Biology 9700/12 — May/June 2015

Background Concept

Cells are the basic structural and functional units of life, and their size varies enormously. Eukaryotic cells (those with a membrane-bound nucleus) are typically much larger than prokaryotic cells (bacteria) and orders of magnitude larger than viruses.

Key biological size references to remember:

• Atoms: ~0.1 nm
• Small molecules (e.g. glucose): ~1 nm
• Proteins: 1–10 nm
• Viruses: 20–300 nm
• Bacteria (prokaryotes): 1–10 µm
• Eukaryotic organelles: 0.5–10 µm (mitochondria, chloroplasts, lysosomes)
• Typical animal cells: 10–30 µm (e.g. a human red blood cell is ~7–8 µm)
• Typical plant cells: 10–100 µm, commonly ~40 µm (larger on average than animal cells because of the large central vacuole)

Unit conversion: $1\ \text{mm} = 10^3\ \text{µm} = 10^6\ \text{nm}$.

Understanding the Question

The question asks for the typical diameter of a plant cell. We need to recall a sensible order-of-magnitude value and convert between units (nm and µm) so all four options can be compared on a common scale. The distractors are designed to test whether you can both remember the right size AND resist being misled by mixed units.

Approach

Recall the standard textbook value for a typical plant cell, then convert every option to a common unit (µm) for a like-for-like comparison. Anything in the tens of µm is a eukaryotic cell; anything in the µm or sub-µm range is much too small.

Step-by-Step Reasoning

1. A typical plant cell diameter is approximately $40\ \text{µm}$ (range $10$–$100\ \text{µm}$).
2. Convert each option to µm using $1\ \text{µm} = 10^3\ \text{nm}$:
   • A: $4.0 \times 10^1\ \text{µm} = 40\ \text{µm}$ — matches the typical plant cell size.
   • B: $1.0 \times 10^0\ \text{µm} = 1\ \text{µm}$ — too small; this is closer to a small bacterium.
   • C: $4.0 \times 10^2\ \text{nm} = 0.4\ \text{µm}$ — far too small; closer to a small organelle or bacterium.
   • D: $1.0 \times 10^2\ \text{nm} = 0.1\ \text{µm}$ — too small; closer to a virus.
3. Only option A falls in the correct size range for a typical plant cell.

Key Takeaways

• A typical plant cell is roughly $40\ \text{µm}$ in diameter and tends to be slightly larger than a typical animal cell because of the large central vacuole.
• The biological size hierarchy: virus (~100 nm) < bacterium (1 µm) < animal cell (10–30 µm) < plant cell (10–100 µm).
• Converting fluently between nm, µm and mm is essential in cell biology: $1\ \text{mm} = 10^3\ \text{µm} = 10^6\ \text{nm}$.

Common Mistakes

• Picking B ($1\ \text{µm}$): confusing a plant cell with a bacterium. Bacteria are typically 1–10 µm; plant cells are an order of magnitude larger.
• Picking C or D: failing to convert nm into µm. $400\ \text{nm} = 0.4\ \text{µm}$ and $100\ \text{nm} = 0.1\ \text{µm}$ — both orders of magnitude too small for a plant cell.
• Treating plant and animal cells as identical in size: plant cells tend to be a little larger than animal cells because of the central vacuole, although both are in the same order of magnitude.

Things to Be Careful About

• Always convert all options to the SAME unit before comparing — MCQ distractors often mix units to mislead.
• Remember the conversion factor: $1\ \text{µm} = 10^3\ \text{nm}$ (not $10^2$ or $10^4$).
• The 'typical' plant cell is ~$40\ \text{µm}$, but anything in the range $10$–$100\ \text{µm}$ is acceptable as a typical cell size.