| Date | May 2011 | Marks available | 6 | Reference code | 11M.3.HL.TZ1.3 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Discuss | Question number | 3 | Adapted from | N/A |
Question
Discuss the endosymbiotic theory for the origin of eukaryotes.
Markscheme
eukaryotes evolved from prokaryotes;
mitochondria/chloroplasts evolved from (independent) prokaryotic cells;
taken in by larger (heterotrophic) cell by endocytosis;
theory supported by characteristics of chloroplasts/mitochondria;
[2 max] for mitochondria/chloroplast characteristics:
mitochondria/chloroplasts have naked DNA;
mitochondria/chloroplasts divide/carry out fission;
mitochondria/chloroplasts have 70S ribosomes / synthezise own proteins;
mitochondria/chloroplasts have double membranes;
cristae similar to mesosomes / thylakoid have similar structures in prokaryotes;
but theory cannot be falsified as it predicts something occurring in the past;
theory does not explain the origins of cilia/flagella/linear chromosomes/meiosis;
weaker evidence that cilia/flagella evolved from attached bacteria/spirochetes;
Examiners report
Many candidates were able to score four or more marks on this question on the endosymbiotic theory by describing it and looking at the supporting evidence shown by the structure of chloroplasts or mitochondria. However, even those candidates scoring well seldom „discussed‟ the fact that evidence was weak for evolution of other organelles such as cilia or flagella and that the theory cannot be falsified.
