Physiology of the Circulatory System - Part 2

Heart Rate and Temperature Study in Daphnia Magna

Hypothesis: If a Daphnia is placed at different temperatures, then its heart rate will either decrease or increase.

Materials:

• Dissecting Microscopes
• Thermometers
• Timer
• Petroleum Jelly
• Triangular File
• Alcohol Swab
• Towel

Procedures

1. Using a large bore plastic transfer pipet, pipet some liquid containing Daphnia from the container your teacher has provided.
2. Hold a 9” glass Pasteur pipet over a petri plate or jar containing the Daphnia culture. Tilt the Pasteur pipet and pipet the Daphnia and liquid into the end with the larger opening. Allow the solution and the Daphnia to flow down into the narrow end of the pipet. The Daphnia should become stuck in the pipet.
3. If needed, use a paper towel to blot out the excess solution from the narrow pipet end until the Daphnia is lodged in the narrow of the Pasteur pipet. Keep the solution just about 1 cm above the Daphnia.
4. To fit into a petri plate, you will have to shorten the pipet a few inches.
5. Score (scratch) the pipet with a triangular file one inch above the point where the Daphnia has become stuck.
6. To protect your hands, wrap a paper towel around the pipet and break it where you have placed the score mark.
7. Seal the bottom, narrow end of the pipet by pushing it slightly into petroleum jelly.
1. Add more Daphnia culture liquid to the pipet so that there is approximately 0.5 inches of liquid above the Daphnia.
2. Seal the other open end of the pipet with petroleum jelly.
8. Place the pipet containing the Daphnia into a petri plate containing 20 ml of water, which is at the same temperature as the Daphnia culture liquid. Record the temperature.
9. Place the petri plate under a dissecting microscope and locate the Daphnia’s Heart. Count the number of heartbeats for 10 seconds. Multiply by 6 to determine the number of beats per minute.

10. Now place the pipet containing the Daphnia into a petri plate containing 20 ml water at approximately 5oC. Measure the temperature and determine the heart rate beats per minute.
11. Record the data for temperature and heartbeats per minute in Table 10.
12. Carefully add warm water to the dish to raise the temperature approximately 5o C. Record the actual temperature and then measure the heart beats per minute.
13. Continue adding additional warm water and record the temperature and heart rate every 5oC.
14. Repeat the procedure until you are unable to measure the heartbeat accurately. Keep recording the data until you stop.
15. Plot the heart rate in beats per minute versus the temperature on linear graph paper. Label the X-axis temperature. Label the Y-axis heart beats per minute.
16. Determine the Q10 for your Daphnia using the following equation:

In this lab, Q10 = 2.2

Table 10: Temperature and Heart Rate
Temperature in Degrees Celsius	Heart Rate
25	72
5	30
10	84
15	132
20	180
25	132

Justification:  This table shows the heart rate of the daphnia at different temperatures. It seems like as the temperature increases, the heart rate does too. Since we did need to use two different daphnias to record the results, the fist two temperature measurements do not fit with the rest of the results. 

Justification: This graph shows the heart rate of a Daphnia in different temperatures. It shows the results of the experiment that tested the hypothesis, “If a Daphnia is placed in different temperatures, then its heart rate will either decrease or increase.” The independent variable (the x-axis) would be the temperature and the dependent variable (the y-axis) would be the heart beats per minute.

Analysis Questions:

1. Why does the Daphnia’s heart slow down at lower temperatures?
1. The Daphnia’s heart rate slows down at lower temperatures because they are cold-blooded. Their body temperature becomes the same as the temperature in their environment. If the environment had a lower temperature, then the Daphnia’s metabolism would also be low which would slow down its heart rate.
2. Why does the heart rate of endothermic, warm-blooded organisms, such as man, remain relatively unaffected by changes in the environmental temperature?
1. The heart rate of endothermic organisms remain unaffected by changes in environmental temperature because they regulate their body temperature in order to maintain homeostasis. When it is cold, endothermic organisms keep their body heat insulated in their body. When we put on a sweater, it isn’t to keep out the cold, it is to keep heat from escaping our bodies. Unlike cold-blooded organisms, warm-blooded organisms do not have fluctuating body temperatures; they have a single body temperature to maintain.
3. How do ectothermic, cold-blooded organisms, such as the Daphnia regulate their temperature? Can you give examples from nature?

Ectothermic organisms regulate their temperature by taking the temperature of their surrounding environment. This means that if it is 20 degrees celsius, the organisms temperature will be 20 degrees celsius. If a lizard wants to increase its metabolism, it will bask in the sun to increase its temperature. When it gets too hot, it goes in the shade and it cools down.

In conclusion, the Daphnia's heart rate is lower at colder temperatures and is higher at warmer temperatures. This proved my hypothesis, "If a Daphnia is placed at different temperatures, then its heart rate will either decrease or increase" was correct. Since Daphnia are cold blooded, they take the temperature of the environment they are in which affects their metabolism. An error that could have occurred was if we counted the heart beats incorrectly. Also, the data we collected does not match well because we had to use to different Daphnia. If this experiment were to be performed again, one thing that could change would be the type of organism that was used.