Demonstrate an understanding of number, size and scale, including the use…

The idea

Cells and organelles are measured in suitable units: millimetres (mm), micrometres (µm) and nanometres (nm). Conversions use powers of ten: 1 mm = 1000 µm, 1 µm = 1000 nm, so 1 mm = 1 000 000 nm. Scale means comparing real size with a drawing or image, often using a magnification factor: magnification = image size ÷ actual size. Estimations are approximate values used when exact measurement is impractical, for example counting cells in a small grid square and multiplying to estimate a whole field, or judging the proportion of a field covered by cells. Estimations are appropriate for quick comparisons, large or irregular samples, or when equipment limits precision; they should be stated as approximate and, where possible, checked against a measured value.

What earns the marks

• Use appropriate units for cell size: mm, µm and nm.
• Convert correctly using factors of 1000 between mm, µm and nm.
• Apply scale and magnification: magnification = image size ÷ actual size.
• Explain that estimations give approximate values when exact measurement is impractical.
• State when estimations are appropriate, e.g. quick comparisons, large or irregular samples, limited equipment.

Common mistakes and how to put them right

• Confusing the direction of conversion between mm and µm.

  Put it right: 1 mm = 1000 µm, so multiply by 1000 to go from mm to µm.

• Treating an estimate as an exact measurement.

  Put it right: An estimate is approximate and should be labelled as such.

• Using magnification as actual size.

  Put it right: Rearrange magnification = image size ÷ actual size to find the required quantity.

Exam tips

• Write the conversion factor before calculating to avoid direction errors.
• Show the magnification equation and rearrange it clearly.
• Justify why an estimate is acceptable in the given context.