Animal and plant cells — AQA GCSE Biology
Test yourself on Animal and plant cells with AQA GCSE practice questions.
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Animal and plant cells explained
Sub-cellular structures are too small to see directly, so biologists estimate their size or area from images and known scales.
Read the full explanation
An estimate is a sensible approximate value, not a guess. To estimate size, compare the structure with a known scale bar or with the field of view. For example, if a scale bar represents 10 µm and a nucleus spans about one fifth of the bar, its diameter is roughly 2 µm. To estimate area, treat a roughly circular structure as a circle and use area = πr², or count squares on a grid. Estimations are used when exact measurement is impractical: structures are irregular, images are low resolution, or only a scale bar is available. They allow quick comparisons, such as judging that a plant cell is larger than a bacterium, and they help decide whether a structure is visible at a given magnification.
Your focus
- Use a scale bar or field of view to estimate the size of a sub-cellular structure.
- Estimate the area of a structure using an appropriate formula or grid.
- Explain why estimations are appropriate when judging the relative size or area of sub-cellular structures.
Animal and plant cells exam tips
Marking Points
- Defines an estimation as an approximate value based on comparison with a known scale, not a random guess.
- Uses a scale bar or the field of view to estimate the size of a named sub-cellular structure.
- Estimates area by treating a structure as a simple shape, for example using area = πr² for a circular nucleus.
- Explains that estimations are used when exact measurement is impractical, such as with irregular or low-resolution images.
- Uses estimates to compare the relative size or area of two sub-cellular structures.
Examiner Tips
- 💡Always show the comparison or calculation you used, such as the fraction of the scale bar covered, so your reasoning is visible.
- 💡Give answers to a sensible number of significant figures and make clear that the value is an estimate.
- 💡Check units carefully and convert mm to µm by multiplying by 1000 before comparing values.
Common Mistakes
- Treating an estimate as an exact measurement and quoting too many decimal places; correct this by rounding sensibly and stating that the value is approximate.
- Forgetting to convert units when comparing a scale bar in µm with a measurement in mm; correct this by converting both to the same unit before calculating.
- Using diameter in the area formula instead of radius; correct this by halving the diameter before applying area = πr².