Cambridge A Level Biology 9700 — 2019 May/June Paper 5 · Variant 3

1 One method of testing a person’s reaction time is to use an online, computer click timer test. Some students carried out a test using the method described. • Each student looks at a computer screen and clicks a start button on the screen and waits for the background colour to change. • As soon as the background colour changes the student clicks a stop button on the screen. • The reaction time appears on the screen. A student in the class thought that boys had a faster reaction time than girls. A group of students from the class decided to repeat the test to investigate the hypothesis: Boys have a faster reaction time than girls. (a) (i) Identify the independent variable and the dependent variable in this investigation. independent ...................................................................................................................... dependent ................................................................................................................... [2] (ii) State the type of qualitative variable which best describes the independent variable. ..................................................................................................................................... [1] (iii) Describe a method the students could use to collect the data needed to test their hypothesis. Your method should be set out in a logical order and be detailed enough for another person to follow. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [6] (iv) Suggest one feature of the online test which may be a source of error. ........................................................................................................................................... ..................................................................................................................................... [1] Another group of students decided to test the hypothesis: Right-handed people have a faster reaction time with their right hand than with their left hand. Table 1.1 shows their results. Table 1.1 student reaction time using right hand / s reaction time using left hand / s A 0.26 0.28 B 0.23 0.23 C 0.22 0.25 D 0.25 0.22 E 0.26 0.27 F 0.24 0.24 G 0.28 0.26 H 0.25 0.30 I 0.21 0.25 J 0.26 0.29 K 0.27 0.31 mean 0.25 0.26 mode 0.26 median 0.26 (b) Complete Table 1.1 to show the mode and median values. [1] (c) (i) The students decided to carry out a t-test to find out if the difference in reaction time was significant. State why a t-test is suitable for these data. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) State a null hypothesis for this test. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) State the number of degrees of freedom for this t-test. ........................................................................................................................................... ..................................................................................................................................... [1] Table 1.2 shows the probability values of t. Table 1.2 degrees of 10 12 14 16 18 20 22 24 26 28 30 40 50 60 freedom probability 2.23 2.18 2.14 2.12 2.10 2.09 2.07 2.06 2.06 2.05 2.04 2.02 2.01 2.00 0.05 (iv) The students concluded that the difference in reaction time was not significantly different. State what this tells you about their calculated value of t. ........................................................................................................................................... ..................................................................................................................................... [1] One student noticed that the more repetitions of the online test that were carried out, the faster the reaction time. The student decided to carry out a different test to investigate the effect of repetition and feedback on the accuracy of carrying out a task. Three students each carried out a task which involved drawing straight lines, without using a ruler, on sheets of paper. The three students did the task in separate rooms. • Each student drew 20 straight lines, as close as possible to 7.5 cm long, on separate pieces of paper. • The teacher measured all the lines and recorded the lengths to the nearest 0.1 cm in a table. • The teacher gave feedback to each student by telling them the actual length of line 20. • Each student then drew another 20 lines which were measured by the teacher and recorded in the table. Table 1.3 shows the data that the teacher recorded. Table 1.3 length of measured line before length of measured line after feedback line feedback / cm / cm student 1 student 2 student 3 student 1 student 2 student 3 1 7.2 5.0 7.3 7.9 7.5 5.2 2 7.5 4.3 7.4 7.0 7.2 5.2 3 7.4 4.0 7.7 7.5 7.7 5.2 4 7.5 3.9 7.9 7.3 7.6 4.9 5 8.0 4.2 8.2 7.0 7.3 6.3 6 7.5 4.3 8.8 7.2 7.7 6.5 7 7.5 4.0 8.8 7.0 7.4 6.3 8 7.2 4.0 9.5 7.2 8.0 6.2 9 7.1 4.4 9.3 7.0 7.6 6.2 10 7.4 3.8 9.0 7.1 7.2 5.7 11 7.2 7.9 9.3 7.0 7.5 6.7 12 7.4 4.0 8.8 7.0 6.8 6.6 13 7.5 4.2 9.7 7.6 7.2 6.6 14 7.5 4.0 9.7 8.0 7.6 6.6 15 7.4 4.2 10.0 8.0 7.4 6.4 16 6.5 4.1 10.0 8.2 7.4 6.6 17 6.7 4.4 9.6 7.9 7.0 6.5 18 6.8 4.0 9.8 8.5 7.5 6.2 19 7.9 4.2 9.7 7.8 7.7 6.9 20 7.5 4.0 10.2 7.5 7.6 6.1 mean ± s 7.3 ± 0.36 4.2 ± 0.26 9.0 ± 0.90 7.5 ± 0.47 7.4 ± 0.27 6.1 ± 0.59 (d) When one of the students was drawing one of their lines, a person entered the room and distracted them. It was decided this result was anomalous. (i) Indicate, by drawing a circle on Table 1.3, the length which is an anomalous result. [1] (ii) Suggest what the students should do with the anomalous result. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Identify the student who drew lines that were most inconsistent in length and give a reason for your choice. student .............................................................................................................................. reason ......................................................................................................................... [1] (iv) The student concluded that receiving feedback on performance leads to an improvement in accuracy. State one way in which the data support the conclusion and one way in which it does not. supports ............................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... does not support ............................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] [Total: 20]

