1[3 marks]
Explain how the development of electron microscopes has increased our understanding of cells.
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- electron microscopes have a higher magnification / higher resolution than light microscopes (1)
- so sub-cellular structures can be seen in more detail / more clearly (1) allow example, such as ribosomes or the inside of mitochondria
- so scientists can understand the roles / functions of sub-cellular structures (1)
2[2 marks]
Explain when it is appropriate to use an estimation in biology. Give one example.
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- when an exact value is not needed / when it is not possible to find an exact value / to check a calculation is sensible (1)
- example, such as estimating the number of cells in a sample by counting a small area / rounding to one significant figure to check an answer (1)
3[1 mark]
A mitochondrion is 2 µm long. What is this length in nanometres?
- 0.002 nm
- 20 nm
- 2000 nm
- 2 000 000 nm
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Answer: C C 2000 nm (1)
4[3 marks]
A student makes a drawing of a plant cell. The drawing is 36 mm wide. The magnification is ×400. Calculate the real width of the cell. Give your answer in µm.
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- real size = image size ÷ magnification (1)
- 36 ÷ 400 = 0.09 mm (1)
- 0.09 × 1000 = 90 µm (1) correct answer in µm gains full marks; 0.09 mm with no conversion gains 2 marks
5
Higher tier
[3 marks]A cell has a real diameter of 3.2 × 10⁻⁵ m. Its image has a diameter of 6.4 × 10⁻³ m. Calculate the magnification. Show your working.
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- magnification = size of image ÷ size of real object (1)
- (6.4 × 10⁻³) ÷ (3.2 × 10⁻⁵) (1)
- ×200 (1) allow 200 / 2 × 10²; correct answer with no working gains full marks
6[6 marks]
Describe how you would prepare a slide of onion epidermis cells, use a light microscope to observe the cells and make a labelled scientific drawing.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A detailed, logical description covering preparing the slide, using the microscope correctly and producing a drawing. Correct technical terms are used throughout. |
| 2 | 3–4 | A description that covers two of the three areas with some detail, or all three areas with limited detail. Some correct technical terms. |
| 1 | 1–2 | A few isolated points about the method, with little detail or order. |
Indicative content
- put a drop of water or stain (for example iodine solution) on a slide
- peel a thin layer of epidermis and lay it flat on the slide using forceps
- lower the coverslip at an angle (with a mounted needle) to avoid trapping air bubbles
- place the slide on the stage and select the lowest power objective lens
- focus using the coarse focus then the fine focus
- change to a higher power objective and use only the fine focus
- draw with a sharp pencil, clear continuous lines, no shading, drawn large
- label with ruled lines that touch the structures, add a title and the magnification