Question 23M.3.SL.TZ2.1
| Date | May 2023 | Marks available | [Maximum mark: 5] | Reference code | 23M.3.SL.TZ2.1 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Calculate, Define, Explain, State | Question number | 1 | Adapted from | N/A |
Define osmolarity.
[1]
measure of solute concentration / total number of solute particles per litre;
Do not accept number of miliosmoles per litre / mOsmL-1 of solution as that is given on the graph.
Very few candidates could define osmolarity, most giving a definition of osmosis.
A study was done on the osmolarity of semen and sperm movement (motility).
[Source: Reproduced from Rossato, M., Balercia , G., Lucarelli, G., Foresta, C. and Mantero, F., 2002.
Role of seminal osmolarity in the regulation of human sperm motility.
International Journal of Andrology 25, p. 6, with permission of Wiley.]
State the relationship between osmolarity and sperm motility.
[1]
negative relationship/correlation / as osmolarity increases, motility decreases / inversely proportional;
The majority of the candidates could identify the relationship between osmolarity and sperm motility.
The same study showed that blood plasma has a lower osmolarity than semen. Explain how that would affect the movement of water between blood plasma and semen.
[1]
water would move from blood plasma to the semen as it follows the concentration gradient / from low osmolarity to higher;
Must include reason/explanation.
Many were unable to explain the movement of water partly due to confusion between hypo and hypertonic.
The electron micrograph shows human sperm.
[Source: https://www.intechopen.com/chapters/32057. Open access.]
Calculate the actual length of the head of the sperm indicated by the arrow, showing your working.
[1]
2.4 μm / 0.0024 mm / 0.00024 cm / 2.2 × 10-4 cm / 2400 nm;
Accept values from 2.2 μm/0.0022 mm/0.00022 cm to 2.6 μm/0.0026 mm/0.00026 cm.
Units required.
Calculation of the magnification of the head of the sperm was generally well done with candidates giving their answers in a variety of units from nanometres to metres. Quite a few candidates did not include the units in their answer. Comments that the quality of the diagram made the question difficult did not appear valid judging by the number of candidates that made the correct measurement. A considerable margin of error for the measurement was included in the markscheme.
Explain a change that would take place in sperm placed in a hypotonic solution.
[1]
- water will enter (by osmosis) causing sperm to swell/enlarge/burst;
- water will enter (by osmosis) causing sperm to become more mobile as osmolarity decreases;
Do not accept turgidity.
Must show movement of water.
The question required the candidates to explain the changes in the sperm placed in a hypotonic solution. Many answered that it would increase in size, others mentioned water would enter but few gave both points required in a question asking for an explanation. A few incorrectly suggested the cell would become turgid.