The effect of caffeine concentration on the heart rate of Daphnia.

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The effect of caffeine concentration on the heart rate of Daphnia

Aim

To conduct an experiment into the relationship between the heart rate of a Daphnia and its exposure to various concentrations of caffeine.

Method

For this experiment we had:

Culture of Daphnia in various concentrations (%) of caffeine (0.0, 0.0025, 0.005, 0.025, 0.05, 0.25, 0.5)

Two cavity slides

A pipette

Stop watch

Microscope

Ice water

Firstly, a job was designated to each member of the group and each had a counter, a recorder and a controller. The controller always set up each slide to ensure that the other members of the group were unaware of the concentration of caffeine that the Daphnia were exposed to, thereby eradicating bias, and also controlled the time. The counter would count the number of heart beats in ten second intervals by stabbing at a piece of paper with a pen, in an 'S' shape. This allowed for easy counting and a control on human error. The recorder counted the dots on the paper to ensure that an extra heart beat was not added here and there in order to make the results match their prediction.

A microscope was then set up and the x10 lens was used to ensure that the Daphnia's heart could be seen clearly. The slide was then set up. Some ice water was then placed on one cavity and the second cavity slide placed on top of the first. This was to help with the regulation of temperature. Placing ice water below the Daphnia helped to counteract the effects of the microscope light in heating up the slide thereby helping to validify our results. One Daphnia (from one of the concentrations) was then placed on the slide in the cavity and positioned under the microscope. If the Daphnia moved around too much some of the caffeine solution was taken off the slide to limit the amount of liquid it had to swim in. The heart beats were then counted in ten second stints. This was felt to be a suitable measurement as this meant that the number of heart beats could be multiplied by six to obtain the heart beat per minute. It also allowed for the fact that the hand and the eye of the counter could not accurately count the heart beats for more time as it was too fast in many cases. A way to improve accuracy in the counting of heart beats would be to film it or use a Stroboscope if one were available. Once a particular Daphnia had had three readings taken from it a change was made to another Daphnia and the ice water replaced. Three readings per Daphnia were taken and five Daphnia per concentration were used. This served two purposes. One, the Daphnia did not succumb to any effects that the heat from the light may have caused and it that it did not suffer any effects of being taken out of the liquid for too long. Secondly, replicates were taken to increase reliability, the more replicates taken, the more reliable the experiment was. Also the more replicates taken the more chance there was to have of spotting a potential inaccuracy or anomaly in the results.
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N.B. The Independent variable is the concentration of the caffeine and the dependent variable is the heart rate.

A Table to show the Relationship between the Caffeine Concentration and the Average Heart Rate of a Daphnia

Concentration of caffeine / %

Number of heart beats of Daphnia per 10 secsonds

Mean number heart rate per a minute

0.0

56

47

44

56

57

50

65

55

312

59

52

49

46

51

46

46

0.0025

...

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