A-LevelBiologyCell structure9700/52 · May/June 2019 · Q213 Marks
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)(i)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 x 0.05 mm x 0.10 mm [Figure 2.1] The semen sample shown in Fig. 2.1 was diluted by a factor of 100. Describe a method by which the number of sperm present in this diluted sample could be estimated.[2]
- (a)(ii)Describe how you could estimate the number of sperm per mm³ in the original, undiluted semen sample.[3]
- (a)(iii)Suggest why this method of counting sperm may not represent the actual number of sperm in the original, undiluted semen.[2]
- (b)(i)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. Identify two variables that have been standardised in this study. 1 2[2]
- (b)(ii)State the reason for including the hormone concentrations of fertile men in Fig. 2.2.[1]
- (b)(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]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- [2] 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 ;
- [3] 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;
- [2] 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] any 2 of:
- (no) smoking;
- (no) alcohol;
- (no) medication ;
- sperm tested within 2 hours of collection ;
- sperm count in a fixed volume (mm³);
- [1] for comparison ;
- [1] 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;
- [2] count / record the number of, live and dead sperm / sperm of the two different colours / live and total ;
- method to calculate the proportion;
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