Fig. 1.1 shows an aquatic crustacean belonging to the genus Daphnia. These animals usually measure between 1-5mm in length. They have a two-chambered heart which can be seen through the exoskeleton when viewed using a low-powered microscope. heart [Figure 1.1] A biology teacher told a class that caffeine can temporarily relieve tiredness in people by increasing heart rate. Table 1.1 shows the caffeine content of various products. Table 1.1 product,caffeine per serving/mg 1 cup of strong coffee,100 50 g dark chocolate,45 1 caffeine tablet,50 1 can of cola drink,20 One group of students in the class used Daphnia to test the hypothesis: Heart rate increases when caffeine concentration increases.

1. (a)Identify the independent and dependent variable in this investigation. independent dependent[2]
2. (b)(i)The group of students decided to use the caffeine tablets to make a 100mgdm-³ caffeine solution. Describe a procedure that the students should use to prepare the 100 mg dm³ caffeine solution.[2]
3. (b)(ii)A culture of healthy Daphnia was supplied to the students in a tank containing fresh water and a suitable food source. Each student followed the same basic procedure. • A Daphnia was placed on some cotton wool fibres in a cavity microscope slide as shown in Fig. 1.2. • A few drops of 100 mg dm-3 caffeine solution were immediately added to the slide. • The Daphnia was observed using the low power objective lens of a microscope. • The number of heart beats of the Daphnia was counted. a Daphnia on cotton wool fibres a few drops of 100 mg dm-3 caffeine solution cavity microscope slide [Figure 1.2] Suggest one reason why the Daphnia was placed on cotton wool fibres.[1]
4. (b)(iii)The students decided to adapt their basic procedure to test their hypothesis that: Heart rate increases when caffeine concentration increases. Describe a method which the students could use to test this hypothesis. Your method should be detailed enough for another person to follow.[6]
5. (b)(iv)Suggest one feature of the procedure which may have added a source of error and suggest how the students could have reduced its effect.[2]
6. (c)The data the students obtained showed that as the concentration of caffeine increased, the heart rate of the Daphnia also increased. The students concluded: To relieve tiredness, people should increase their caffeine intake so that their heart rate will increase. Suggest why the students' conclusion was not valid.[1]
7. (d)(i)Another group of students decided to use cola as their source of caffeine. They diluted the cola to give a range of caffeine concentrations. They counted the heart rates of Daphnia in each of these concentrations. Table 1.2 shows their results. Table 1.2 concentration of caffeine/ mg 100 cm-3,heart rate of Daphnia/beats per minute,mean ,1,2,3,4,5, 2.4,307,284,228,275,301,279 3.6,325,331,319,328,322,325 4.8,361,355,349,358,362,357 6.0,377,365,357,360,363,364 On Table 1.2 indicate, by placing a circle around the value, one result that may be anomalous.[1]
8. (d)(ii)These students concluded that: The heart rate of Daphnia increased proportionally to the concentration of caffeine. Suggest two reasons why the data collected was not sufficient to make this conclusion.[2]

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