Investigating the Effect of Caffeine on the Heart Rate of Daphnia Daphnia are extremely small aquatic invertebrates. They are transparent, so you are able to see their internal organs; this means it is easy to monitor their heart rate by observing them through a microscope . Physiological results from caffeine consumption include the stimulation of heart rate, blood pressure, metabolism, body temperature and alertness. 2] In this practical investigation the aim was to determine whether the hypothesis that caffeine increases the heart rate of Daphnia, was true or false or whether caffeine decreased the heart rate of Daphnia. To determine this you need to test this hypothesis practically; in order to do this you have to make up a range of caffeine solutions of different concentrations; this is the independent variable – the thing that you change during the investigation.
You also need a control variable, which would be a solution which has no caffeine in at all, to compare the results from the caffeine solutions. In addition to this, you need a dependent variable; this is what you measure in the experiment and what is affected during the experiment, in the case of this investigation it would be the Daphnia’s heart rate which you would measure, and is affected by the independent variable – the concentration of caffeine solution.
In all practical investigations you need to take into account any hazards (situations that pose a level of threat to life, health, property, or environment)  that you may encounter: Hazard| Outcome of hazard if it is realised| How to control/reduce hazard| Breakage of cavity slide| Less risk of the cavity slide breaking – less chance of getting cut by glass| Handle slide very carefully, do not squash with microscope lenses| Spillage of water/solutions| Less of a chance of falling over due to spillages| Place water/solutions away from edge of desk, and clear desks of unnecessary things to avoid knocking them over| Glass (beaker) smashing| Less risk of getting cut by broken glass| Don’t pick up glass with wet hands, place glass away from edge of desk| When you are dealing with live organisms you need to take into consideration the ethical issues involved. Using Daphnia in the investigation allows us to study the effect of caffeine, as it would be deemed unethical to study this effect on humans. This is because Daphnia are invertebrates, rather than vertebrates; they are considered to be simpler organisms than vertebrates due to them having a much less sophisticated nervous system, which could mean that they feel less/no pain, they are also more distantly relation to humans than vertebrates are.
Though many people still believe that it is unethical to carry out experiments on invertebrates as they cannot give their consent to be experimented on, and they believe that it is unethical to cause distress or suffering to any living organism.  To ensure that the Daphnia’s well-being was ensured during the investigation, they were only subjected to the caffeine solution for a very short period of time (about 30-60 seconds), then after this they were placed back into fresh water so their heart rate could return to normal. The Daphnia were not left under the light of the microscope for longer than around 30 seconds, as the light from the light microspore could raise their body temperature to dangerous levels, which could kill them. They were also only subjected to caffeine solutions that were not strong enough to kill them.
During all practical investigations you must use the apparatus skilfully and safely; in the investigation this was achieved by adjusting the microscope in the correct fashion in order to see the heart of the Daphnia clearly, without squashing the Daphnia with the lenses. As well as this, the Daphnia were placed into a very small volume of caffeine solution, to prevent them from being washed off the cavity slide. The apparatus used for this investigation included: * A cavity slide * A stop-clock * A light microscope * Daphnia * Caffeine solutions of different concentrations * Water (control solution) * A pipette * Cotton wool * Pen * Paper * Beakers Method: 1. Get caffeine solutions of concentrations of 0. 0%, 0. 25%, and 0. 5%. 2. Set up the light microscope. 3. Get one Daphnia from its tank by sucking it up with a pipette, ensuring it has a sufficient supply of water. 4.
Put cotton wool around the concave side of the dip in the cavity slide. 5. Transfer the Daphnia into this dip, ensuring there is ensure water transferred as well to allow it to live. 6. Place the cavity slide onto the microscope stage and hold it in position using the stage clips. 7. Focus the microscope by adjusting the coarse and fine focusing knobs until you can clearly see the image of Daphnia. 8. Adjust the cavity slide until the heart of the Daphnia can be seen clearly. 9. One person must observe the Daphnia using the microscope and count the Daphnia’s heart beat for a period 15 seconds and make dots on a piece of paper using a pen for every beat of the Daphnia’s heart.
Another person records the time by using a stop clock. Repeat this eight more times and find an average of the results. 10. Return the Daphnia to the beaker containing fresh water for one minute. 11. Repeat steps 4 to 9 but by adding caffeine solution of concentrations 0. 0%, 0. 25%and 0. 5% to the cavity slide. 12. Multiply each average value of heart beat of Daphnia by four to obtain the heart beat of Daphnia in unit of beats per minute.  Results: Concentration of| Number of heartbeats in 15 seconds| caffeine solution (%)| Test 1| Repeat 1| Repeat 2| Repeat 3| Repeat 4| Repeat 5| Repeat 6| Repeat 7| Average| 0. 00| 48| 44| 38| 42| 40| 37| 45| 44| 42| 0. 5| 73| 77| 80| 75| 71| 70| 75| 74| 74| 0. 50| 90| 86| 95| 91| 89| 84| 91| 96| 90| The results show a positive correlation; that is, when the concentration of caffeine solution is increased, the Daphnia’s heart rate also increases. This is due to caffeine being a stimulant, once absorbed into the bloodstream the caffeine can make its way to the heart, where it acts as a stimulant by binding, or attaching itself to, specific receptors on the heart tissues causing the heart rate to increase.  The heart rate of the Daphnia increases more from 0-0. 25% caffeine solution, than from 0. 25-0. 5% caffeine solution due to the heart beating too fast to allow accurate measurements.
Due to this trend in the data it is possible to accept the hypothesis that caffeine does in fact increase the heart rate of Daphnia, and the null hypothesis that heart rate decreases heart rate is false. Anomalies are values 10% greater than or less than the mean value, or values which do not follow the trend or pattern shown by the data. For the 0. 0% caffeine solution, the values of 48 and 37 were not within 10% of the mean, this is likely to be due to miscounting the Daphnia’s heart rate (human error). There were no anomalous results for the 0. 25% caffeine solution and the 0. 5% caffeine solution. The results were more than likely not very accurate due to having to count the heart beats by eye; this can easily be distorted by human error, which could mean that the results obtained are invalid.
The results are fairly reliable due to them all being relatively similar, the results are repeatable, other researchers would be able to follow this method, and should obtain similar results, they are also valid as there was only one independent variable and the experiment also included a control variable to compare the results against. To improve the accuracy of this method more accurate measuring equipment could be used, more repeats could be done. References  – CGP AS-Level Biology complete revision and practical (Edexcel exam board) pg. 16  – http://www. livestrong. com/article/321769-why-does-caffeine-raise-the-heart-rate/#ixzz284jZnaO3  – http://en. wikipedia. org/wiki/Hazard  – AS-Level Biology complete revision and practical (Edexcel exam board) pg. 17  – http://www. scribd. com/doc/29218203/Heart-Rate-of-Daphnia  – http://www. ehow. com/how-does_5185453_caffeine-raise-heart-rate_. html