Biology in Daphnia

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Teachers Guide.qxd

25/1/08

8:00

Page 1

Teacher/Lecturer A1.21L CORE

Activity 1.21 Does caffeine affect heart rate?
Purpose
• • To investigate the effect of caffeine on the heart rate of Daphnia (water fleas). To highlight experimental and investigative assessment objectives.

Safety
Any student known to suffer from photosensitive epilepsy should be warned before a stroboscopic light is used. Flickering lights in the range 5–30 Hz are thought to trigger seizures in sensitive individuals.

Notes on the procedure
The student sheet that accompanies this activity guides students through planning and writing up this investigation. After students plan the investigation their plans can then be discussed with the group. To support less able students, or to help with the organisation of practical work for a large group, students could be given the practical procedure at the end of these notes (see page 3). It provides a basic outline and will need to be read by students before starting the practical with decisions and modifications made as appropriate. The procedure is for a simple experiment; more able students could complete a more complex experiment using serial dilutions, with several Daphnia used at each concentration. Note that high concentrations of caffeine can be fatal for Daphnia. The Daphnia hearts are fairly easily seen but counting the number of beats can be difficult. Counting is easier if each heartbeat is recorded by tapping a pencil on a piece of paper and counting up the pencil marks after the specified time. In addition, cooling the Daphnia before the experiment may help slow their heart rate. An ICam above the eye-piece of the microscope to project an image of the slide onto a large screen may also help with counting. Foster, in the Journal of Biological Education (1997) 31, pp. 253–255, provides a method using a stroboscope to freeze the motion. Use of the stroboscope may overcome the problems of counting faster heart rates. However, we would not recommend this method. Positioning the light sources and strobe is tricky. It is very difficult to freeze the motion and viewing with a strobe light can cause eye strain and dizziness. A simpler approach is to count the rate at which the legs beat. This rate is proportional to the rate at which the heart beats. A dissecting microscope with a light source under the stage works well for this experiment. To prevent the Daphnia from overheating while on the microscope turn off the microscope light between observations and use a heat sink – a cavity tile filled with iced water placed on the microscope under the slide. Using pond water/Daphnia culture solution is recommended for both the control group and to dissolve the caffeine as this may give more valid results and be less stressful to the Daphnia. In distilled water the heart rate may rise due to lack of oxygen.

Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

1

Teachers Guide.qxd

25/1/08

8:00

Page 2

Teacher/Lecturer A1.21L CORE

Activity 1.21 Does caffeine affect heart rate?
In the trial of the experiment caffeine was used at 0.1% and 0.5% w/v with no ill effect. At 1% the Daphnia stopped swimming after 5 minutes. It is suggested that a ‘blind’ study is done. This means that the person counting the heart rate is unaware as to whether the Daphnia is in water with or without caffeine. It has been shown that observer expectations influence the result. It is difficult to get clear-cut results from this experiment and significant differences between treatments may not be found. The sets of results below indicate the sorts of results that can be expected. It should be impressed upon students that it does not matter if they do not get differences between treatments. Indeed, the experiment provides plenty of opportunity to evaluate critically the technique used and really focus attention on the experimental...
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