Mark scheme: 1(a)(i) independent: sex / male or female / boy or girl ; dependent: reaction time ; 2 1(a)(ii) categoric ; 1 1(a)(iii) any 6 of: 1. ref. to suitable number of people in total ; 2. same / similar ages ; 3. same number boys and girls in each group ; 4. ref. to consent form (e.g. using data / children / AW) ; 5. ref. to standard conditions for the test, e.g. same (type of) computer / tablet / software / same colour change ; 6. ref. to same, stated, room conditions ; 7. ref. to avoiding people with conditions affecting reaction time e.g. poor eyesight / colour-blind / neurological disorders ; 8. no, (named) drugs / medications / caffeine / alcohol ; 9. ref. to testing at same time of day / stated time ; 10. ref. to using same hand to push the button ; 11. minimum of three tests on each subject and taking a mean OR minimum of three subjects of each sex and taking a mean ; 12. low / medium risk experiment / parent or guardian present for children / carer for vulnerable adults ; 6 1(a)(iv) idea of processing delay from computer / AW ; 1 1(b) mode 0.26 0.25 median 0.25 ; 0.26 1 Question Answer Marks 1(c)(i) comparing (two) means / continuous data / normal distribution ; 1 1(c)(ii) there is no (significant) difference in the (mean) reaction time (in right handed people) when using the left or right hand ; 1 1(c)(iii) 20 ; 1 1(c)(iv) less than 2.09 / less than the critical value (at p = 0.05) ; 1 1(d)(i) 7.9 (trial 11, student 2, before measurement) ; 1 1(d)(ii) idea that affected data should be left out ; 1 1(d)(iii) student 3 (no mark) (as their) standard deviations / s, are largest ; 1 1(d)(iv) supports any 1 of: 1. Idea that mean measurement for students was close(r) to, the actual measurement / 7.5, after feedback had been given ; 2. mean +/- s before feedback does not overlap with mean +/- s after feedback for, students 2 / 3 ; 3. AVP ; does not support any 1 of: 4. small sample size / only 3 students tested ; 5. mean +/- s for student 1 before and after feedback overlap 6. idea that improvement in accuracy in student 3 is small / 1.5 to 1.4 (cm) change in closeness to mean ; 7. idea of no statistical analysis to determine if difference in results is significant or not ; 8. AVP ; 2

