Cambridge A Level Biology 9700 — 2019 May/June Paper 5 · Variant 2

1 (a) Some students wanted to determine the reaction time of a sample of people aged from 5–80 years. The students found three different methods of testing reaction time, as described in Fig. 1.1. Method 1: Ruler test Method 2: Computer click Method 3: Light board test timer • The subject rests their elbow • The subject stands in on a table so that their wrist • The subject looks at a front of a board with extends over the side. computer screen and buttons arranged in a clicks a start button on pattern. The buttons • The assessor holds the ruler the screen and waits for light up randomly. vertically in the air between the background colour the thumb and first finger of to change. • As soon as a button the subject. lights up the subject • As soon as the presses the button to • The zero mark on the ruler is background colour turn it off. lined up with the first finger changes the subject of the subject. The ruler is clicks a stop button on • As soon as a button is released without warning. the screen. pressed another button lights up. • As soon as the subject sees • The reaction time, in the ruler fall, they catch it as seconds, appears on • The number of lights quickly as possible. the screen. turned off in a set time is recorded. • The distance the ruler falls is recorded in metres. Fig. 1.1 (i) The students decided that Method 2 was the most suitable to use for testing subjects aged from 5–80 years. Suggest two reasons why Method 2 is the most suitable. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) State the independent variable and the dependent variable in Method 2. independent variable ......................................................................................................... ........................................................................................................................................... dependent variable ............................................................................................................ ........................................................................................................................................... [2] (iii) Describe how the students could investigate the reaction time of a sample of people aged from 5–80 years, using Method 2. Your description should be set out in a logical order and be detailed enough to let another person follow it. Details of Method 2, as described on page 2, are not required. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [6] (b) The students found some results on the internet for a reaction time test, as shown in Fig. 1.2. The error bars show ± 1 standard deviation (s). 400 Key male female 300 mean reaction time / ms 200 100 0 10 20 30 40 50 60 70 80 age / years Fig. 1.2 (i) The percentage change in mean reaction time for males between the ages of 5 years and 10 years is 47%. Calculate the percentage change in mean reaction time for females between the ages of 5 years and 10 years. Show your working and give your answer to the nearest whole number. .................% [2] (ii) State two conclusions about the trends shown by the results in Fig. 1.2. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (iii) State a suitable statistical test that the students could use to find out if the difference in reaction times between males and females of the same age was significant. Give a reason for your answer. name of test ...................................................................................................................... reason ............................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (iv) Suggest a null hypothesis for the test that you have chosen in (iii). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 17]

Mark scheme: 1(a)(i) any 2 of: 1. easy to explain / easy to do / no complex apparatus ; 2. mobility / physical strength / stamina (of subject), is not necessary ; 3. no calculations needed to get reaction time AW ; 4. results are more accurate / precise ; 2 1(a)(ii) independent variable: age ; dependent variable: reaction time ; 2 1(a)(iii) any 6 of: 1. ref. to suitable number of people in total ; 2. ref. to equally spread, age categories ; 3. ref. to same number males and females 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 in same age category and taking a mean ; 12. low / medium risk experiment / parent or guardian present for children / carer for vulnerable adults ; 6 1(b)(i) (340 210) 340 − × 100 = 38% ;; 2 Question Answer Marks 1(b)(ii) any 2 of: (mean) reaction time in females is higher than in males (at most ages) ; reaction time decreases (to age 20 years), then starts to increase ; (due to overlaps of SD lines) there is no significant difference between (male and female) reaction times ; 2 1(b)(iii) t-test ; data is continuous / comparing means / standard deviations are approximately the same ; 2 1(b)(iv) there is no (significant) difference in the (mean) reaction time between males and females (at same age) ; 1

