Biology in Daphnia Essay Example For Students

Biology in Daphnia Essay 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 th e 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 Ultimate Revenge from Medea EssayIf 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