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Date May 2019 Marks available 1 Reference code 19M.2.SL.TZ1.2
Level Standard level Paper Paper 2 Time zone Time zone 1
Command term Identify Question number 2 Adapted from N/A

Question

Identify which electron micrograph shows a mitochondrion, providing one observation to support your choice.

[1]
a.

Discuss the evidence for the theory that mitochondria may have evolved from free-living prokaryotes by endosymbiosis.

[3]
b.

Markscheme

«micrograph» C cristae/double membrane is visible/«folds of» membranes inside ✔

Must state observation

a.

a. «double» membrane may have formed when engulfed ✔

b. replicate by binary fission like free-living prokaryotes
OR
reproduce separate from «host» cell replication ✔

c. they have their own «circular» DNA AND reproduce on their own ✔

d. they have «70s» ribosomes AND can manufacture «their own» proteins ✔

e. have organelle«s» similar to free-living prokaryotes ✔

f. similar in size to free-living prokaryotes ✔

g. mitochondrial inner membranes manufacture ATP like bacterial membranes ✔

h. currently there is no free-living prokaryote like a mitochondrion ✔

b.

Examiners report

Most candidates identified the correct micrograph but for the wrong reason such as 70S ribosomes visible, has a matrix, or correct shape. Only starch granules were visible in the image, not ribosomes.

a.

The command term for this question was “discuss”. Connections were not made as many candidates just gave a list of features of mitochondria with no attempt at explanation or discussion. There was some confusion between the words endocytosis and endosymbiosis. In spite of the many shortcomings, there were a few brilliant responses

b.

Syllabus sections

Core » Topic 1: Cell biology » 1.2 Ultrastructure of cells
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Core » Topic 1: Cell biology
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