2 For many years, farmers have used artificial insemination (AI) as a method to improve the milk yield of dairy cattle. In cattle, sperm cells carrying the X chromosome contain 3.8% more DNA than the sperm cells carrying the Y chromosome. Female cattle have the sex chromosomes XX. Assisted reproduction programmes allow dairy farmers to increase the number of female offspring produced. One technique used to identify which sex chromosome a sperm cell contains is centrifugation. Some semen samples were collected from male cattle. • 10 cm3 samples of semen were placed in centrifuge tubes with 15 cm3 of a buffer solution added to each tube. • The tubes were placed in a centrifuge and left to spin for 20 minutes at 1275 revolutions per minute. • The spinning causes the sperm cells containing the X chromosome to fall to the bottom of the tubes. The sperm cells containing the Y chromosome remain in the fluid at the top. Semen samples which have been processed in this way are referred to as sexed samples. (a) (i) State three variables which were standardised in this technique. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 3 ........................................................................................................................................ ........................................................................................................................................... ..................................................................................................................................... [3] (ii) Suggest one reason why it is important that the semen sample is not spun at a very high speed. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] Another method of sexing semen samples is to use a technique called flow cytometry as shown in Fig. 2.1. semen sample is diluted droplets of the sample are passed through a fine tube each droplet contains a sperm cell a computer analyses the sperm cell and detects if it contains the X or Y chromosome sperm cells containing X chromosomes sperm cells containing Y chromosomes are sprayed with a positive electrical charge are sprayed with a negative electrical charge sperm cells with X chromosomes are attracted sperm cells with Y chromosomes are attracted to a negatively charged detection plate to a positively charged detection plate sperm cells containing X chromosomes are sperm cells containing Y chromosomes are collected collected Fig. 2.1 The samples containing X chromosomes are frozen in small plastic straws, allowing semen samples to be stored for many years and transported all over the world. • Flow cytometry can process 8000 sperm cells every second. • One straw can hold 2.0 × 106 sperm cells. (b) Calculate how many straws can be filled every hour by flow cytometry. Number of straws filled every hour = ........................................................................ [1] The semen is analysed by technicians to determine its quality before it is sexed. Technicians use microscopy to look at the appearance and motility of the sperm cells and to estimate the sperm count. Fig. 2.2 shows the surface view of a haemocytometer used to estimate the number of sperm cells in a sample. The depth of the haemocytometer is 0.1 mm. 0.25 mm 0.25 mm sperm Fig. 2.2

Mark scheme: 2(a)(i) any 3 of: 1. (same) volume / 10 cm3, of semen ; 2. (same) speed of spinning / spun at 1275 revolutions per minute ; 3. (same) time for spinning / spun for 20 minutes; 4. (same) volume / 15 cm3, of buffer ; 3 2(a)(ii) any 1 of: idea that: some sperm cells may get damaged ; separation of X-bearing and Y-bearing sperm is not achieved / all sperm collect at the bottom of the tube ; AVP ; 1 2(b) 14 ; 1 Question Answer Marks 2(c) any 5 of: estimate of numbers: 1. idea of find the volume of the sample or (volume of sample is) – (for mm3) 0.25 × 0.25 × 0.1 / 0.00625 / 6.25 × 10–3 (for cm3) 0.025 × 0.025 × 0.01 / 0.00000625 / 6.25 × 10–6 ; 2. divide the number of sperm by the volume or divide 1 mm3 by the volume and multiply by the number of sperm counted ; 3. evidence of correct method of converting to cm3 ; deciding which sperm to include: 4. idea of: count those where sperm (fully) inside, the square / 0.0625 (mm2) / all (small) squares / haemocytometer / quadrant / grid / boxes ; 5. idea of: including those in or partially in the inner double lines ; 6. idea of: including those (in edge region) top and left / up and right or exclude those in edge region) bottom and right / down and left ; 7. AVP detail of method: e.g. ref. to, dilution / stirring / even spreading / immobilising sperm / use of coverslip / screenshot for counting / idea of only counting non- deformed / sperm ; 5

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