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Origins of Cells · Paper 2 — Free Biology HL Practice Questions

1MasterySAQ-SEndosymbiotic theory9 marksPaper 2~14 min
The endosymbiotic theory proposes that mitochondria originated from free-living prokaryotes that were engulfed by a host cell.
(a)
State the type of prokaryote that is believed to have given rise to mitochondria. [1 mark]
(b)
A mitochondrion appears an image of length 15 mm15\,\text{mm} at a magnification of ×5000\times 5000. Calculate the actual length of the mitochondrion in micrometres (μm\mu\text{m}). [2 marks]
(c)
Explain how the double membrane of mitochondria provides evidence for the endosymbiotic theory. [2 marks]
(d)
Evaluate the strength of the evidence for the endosymbiotic theory by discussing two additional features of mitochondria, other than the double membrane, that support this theory. [4 marks]
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Solutions

2MasterySAQ-SEndosymbiotic theory6 marksPaper 2~9 min
Endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by a host eukaryotic cell.
(a)
State two features of mitochondrial DNA that are similar to prokaryotic DNA. [2 marks]
(b)
A chloroplast genome is found to be 120 kb120\,\text{kb} in length and replicates independently of the nuclear genome. Deduce how independent replication of the chloroplast genome supports the endosymbiotic theory. [2 marks]
(c)
Explain how the presence of 70S70\text{S} ribosomes in chloroplasts provides evidence for the endosymbiotic theory. [2 marks]
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Solutions

3ChallengeLAQOrigin of eukaryotic cells10 marksPaper 2~15 min
The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by a host cell. The following data were obtained from comparative genomic studies. Parameter — Value Genome size of free-living α\alpha-proteobacterium (ancestral to mitochondria) — ≈4.0×106 bp\approx 4.0 \times 10^{6}\,\text{bp} Genome size of modern human mitochondrial DNA (mtDNA) — ≈1.66×104 bp\approx 1.66 \times 10^{4}\,\text{bp} Genome size of free-living cyanobacterium (ancestral to chloroplasts) — ≈3.5×106 bp\approx 3.5 \times 10^{6}\,\text{bp} Genome size of modern chloroplast DNA (cpDNA) in Arabidopsis thaliana — ≈1.54×105 bp\approx 1.54 \times 10^{5}\,\text{bp} Protein-coding genes in free-living α\alpha-proteobacterium — ≈3,000\approx 3{,}000 Protein-coding genes in modern human mtDNA — 3737 Mitochondrial proteins in humans (nuclear-encoded) — ≈1,100\approx 1{,}100
(a)
Using only the data in the table, analyse the evidence that supports the endosymbiotic theory. [4 marks]
(b)
Explain the discrepancy between the number of protein-coding genes retained in modern human mtDNA and the total number of mitochondrial proteins found in human cells. [2 marks]
(c)
Evaluate the strengths and limitations of the endosymbiotic theory in explaining the origin of eukaryotic cells, with reference to both mitochondria and chloroplasts. [4 marks]
diagram

Solutions

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