Which structure is a small ring of DNA found in a bacterial cell?
- Cell wall
- Plasmid
- Nucleus
- Ribosome
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Answer: B B: plasmid (1)
State two differences between a bacterial cell and an animal cell.
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- bacterial cell is much smaller / animal cell is larger (1)
- bacterial cell has a cell wall, animal cell does not (1)
- bacterial DNA is a single loop / not enclosed in a nucleus, animal cell DNA is in a nucleus (1)
- bacterial cell may have plasmids, animal cell does not (1)
- Max 2. Ignore references to mitochondria.
A plant cell is 1 × 10⁻⁴ m long. A bacterial cell is 1 × 10⁻⁶ m long. How many orders of magnitude longer is the plant cell?
- 1
- 2
- 4
- 6
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Answer: B B: 2 (10⁻⁴ ÷ 10⁻⁶ = 10²) (1)
Describe the function of each of these sub-cellular structures: nucleus, mitochondria, ribosomes.
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- nucleus contains genetic material / DNA / controls the activities of the cell (1)
- mitochondria: where aerobic respiration takes place / release energy (1)
- ribosomes: where proteins are made / protein synthesis (1)
A student draws a plant cell. The drawing is 45 mm long and the magnification is × 500. Calculate the real length of the cell. Give your answer in micrometres (µm).
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- real size = image size ÷ magnification (1)
- 45 ÷ 500 = 0.09 mm (1)
- 0.09 × 1000 = 90 µm (1)
- Correct answer of 90 µm gains all 3 marks. Allow 9 × 10¹ µm. 0.09 mm with no conversion gains 2 marks.
Explain how the development of the electron microscope has increased our understanding of sub-cellular structures.
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- electron microscopes have a much higher magnification (than light microscopes) (1)
- and a higher resolving power / resolution (1)
- so smaller sub-cellular structures (such as ribosomes) / more detail of structures (such as the inside of mitochondria) can be seen (1)
A muscle cell contains many mitochondria and protein fibres that can shorten. Explain how these features help the muscle cell to carry out its function.
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- protein fibres shorten so the cell contracts (1)
- (contraction) causes movement (1)
- mitochondria release energy by respiration (1)
- energy is needed for contraction (1)
- Max 3.
Explain why cell differentiation is important in a multicellular organism. Describe one difference in differentiation between animals and plants.
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- cells acquire different sub-cellular structures (1)
- so they can carry out specific functions / become specialised (1)
- most animal cells differentiate at an early stage, but many plant cells keep the ability to differentiate throughout life (1)
Describe how you would use a light microscope to observe a plant cell and make a labelled drawing, including a magnification.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, logical and complete method covering preparation of the slide, focusing from low power, and drawing with labels and a magnification. Scientific terms are used correctly and there are few or no errors. |
| 2 | 3–4 | A method that covers most stages in a sensible order, but with some detail missing, such as focusing or the drawing technique. |
| 1 | 1–2 | Some relevant points are made, but the method is incomplete or in the wrong order. |
Indicative content
- thin slice of plant tissue placed on a slide with a drop of water or a stain (such as iodine)
- cover slip lowered at an angle to avoid air bubbles
- start with the lowest-power objective lens and focus using the coarse focus knob
- then use the fine focus knob to get a sharp image, moving to a higher-power lens if needed
- draw with a sharp pencil, clear lines and no shading
- label the cell structures (such as cell wall, nucleus, vacuole)
- calculate and write the magnification (size of drawing ÷ real size), or draw a scale bar
A bacterium divides once every 30 minutes. Calculate the number of bacteria produced from one bacterium in 4 hours.
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- 8 divisions (240 ÷ 30) (1)
- 2⁸ = 256 bacteria (1)
- Correct answer of 256 gains both marks.
A bacterium divides once every 20 minutes. Calculate the number of bacteria produced from one bacterium in 10 hours. Give your answer in standard form to 2 significant figures.
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- 10 hours = 600 minutes, so 30 divisions (1)
- 2³⁰ = 1 073 741 824 (1)
- 1.1 × 10⁹ (1)
- 1 073 741 824 not in standard form, or 1.07 × 10⁹ (wrong number of significant figures), gains 2 marks.
Explain why, when preparing an uncontaminated culture of bacteria in a school laboratory: (a) the inoculating loop is passed through a flame, (b) the lid of the Petri dish is secured with adhesive tape, (c) the culture is incubated at 25 °C and not at a higher temperature.
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- (a) to sterilise it / kill other microorganisms (1)
- (b) to stop microorganisms from the air getting in / the cultured microorganisms escaping (1)
- (c) to prevent growth of pathogens (harmful to humans) (1)
A student places a disc soaked in an antibiotic on an agar plate covered in bacteria. After incubation, the clear zone around the disc has a diameter of 16 mm. Calculate the area of the clear zone. Use the equation area = π × r² and π = 3.14.
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- radius = 16 ÷ 2 = 8 mm (1)
- area = 3.14 × 8² (1)
- = 201 mm² (allow 200.96 mm², 200 mm² or 2.0 × 10² mm²) (1)
- Correct answer gains all 3 marks. Using the diameter (3.14 × 16² = 804 mm²) gains 1 mark only.