the daphnia was left in the solution.
The age of the daphnia.
The gender of the daphnia.
The genetic makeup of the daphnia.

Controlled Variables:

Concentration of caffeine
Daphnia (same daphnia on all trials)
Distilled water (no caffeine)
Time daphnia is given a rest

Improvements

To improve the experiment, I think instead of clicking the number of heart beats for 15 seconds. It would be more accurate if the view of the microscope was recorded to a computer, so it can be slowed down and counted more accurately.

In addition to that I think that the daphnia should not be immediately tested on after the last testing. It should be given time for it to relax such as 10 minutes. Otherwise the effects of the previous test will still be apparent.