| Date | May 2013 | Marks available | 2 | Reference code | 13M.2.SL.TZ2.4 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 2 |
| Command term | Explain | Question number | 4 | Adapted from | N/A |
Question
The electron micrograph is of a spore of a fungus (Tilletia controversa) which affects wheat.
Determine the magnification of the spore in the electron micrograph. The scale bar represents 1 µm. Show your working.
Explain the importance of surface area to volume ratio as a limit to cell size.
Markscheme
a. magnification measured length of bar actual size bar represents/4 mm 1 μm
/ spore micrograph size real size / OWTTE;
b. 4000;
a. rate of exchange of materials/gas/energy is a function of its surface area;
b. rate of production of heat/waste/resource consumption is a function of its volume;
c. surface area to volume ratio decreases with increase in size / OWTTE;
d. at low surface area to volume ratios, exchange of materials takes longer/reduced efficiency of exchange / vice versa;
Examiners report
Candidates scored marks for a good working, but did not measure correctly, so failed to obtain the second mark. This question seemed to cause some confusion with candidates measuring the picture rather than the scale bar. Also, many wrote down very large magnifications without thinking about plausibility.
Confusing answers led to interpret that many candidates do not clearly understand this topic, writing wrong memorized concepts instead.
