Question 23M.3.SL.TZ1.2
| Date | May 2023 | Marks available | [Maximum mark: 7] | Reference code | 23M.3.SL.TZ1.2 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Calculate, Explain, Identify, Outline, State | Question number | 2 | Adapted from | N/A |
The electron micrograph shows the surface membrane of an animal cell.
[Source: Dunant, Y., Garcia-Segura, L.M., Muller, D. and Parducz, A., 1989. Momentary alteration of the postsynaptic membrane
during transmission of a single nerve impulse. Proceedings of the National Academy of Sciences of the United States of
America, [e-journal] 86(5), pp. 1717–1720. https://doi.org/10.1073/pnas.86.5.1717.]
State the technique used to obtain this electron micrograph.
[1]
freeze fracture/etching
OR
involves freezing and fracturing cells;
Many candidates stated an electron microscope, a few recognised freeze fracturing.
Using the scale, calculate the magnification of the image.
[1]
140 000 to 150 000 (X);
There were few correct answers; this also showed the lack of practical work. The scale bar was usually measured correctly and sometimes the correct calculation was known; however the correct units were converted. This skill is practical 1 in the syllabus.
Identify the type of molecule that can be seen at region X.
[1]
protein/named membrane protein;
Accept first answer only
About half the cohort answered proteins correctly but lipids and water were sometimes mentioned.
Outline the reason that images such as this falsified the Davson-Danielli model.
[1]
the phospholipid bilayer was not coated by/sandwiched between two protein layers
OR
(transmembrane/integral) proteins are found within the phospholipid bilayer;
OWTTE
There were some good answers here, either referring to continuous protein layers or naming specific proteins such as integral.
Explain the reason that animal cells and tissues under investigation must be maintained in solutions with the same osmolarity.
[3]
- to prevent osmosis;
- cells placed in the in the incorrect osmolarity might swell/burst/shrink;
- hypotonic solution would cause water to enter cells/tissues;
- hypertonic solutions would cause water to leave cells/tissues;
- water loss would hinder (metabolic) reactions in cell cytoplasm
OR
distort appearance of the cells (for the investigation);
Mp c and d must indicate direction of water movement
Accept WTTE for hypo- and hyper-tonic
For making point b "bursting" was often awarded, so most candidates scored at least 1 mark. The terms hypo- and hyper- tonic were frequently used without the direction of water movement being stated. Higher achieving candidates answered the question clearly and succinctly. Marking point e was awarded generously if the student referred to the investigation.
