Experiment. Hypotheses: The higher the concentration of caffeine the higher the heart rate of the daphnia.

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The effect of caffeine concentration on daphnia’s heart rate

Hypotheses: The higher the concentration of caffeine the higher the heart rate of the daphnia.

 Biological information: caffeine speeds heart rate, and circulation. Caffeine is a stimulant drug, which causes increased amounts of stimulatory neurotransmitters to be released. It belongs to a Group of chemicals called methylxanthines. Caffeine and similar compounds also inhibit a class of enzymes known as cyclic nucleotide Phosphodiesterases. These enzymes are, in part responsible for degrading a stimulatory signal produced when excitatory neurotransmitters activate different neurons in the central nervous system (CNS). When they are inhibited by caffeine, the stimulatory signal remains active for a longer period of time resulting in a greater sense of alertness (a CNS effect).


Independent variable: concentration of caffeine 0- 0.5%

 Dependant variable: this is the heart rate (BPM)

 Controls: 0% caffeine to check that water alone does not have an effect on the heart rate compared to pond water.

 Other fixed/controlled variables: The temperature must always stay constant this means that it must be fixed at room temperature. This can be done by removing the light source from the microscope when not counting because it increases the temperature which leads to an increase in metabolism and therefore an increase in the heart rate.

Another variable is the size of the daphnia, this means you should always try to choose large animals of similar size this is done using the same daphnia for each of the different concentrations throughout the experiment, it is important because the size of the daphnia effects the heart rate. Larger animals have slower heart rate.                        Another reason for this could be that if you are using the same daphnia the results that you get can be said to be the effect of caffeine rather than other factors.                                       The time for acclimatisation to test solution must be fixed ate two minutes, so you would have to use a stop watch for accuracy because it gives the daphnia time to climatise to new caffeine environment.                                                                      The time for counting the heart rate for the daphnia should be no more than 30 seconds; this should also be counted by a stop watch. This is because errors are introduced when time is too short or long.                                                                                                       The oxygen concentration of test solution must be maximised as far as possible, this can be done by using the same solvent (pond water) and using as much solution as possible to fill the cavity on the slide without a cover slip. Take care not to wet the objective lens this is an essential part of the experiment because if oxygen levels decrease the heart rate increase.                                                                             Finally, the last variable the test solution of the pH, the pH of the pond water has to be kept neutral. To test the pH of the pond water you can use a variety of indicators. You can use universal indicator or litmus paper and limewater. The PH has to be neutral so that the enzymes will not denature at a low or high pH and this too will effect the rate of metabolism and the heart rate.

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 Reliability: To make this experiment a fair test the same daphnia should be used for all of the different caffeine concentrations. Also I will repeat the experiment to accurate results, and do blind counting; which is when the person counting the beats should not know which concentration of caffeine has been used so that it will not be biased.

 Ethical issues: to ensure that the daphnia is not harmed we need to cut off the end of the pipette so that it will nor be harmed when it is squeezed in. also that water flea should be returned back in ...

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