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. 53–255, 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 5/1/08 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 skills aspect of this activity. Experiment 1 Daphnia were cooled on ice before the experiment. Beakers containing Daphnia in pond water were put on ice for about half an hour. This had the effect of slowing the heart rate and thus facilitating counting.
The temperature of the pond water in which the Daphnia were swimming fell to about 5 oC. A single Daphnia was placed in a beaker containing test solution for 5 minutes: either pond water or pond water + 0. 5% caffeine. After the 5 minutes, the Daphnia (in a few drops of test solution) was transferred to the slide for measurement of heart rate. Each individual was counted for 4 _ 30 seconds. A blind counting method was used. Overall means: Caffeine – 173 beats per minute Control – 172 beats per minute There were no immediately observable ill effects of caffeine at this concentration (0. 5% w/v). Experiment 2
The experiment was conducted at room temperature. A single Daphnia was transferred to the slide. A paper towel was used to remove the pond water from the slide. A few drops of test solution – either pond water or pond water + 0. 5% caffeine – were dripped onto the Daphnia. The clock was started immediately and the heart rate recorded for 15 seconds at 2, 4, 6, 8 and 10 minutes. An ICam was used to facilitate counting. The results from the two experiments do not show that caffeine increases heart rate in Daphnia. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group. Teachers Guide. qxd 25/1/08 8:00 Page 3 Teacher/Lecturer A1. 21L CORE Activity 1. 21 Does caffeine affect heart rate? Table 1 Results for Experiment 1 Treatment Heart rate/beats 30 s-1 Trial 1 Trial 2 85 84 86 91 87 70 101 75 98 91 Trial 3 81 78 86 93 90 73 74 98 85 Trial 4 88 82 84 99 90 74 85 100 94 Mean 85 82 85 93 88 70 91 81 97 90 Mean for individual Beats min-1 Caffeine Caffeine Caffeine Caffeine Caffeine Control Control Control Control Control 87 83 84 89 86 62 81 88 93 89 170 164 170 186 176 140 182 162 194 180 Table 1 Results for Experiment 1 Source: Hardie and Tranter (1983) Biology Practicals: Teacher’s Guide
Number of heartbeats in 15 seconds Length of time in solution/minutes Treatment Caffeine Caffeine Caffeine Caffeine Mean Control Control 2 49 54 54 59 54 57 58 4 53 54 59 65 58 55 60 6 49 57 55 64 56 59 54 8 44 51 61 68 56 50 58 10 48 55 64 62 57 58 60 3 Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group. Teachers Guide. qxd 25/1/08 8:00 Page 4 Technician 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). This is an activity that students may plan themselves.
There is also a support sheet giving a suggested method. The list below gives the apparatus and approximate quantities needed for the suggested method. However, students may ask for things outside of this list. Requirements per student or group of students Culture of Daphnia Notes • Daphnia may be killed by chlorine in tap water. Any tap water used should be left to stand for 24 hours first to let the chlorine dissipate. • Allow at least nine Daphnia per student or pair. They should be unharmed by the activity, but may need too long to recover after the caffeine to use the same one for different concentrations in the same practical.
They can be bought from an aquarist or Blades Biological Supplies. • To catch them use a plastic pipette with the tip cut off to make it wider. Goldfish enjoy disposing of Daphnia. • This will stop them getting mixed up with the ‘fresh’ ones. • Students can wash and reuse the slides if needed. Container for ‘used’ Daphnia 6 cavity slides Approximately 5 (0. 5%) cm3 caffeine solution • • • • Dissolve 0. 5 g caffeine in 100 cm3 water. Instant coffee can be used. Make up the solution with culture water or distilled water. Students will need to dilute this to make weaker solutions.
If time is limited it may be easier to provide made-up solutions of 0. 25%. (Consult with the teacher/lecturer – they may prefer to use more concentrations, e. g. 0. 1%, 0. 2%, 0. 3%, 0. 4% if time permits. ) Cotton wool • Students will only need a small amount – about as much as is in a single cotton wool ball will supply the whole class. They put a few strands on the slide to stop the Daphnia moving. 3 dropping pipettes 5–10 cm3 distilled water or water from the Daphnia culture Test tubes/small beakers Small measuring cylinder or 5 cm3 syringe Approximately 100 pond water cm3 distilled or • For adding to the Daphnia during the experiment. For students to collect or mix their caffeine solutions in. They will need more if they are doing more concentrations. • For making dilutions of caffeine solution. • For making dilutions of caffeine solution. Amount required will depend on the dilutions made Stopclock Paper towels or filter paper Microscope Stroboscope • For use at low power. Some centres find that it is useful, if available, to use a microscope-mounted video camera to show students the heart. • If this is available, it can be used to get a more accurate measure of how fast the heart is beating, but is not recommended. Safety Important! Stroboscopes can trigger epileptic fits in some people. Anyone known to be susceptible should be warned. Background lighting should be retained and exposure times minimised. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group. 4 Teachers Guide. qxd 25/1/08 8:00 Page 5 Student 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). Safety
If a stroboscope is used to show the Daphnia’s heart rate and you know you suffer from photosensitive epilepsy tell your teacher and take appropriate precautions. Caffeine Plants produce caffeine as an insecticide. Cocoa in South America, coffee in Africa and tea in Asia have all been used for hundreds of years to produce ‘pick me up’ drinks containing caffeine. These days, caffeine is also used as a flavour enhancer in a wide range of cola and other soft drinks. In addition, it has medicinal uses in aspirin preparations, and is found in weight-loss drugs and as a stimulant in students’ exam-time favourites like Pro-plus and Red Bull.
At high levels of consumption caffeine has been linked to restlessness, insomnia and anxiety, causing raised stress and blood pressure. This can lead to heart and circulation problems. Procedure Making a hypothesis What do you think will be the effect of caffeine on the heart rate of water fleas? Write down your ideas and support your prediction by presenting biological knowledge which supports the idea. You now have an idea (hypothesis) to test. Planning The beating heart of a water flea can be seen through its translucent body, by placing the flea in a few drops of water in a cavity slide.
A cover slip helps stop the water evaporating. The following equipment will be available: • • • • • Culture of Daphnia (water fleas) Cavity slides Dropping pipettes Distilled water Caffeine tablets • • • • • Cotton wool Standard glassware (beakers, measuring cylinders, etc. ) Stopclock Paper towels or filter paper Microscope Produce a detailed plan for an experiment that allows you to test your hypothesis about the effect of caffeine on the heart rate of water fleas. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group. 5 Teachers Guide. qxd 25/1/08 8:00
Page 6 Student A1. 21L CORE Activity 1. 21 Does caffeine affect heart rate? In your plan, make sure you include the following: • Select suitable apparatus that will give you measurements which will validly test your hypothesis. Explain why the apparatus is suitable and how the results will let you test the hypothesis. Include a risk assessment, identifying any risks and explaining any safety precautions that need to be taken so as to reduce those risks. Comment on the ethical issues that arise from using living organisms in the experiment and explain how these will be taken into account in the practical method used.
Identify the dependent and independent variables, and indicate how relevant variables will be controlled. Show how you will ensure that reliable and valid results are produced, and describe what you will do to ensure that measurements are precise, accurate and repeatable. Identify any potential errors in readings that can be systematic (values differing from the true value by the same amount) or random (values equally likely to lie above or below the true value). • • • • • Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group. 6