Biology 9700/11 — October/November 2013

Background Concept

Microscopes are limited not by magnification but by resolution — the smallest distance between two points at which they can still be distinguished as separate. The resolution of a good light microscope is about 200 nm (0.2 µm); an electron microscope has a resolution of about 0.2 nm, because electrons have a much shorter wavelength than visible light.

Any cellular structure smaller than ~200 nm cannot be seen with a light microscope no matter how much it is magnified — you only see a blurred blob. Organelles that are below this threshold — ribosomes (20–30 nm), the endoplasmic reticulum, the Golgi apparatus as fine cisternae, and details of membranes — require an electron microscope.

Understanding the Question

The command word "only" is critical. The question is not asking which structure is best seen with an electron microscope, but which one cannot be seen at all with a light microscope. We need to test each option against the 200 nm threshold.

• A — cell wall: in plant cells, this is hundreds of nanometres to micrometres thick. Clearly visible under a light microscope.
• B — chromosome: condensed chromosomes during mitosis are several micrometres long and easily seen with a light microscope (this is how mitotic stages are routinely identified on slides).
• C — nucleolus: a dense, rounded body inside the nucleus, typically 1–7 µm across. Visible with a light microscope as a darker region within the nucleus.
• D — ribosome: at only 20–30 nm in diameter, it is well below the light-microscope resolution limit.

Approach

Compare the size of each named structure to the resolution limit of a light microscope (~200 nm). Any structure substantially smaller than 200 nm requires an electron microscope.

Step-by-Step Reasoning

1. State the resolution limit of a light microscope (≈ 200 nm) and that ribosomes (~20–30 nm) fall well below it.
2. Eliminate A, B and C on the grounds that all three are large enough to be resolved by a light microscope.
3. Conclude D is the only structure that can be seen only with an electron microscope.

Key Takeaways

• Resolution, not magnification, determines what can be seen.
• Ribosomes, the endoplasmic reticulum and details of internal membranes require an electron microscope.
• Nucleoli, chromosomes, chloroplasts, mitochondria and cell walls are all within light-microscope range.

Common Mistakes

• Confusing magnification with resolution — increasing magnification of a light microscope does not make ribosomes visible, it only enlarges the blur.
• Choosing C (nucleolus) because it is "inside" the nucleus and feels small, even though it is actually several micrometres across.
• Choosing B (chromosome) because chromosomes are described as containing DNA; the chromosome as a whole structure, however, is large enough to see.

Things to Be Careful About

• Read the word "only" in the question; a structure that is better seen with an electron microscope is not the same as one that can only be seen with one.
• Remember approximate sizes: cell wall (µm), chromosome (µm), nucleolus (µm), ribosome (nm).