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Date November 2020 Marks available 1 Reference code 20N.1.HL.TZ0.1
Level Higher level Paper Paper 1 Time zone TZ0 / no time zone
Command term Question number 1 Adapted from N/A

Question

Chlorella and Paramecium are both unicellular eukaryotic organisms living in freshwater. Chlorella is photosynthetic and has a cell wall. Which organelle will be found in Paramecium but not in Chlorella?

A. Chloroplast

B. Contractile vacuole

C. Rough endoplasmic reticulum

D. Mitochondrion

Markscheme

B

Examiners report

Only about half of candidates chose the contractile vacuole for a structure in the cells of Paramecium but not Chlorella. The expected reasoning was that the cell wall of Chlorella protects it against bursting due to excessive entry of water by osmosis, so a contractile vacuole is not needed. The commonest other answer was rough endoplasmic reticulum. While a photosynthetic cell is not likely to secrete much protein and so will not have an extensive rough ER, generally eukaryotic cells have at least some. This question was thought by the examining team to be just about acceptable, though the issue of whether unicellular algae have contractile vacuoles is complicated by their presence in some species. If there is a flagellum, there is a gap in the cell wall, and the flagellum itself is not protected by cell wall, so as with animal cells there is a danger of bursting and a contractile vacuole is generally needed.

Syllabus sections

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