| Date | May 2009 | Marks available | 3 | Reference code | 09M.2.SL.TZ1.2 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Compare | Question number | 2 | Adapted from | N/A |
Question
The electron micrograph below shows an E. coli cell.
Identify the structures labelled A and B in the electron micrograph above and state one function of each.
A: Name
Function
B: Name:
Function:
Compare prokaryotic and eukaryotic cells.
Markscheme
Both name and function required to achieve [1].
A: name: flagella/flagellum
function: used for locomotion / beats in whip-like action to propel cell;
B: name: pili/pilus
function: used for adhesion (to another cell/surface) / transfer of genetic material (between cells);
ECF, for one mark, can be applied if both parts of the pair are reversed.
Award [1] for a similarity.
both have a plasma/cell membrane/ribosomes/cytoplasm/genetic material;
Award up to [2] for differences. Candidate must make a valid comparison, not simply describe each. Award [2 max] if features of prokaryotic and eukaryotic cells are not compared directly, item by item, although a table is not necessary.
Examiners report
The flagella and its function were usually known. Although the pilus was often known, its function was not. Low achievers sometimes labeled the pili as cilia.
It was essential for candidates to make valid precise comparisons of the features for prokaryote and eukaryote. For example,
- “naked DNA” (prokaryote) was confused to mean DNA outside a nucleus so it was paired with “DNA enclosed in nuclear envelope
- (eukaryote) instead of “DNA associated with proteins” (eukaryote). For organelles, there had to be reference to membranes as in “no”
- membrane-bound organelles” (prokaryotes) paired with “membrane-bound organelles” (eukaryotes).
- When referring to differences in ribosomes or cell sizes, a quantified answer was required e.g. “70S ribosomes” (prokaryotes) paired with “80S ribosomes (eukaryotes)
- “smaller than 10 microns” (prokaryotes) paired with “larger in size, up to 100 microns (eukaryotes). Most candidates did not provide similarities,
- the command term “compare” includes similarities and differences. A few candidates produced tables which clearly represented similarities and differences.
