1.1 Eukaryotic and prokaryotic cells
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Which of these describes the genetic material of a bacterial cell?
Choose one answer.
Correct Not quite: the answer is A single loop of DNA, free in the cytoplasm, and sometimes plasmids
Mark scheme
B (A single loop of DNA, free in the cytoplasm, and sometimes plasmids)
Model answer
B: A single loop of DNA, free in the cytoplasm, and sometimes plasmids. Bacteria have no nucleus. Their DNA is a loop in the cytoplasm, with extra rings called plasmids.
Give two differences between a prokaryotic cell and a eukaryotic cell.
Mark scheme
Any two from:
- a prokaryotic cell has no nucleus / its DNA is free in the cytoplasm; a eukaryotic cell has its DNA in a nucleus (1)
- a prokaryotic cell has a loop of DNA (and plasmids); a eukaryotic cell has chromosomes (1)
- a prokaryotic cell is much smaller (1)
- a prokaryotic cell has no mitochondria (1)
- the cell wall of a prokaryotic cell is not made of cellulose (1)
- each point must make clear which type of cell it refers to, or use a comparison such as 'smaller'
Model answer
A prokaryotic cell has no nucleus, so its DNA lies free in the cytoplasm, whereas a eukaryotic cell keeps its DNA in a nucleus. A prokaryotic cell is also much smaller than a eukaryotic cell.
Name two structures that are found in a bacterial cell but not in an animal cell.
Mark scheme
Any two from:
- cell wall (1)
- plasmid(s) (1)
- flagellum / flagella (1)
- a circular loop of DNA (1)
- do not accept: DNA, ribosomes, cytoplasm or cell membrane, because animal cells have these too
Model answer
A cell wall and plasmids.
A bacterium is 2 µm long. A human cheek cell is 40 µm long.
(a) Calculate how many times longer the cheek cell is than the bacterium. (2 marks)
(b) Write the length of the bacterium in metres, in standard form. (1 mark)
Mark scheme
(a) 40 ÷ 2 (1)
- = 20 (times) (1)
- allow 20 for 2 marks with no working
(b) 2 × 10-6 (m) (1)
Model answer
(a) 40 ÷ 2 = 20, so the cheek cell is 20 times longer.
(b) 2 µm = 2 × 10-6 m.
Explain why a bacterial cell is described as prokaryotic.
Mark scheme
- it has no nucleus / its genetic material is not enclosed in a nucleus (1)
- its DNA is free in the cytoplasm (as a loop) (1)
Model answer
A prokaryotic cell has no nucleus. The bacterium's DNA is not enclosed in a nucleus but lies free in the cytoplasm as a loop.
Compare the structure of a bacterial cell with the structure of a plant cell.
Mark scheme
Any four from:
- similarity: both have a cell wall / cell membrane / cytoplasm / ribosomes (1) (maximum 1 mark for similarities)
- a plant cell has a nucleus; a bacterial cell does not (1)
- bacterial DNA is a loop (and plasmids) in the cytoplasm; plant DNA is in chromosomes in the nucleus (1)
- a plant cell has chloroplasts / mitochondria / a permanent vacuole; a bacterial cell has none (1)
- the plant cell wall is made of cellulose; the bacterial cell wall is not (1)
- a bacterial cell is much smaller (1)
Model answer
Both cells have a cell wall, a cell membrane, cytoplasm and ribosomes. A plant cell has a nucleus containing its DNA, but a bacterial cell has no nucleus and its DNA is a loop lying free in the cytoplasm. A plant cell also has chloroplasts, mitochondria and a permanent vacuole, which a bacterial cell does not. A bacterial cell is much smaller than a plant cell.
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