An Investigation to Show the Effect of Temperature on Daphnia

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An Investigation to Show the Effect of Temperature on Daphnia


I will submerge some daphnia contained in a test tube in water of
differing temperatures and measure their subsequent heart rates.


I think that the heart rate of the daphnia will increase up until
around 40ËšC at which point most of the daphnia's enzymes will have
denatured and rate of metabolism will have stopped or decreased
sufficiently to have stopped the daphnia's heart rate.

Apparatus list

Test tube


Different temperatures of water



Stop clock




Follow usual lab safety rules. There are no major safety
considerations in this experiment as the daphnia aren't harmful and
water above 40ËšC will not be used.


To determine if temperature does have an effect on the daphnia, I
intend to carry out the following experiment. The experiment will
involve measuring the cardiac activity of the daphnia at different
temperatures, ranging from very cold (approx. 5 degrees Celsius) to
quite warm (approx. 40 degrees Celsius).

· A selection of similar sized/age Daphnia will be taken.

· A variety of different temperatures of water will be set up, these
temperatures will be kept constant whilst the daphnia are submersed in
them. The temperatures will be set up by using ice to cool tap water
down to lower temperatures and boiling water to heat it up. The
temperature will be measured to within 0.1°C on a mercury thermometer
to ensure accuracy. The temperatures will range from 10°C to
approximately 40°C and it will be done at 5°C intervals.

· The daphnia will be submersed in the water and left to equilibrate
for 3 minutes, after this time one of the Daphnia will be removed and
put onto a microscope slide, this will then be quickly put under the


· Under the microscope, the Daphnia will be observed for 20 seconds,
this heart rate will be counted and recorded, this can the be

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"An Investigation to Show the Effect of Temperature on Daphnia." 25 Jun 2018
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multiplied by 3 to give a beats per minute. This way, the beats
counted will be at as close temperature as possible, as they won't
have time to cool / warm significantly.

· The experiment will be repeated twice for each temperature.

Dependant variable:

It is important to ensure that all variables are kept constant,
allowing the experiment to be a fair one, to do this the following
precautions should be taken:

· The Daphnia used should be of a similar size and age to ensure that
they react in the same way to the different temperatures.

· The heartbeat should be counted as soon as possible after the
Daphnia is placed under the microscope to ensure that the temperature
it has equilibrated to remains as close as possible and is not
affected by the light source of the microscope.

· All of the Daphnia should be allowed the same amount of time to
equilibrate to the temperatures, thus they all experience the same
conditions and this makes the experiment more accurate.

· The same person should always count the heartbeats to ensure that
judgement errors do not affect the overall results.

· Each daphnia should have the same transfer time from water to
microscope. (If time exceeds 15 seconds, void that part of experiment
and repeat.)

· The temperature of the water needs to be monitored constantly to
ensure that the results are being recorded at consistent temperatures.

Independent variable

· The temperature of the daphnia will be changed from 10˚C to 40˚C
with 5ËšC increments. This will be done by allowing the daphnia to
acclimatise to different temperatures by putting their container in
different temperatures of water. The results for experiment will each
be repeated three times.

· The heart rate of the daphnia will be counted by looking at them
under a microscope with 100x magnification and taking heart beat for
20 seconds and multiplying this figure by 3 to give beats per minute.




Heart rate of daphnia in beats per minute

In ºC








































The underlined result for 30º C is clearly anomalous and therefore has
not been included in the mean average beats per minute for that

Interpretation of results

The graph shows a steep increase in heart rate as the temperature goes
up, and from the graph, this looks linearly proportional. This is from
10ºC to 35ºC as the mean BPM goes from 72.7 to 129.3. The graph peaks
at 35ºC where BPM is 129.3. At 40ºC all the heart rates fell to 0.

Heart rate, along with most metabolisms in a living organism, is
controlled by the action of enzymes. The hearts pacemaker,
(sino-atrial node (SAN), also controls the Daphnia's heart rate. The
pacemaker sends out an electrical signal across the heart that makes
it contract. Hormones and transmitters control the rate set by the
pacemaker. The hormones communicate with the pacemaker in the
membrane. The transmitter substance and hormone fit onto a protein
molecule of the cell membrane, this causes the pacemaker to react. The
theory is very similar to that of enzymes, more heat produces more
kinetic energy and thus the hormones move more rapidly, this increases
the chance that it will collide with the protein molecule on the cell
membrane. Temperature, pH, enzyme concentration, substrate
concentration and inhibitors affect enzymes. For a non-enzymic
controlled reaction, the general rule is the higher the temperature,
the faster the reaction. The same rule is true for a reaction
catalysed by an enzyme, but only up to about 40ËšC. This is shown
between 10 and 35ºC. At 35ºC, the heart rate is at its fastest as this
is when the enzymes are working at their fastest due to the large
amount of kinetic energy the substrates have. Above 35ºC, the enzyme
molecules begin to vibrate so violently that the delicate bonds that
maintain tertiary and quaternary structure are broken, irreversibly
changing the shape of the molecule. When this happens, the active site
shape has changed and therefore an enzyme-substrate complex is no
longer possible. This means that the enzymes and other protein
structures controlling heart rate can no longer work. We then say it
is denatured. This is shown at 40ºC. Although not all of the enzymes
were denatured right after 35ºC, enough will have been denatured to
slow down to process sufficiently to stop almost all metabolisms.

Evaluation of practical work

The experiment was carried out as accurately and safely as possible.
Under school laboratory conditions and with the equipment available
the experiment was flawed. There were problems in several aspects of
the experiment; firstly the daphnia's


heart rate is very fast most of the time, this gives the person
observing a distinct problem

in counting. We decided to count the heart rate for 20 seconds and
then multiply that total by 3 to give beats per minute. This of course
solved the problem of getting into very high unmanageable numbers that
the observer would find difficult to cope with in a very short space
of time. Unfortunately this in itself presented more problems. In a
very small amount of time which twenty seconds is, the percentage
error will be much greater than it would be overall in a much larger
time frame as each error would be multiplied by three. If I was to
repeat this experiment I would try a slightly larger time frame such
as 30 seconds, this would reduces the percentage error whilst still
keeping the counting to manageable figures.

Another area in which error could have been introduced was using the
microscope. The microscope requires a light source to function.
Unfortunately the light source is also a source of heat, and whilst
the daphnia are under it, the temperature they have been equilibrated
to will rise, particularly the daphnia which were at very low
temperatures will feel the effect of the light source. This problem
was however minimised as the daphnia were only exposed to the
microscope light for the twenty seconds need for counting and the five
seconds needed to position the daphnia under then lens - the
microscope was focused correctly before commencing the experiment.

The temperature of the water used was kept constant by topping up with
hot / cold water, but when the daphnia were removed from the test
tube, the ones at higher temperatures would have cooled faster than
those at lower temperatures (above room temperature.) as there was a
larger difference between them and their surroundings. Also, the
stress of the changes of temperature and surrounding could have
increased heart rate but this could not be avoided, as their heart
rates could not have been taken in their natural environment as they
would not normally be subjected to these temperatures.

There was one clear anomalous result which was in experiment 3 at
30ºC. This result was higher than the others by at least 12 BPM and
was equal to one of the BPM for the next temperature up.

Despite the difficulties in this experiment, the overall results I
gained were to a good degree of accuracy and were sufficiently
conclusive to my original hypothesis.

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