Biology 9700/11 — May/June 2015

Background Concept

In biology we choose units that match the size of the structure being measured. The three metric length units used in the A-level course are:

• Millimetre (mm) = $10^{-3}$ m, suitable for features visible to the naked eye (e.g. large organs, seeds, whole small organisms).
• Micrometre (μm) = $10^{-6}$ m, the working unit of light microscopy, suitable for cells and many tissues.
• Nanometre (nm) = $10^{-9}$ m, required for sub-cellular detail such as membranes, viruses, and very thin structures like cell walls — these are usually beyond the resolving power of the light microscope and need an electron microscope.

A good rule of thumb: if the structure is on the scale of a cell → μm; if it is a thin layer within or around a cell → nm; if it is visible without a microscope → mm.

The structures to be measured here are very different in size:

• Alveoli are tiny air sacs in the lungs, with diameters of roughly 200–300 μm — far too small to measure in mm, but ideal in μm.
• White blood cells (leucocytes) are typically 10–25 μm across — again ideal in μm.
• Plant cell walls are thin layers around each cell, typically 100–1000 nm (0.1–1 μm) thick — best expressed in nm because the μm value would be an awkward decimal such as 0.5 μm.

Understanding the Question

The question presents a table in which each row offers a different combination of units for the three structures. We must pick the row in which every column uses the unit that best fits the size of that structure. It is a single-mark multiple choice and tests practical familiarity with the metric prefixes rather than detailed calculation.

Approach

For each structure, recall (or estimate) its actual size, then choose the unit that gives a sensible, non-awkward number:

• Alveoli → hundreds of μm → μm (not mm, which would be 0.2–0.3; not nm, which would be 200 000–300 000).
• White blood cells → tens of μm → μm (not mm, which would be 0.01–0.025; not nm, which would be 10 000–25 000).
• Cell walls → fractions of a μm → nm (not mm or μm, which would give tiny decimals).

Any option that places mm against white blood cells or cell walls, or μm against cell walls, is immediately wrong. Only option C has μm, μm, nm across the three columns.

Step-by-Step Reasoning

1. Reject option A because alveoli are not measured in mm — they are roughly 0.2–0.3 mm across, and biologists use μm for structures that need a microscope.
2. Reject option B because white blood cells are not measured in mm — they are roughly 0.01–0.025 mm across, far too small for the mm scale.
3. Reject option D because alveoli and white blood cells are not measured in mm, and although cell walls should indeed be in nm, the other two columns are wrong.
4. Accept option C: alveoli in μm, white blood cells in μm, cell walls in nm — all three units are appropriate to the size of each structure.

Key Takeaways

• The unit chosen should reflect the actual size of the structure and produce a reasonable number.
• Light-microscope structures (cells, alveoli, many tissues) are usually measured in μm.
• Sub-cellular structures and very thin layers (membranes, cell walls, viruses) are usually measured in nm.
• mm is reserved for features visible without a microscope (whole organs, large specimens).

Common Mistakes

• Treating "small" as the only criterion: nm is not always correct simply because something is small — using nm for whole cells (e.g. 10 000 nm) is technically valid but impractical, so μm is preferred.
• Confusing the unit appropriate for diameter with the unit for thickness — a white blood cell's diameter (10–25 μm) and a cell wall's thickness (0.1–1 μm) differ in scale, and the latter is far better expressed in nm.
• Choosing mm for any microscopic structure, simply because mm is a familiar unit.

Things to Be Careful About

• Remember the conversions: $1\ \text{mm} = 1000\ \mu\text{m} = 1\,000\,000\ \text{nm}$.
• Cell walls are much thinner than cells — do not assume "cell structure = μm" automatically.
• The question rewards matching units to all three structures; one wrong column loses the mark, so check every column before committing to an option.