2 Sperm production in humans is controlled by the hormones FSH and LH. These hormones are released from the pituitary gland in males to maintain a constant concentration in the blood. Research has shown that sperm production in humans has decreased during the last 75 years. This research has highlighted concern about the fertility of future generations. (a) Assessments of fertility are carried out on diluted semen samples. The samples are checked for number of sperm per mm3, motility and abnormal structure. Fig. 2.1 shows sperm in one part of a haemocytometer grid used for counting sperm. The microscope illumination causes the sperm heads to fluoresce. sperm one complete counting square 0.05 mm × 0.05 mm × 0.10 mm Fig. 2.1 The semen sample shown in Fig. 2.1 was diluted by a factor of 100. (i) Describe a method by which the number of sperm present in this diluted sample could be estimated. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Describe how you could estimate the number of sperm per mm3 in the original, undiluted semen sample. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (iii) Suggest why this method of counting sperm may not represent the actual number of sperm in the original, undiluted semen. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) Hormone imbalances can result in reduced sperm production and immobile sperm, leading to infertility. A study was carried out on 120 men with different causes of infertility. All the subjects were non-smokers, did not drink alcohol and did not take any medication for 6 months before the study. The concentrations of the hormones FSH and LH were measured in each man. Semen samples were collected and tested within 2 hours of collection. The number of sperm per mm3 were estimated and the motility of the sperm assessed. (i) Identify two variables that have been standardised in this study. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] Fig. 2.2 shows the concentration of FSH and LH in four test groups. In one test group, the men are fertile and in the other three groups, the men show different types of infertility. Key for hormones Key for test groups 70 FSH 1 fertile LH 60 2 infertile, due to 50 mean non-motile sperm concentration 40 3 infertile, due to low of hormone 30 sperm number in semen/ arbitrary units 20 4 infertile, due to no sperm present in semen 10 0 1 2 3 4 test groups Fig. 2.2 (ii) State the reason for including the hormone concentrations of fertile men in Fig. 2.2. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) State one conclusion about the effect of FSH and LH on infertility in men that is consistent with the results of this investigation. ........................................................................................................................................... ..................................................................................................................................... [1] (c) Semen samples may also be tested for viability of sperm. Viability is the proportion of live sperm present in semen samples. Viability of sperm can be tested using a chemical which stains living cells a different colour to dead cells. Suggest a method, using this stain and a haemocytometer, that could be used to test the viability of sperm in a semen sample. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] [Total: 13]

Mark scheme: 2(a)(i) award mp4 only if 2 marks not already awarded any 2 of: 1. count the sperm inside the (complete) square(s) ; 2. include sperm with heads inside the square(s) ; 3. include those touching the line (of square) top and left (and exclude those touching bottom and right) ; 4. (calculate the) volume of 1 square / 0.05 × 0.05 × 0.10 / 2.5 × 10–4 mm3 ; 2 2(a)(ii) 1. (calculate the) volume of 1 square / 0.05 × 0.05 × 0.10 / 2.5 × 10–4 mm3 ; 2. divide number sperm (in n squares) by volume (in n squares) or divide 1 mm3 by the volume (in n squares) and multiply by the number of sperm counted (in n squares) ; 3. multiply by, the dilution factor / 100 ; 3 2(a)(iii) any 2 of: idea of small volume / only a sample (volume) is taken ; (sperm moving so) easy to miscount / harder to count ; sperm, not evenly distributed / forms clumps / overlap ; not repeated ; 2 2(b)(i) any 2 of: (no) smoking ; (no) alcohol ; (no) medication ; sperm tested within 2 hours of collection ; sperm count in a fixed volume (mm3) ; 2 2(b)(ii) for comparison ; 1 Question Answer Marks 2(b)(iii) any 1 of: 1. high(er) FSH and low(er) LH, reduces sperm motility / causes group 2 infertility ; 2. only high(er) FSH / high FHS and normal LH, reduces sperm number / causes group 3 infertility ; 3. FSH and LH, both high, prevents sperm release / prevents sperm production / causes no sperm / causes group 4 infertility ; 1 2(c) count / record the number of, live and dead sperm / sperm of the two different colours / live and total ; method to calculate the proportion ; 2

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