My hypothesis is that with an increase in temperature a Daphnia's heart rate willincrease.

HEFC BIOLOGICAL SCIENCE - UNIT 2 Joseph Colledge

ASSESSED PRACTICAL

VARIATION OF DAPHNIA HEART BEAT WITH TEMPERATURE

My hypothesis is that with an increase in temperature a Daphnia’s heart rate will

increase, likewise with a reduction in temperature the heart rate will decrease. This is due to the Daphnia being an ectothermic organism, so its body heat is determined by its environment, namely that their body temperature is the same as the water they are floating in. Thereby having a direct effect on its metabolic rate.

A Daphnia was taken via a teat pipette from a beaker of room temperature water

(Measured at 20°C) and placed onto a cavity slide, this was then placed under a microscope. The heart was then located via the use of diagrams and confirmation sought by a person who had previously carried out the experiment. It was then decided that to get the most accurate results a group of 2 people would be most efficient, one person would be responsible for counting the beats per minute whilst the other would govern the time. It was also decided that as the heart was beating so quickly it would be wise to have a pencil and just tap a piece of paper each time the heart beats for 20 seconds, then multiply the result by 3, these were timed with a stopwatch. As there was the possibility of slight differences in the readings of bpm, the heartbeats were recorded 5 times at each temperature, an average calculated and that was the figure used.

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A beaker was then half filled with warmer water, the temperature measured with a thermometer. The Daphnia was taken once more with a teat pipette, into test tube of water that was placed into the beaker. It was decided that the Daphnia would be left for 5 minutes in the new temperature water to adjust, which was timed with a stopwatch. After 5 minutes, once again the Daphnia was taken (via the teat pipette) back to the cavity slide, but this time in an effort to maintain the temperature the slide was placed into a petri dish. The cavity slide this time was held slightly aloft in the petri dish on two balls of plasticine, then the water used to heat the Daphnia was poured into the petri dish up to the point were it was touching the base of the cavity slide. The timing of the heartbeat was once again repeated as in the first occasion. After once again getting the 5 times of the bpm the temperature in the beaker was reduced, firstly by using cold tap water, then in subsequent experiments ice was used to further lower the temperature. Each time the thermometer was used to record the temperature and each time the petri dish was filled with the new water samples to maintain the temperature. This was to be carried out at 5 different temperatures and the average bpm results documented.

Table to show how temperature effects the heart rate of a Daphnia

Joseph Colledge

The results show that due to the Daphnia being Ectothermic (cold-blooded) and

therefore not able to thermo-regulate itself, the temperature of the water in which it habituates has a direct effect on its heart rate. The reason for this is that the chemical reactions that occur in the Daphnia are dependent on certain enzymes, or proteins to help them proceed. As the temperature of the water is increased, the metabolism of the Daphnia increases also; this is due to chemical reactions occurring faster at higher temperatures due to an increase in kinetic energy. As a consequence of this increase, the heart has to beat faster as more Carbon Dioxide is produced from the cells during respiration. This can cause acidic conditions which will eventually denature enzymes, so more oxygen needs to circulated which is why the heart speeds up.

The results also show that with the decrease in temperature less cell respiration is occurring and so the heart beats at a slower pace. This is due to the metabolism of the Daphnia slowing down as the kinetic energy reduces. Also the enzymes are less effective at the lower temperatures. The movements of the Daphnia were also seen to slow down compared to when the temperature was higher, the Daphnia appearing sluggish compared to earlier, as with a slower metabolism its is not as efficient at producing energy so its energy levels were not as high.

There are a number of possible sources of error in the experiment. The first is the

most obvious and that is the method in which the heart rate is measured. As the graph and table show, the amount of beats per minute is very great and so fast that it is near impossible to register each one, as the human reflexes just are not quick enough. Another is the side effect of using a lamp as a light source for the microscope and that is heat. The hypothesis of the experiment was to test if temperature affects heart rate, and thus the Daphnia’s heart rate is going to increase the longer the amount of time it spends on the microscope. And due to the small amount of water which the Daphnia was placed in on the cavity slide it would take a very small amount of time for the temperature to rise even a few degrees. Another thing which could not be accounted for in the experiment was the age of the Daphnia, the metabolic rate will vary greatly depending on the age and size, the sex of the Daphnia also could not be clearly determined. It would also greatly change the amount of bpm if the Daphnia were pregnant and that was very hard to determine. Another would be the amount of stress, which the Daphnia is placed under during the experiment. The final error is the fact that in some cases the Daphnia being used would die and another used to carry out the remainder of the experiment, which would invalidate previous heart rates.

f) To improve the experiment I would suggest that natural light was used for the microscopes rather than lamps to reduce heat. Another way to improve it would be to film the beats of the heart with a video camera then play them back in slow motion to get a more accurate count. Getting a person who is experienced with them to say whether it’s a male/female would give more accurate results also. Also if a Daphnia dies during the experiment it would be more accurate to just start again rather than continuing with the same results from the previous one.