Biology 9700/13 — May/June 2023

Background Concept

The key concept here is resolution (resolving power), not magnification. Resolution is the minimum distance between two points at which they can still be seen as two separate objects. For a light microscope using visible light, the practical resolution limit is approximately $200\ \text{nm}$ ($0.2\ \text{µm}$). This limit is set by the wavelength of visible light and the numerical aperture of the objective lens; it cannot be improved simply by increasing magnification. Any object (or detail within an object) smaller than this limit appears as a single blurred point and cannot be made out, however much the image is enlarged.

By contrast, the transmission electron microscope (TEM) has a resolution of about $0.2\ \text{nm}$ because it uses a beam of electrons, whose effective wavelength is far shorter than that of visible light. This is why sub-cellular structures such as ribosomes, membranes and DNA must be studied with electron microscopy.

Understanding the Question

The question asks which of four biological features can be seen with a light microscope using a natural (visible) light source. The four options give the characteristic size of each feature, so the test is essentially: is the size greater than the resolution limit of the light microscope?

Approach

Compare each stated diameter/width with the $\approx 200\ \text{nm}$ resolution of the light microscope. Anything at or above that size is visible; anything clearly below it is not.

Unit conversion is helpful so all four values can be compared on the same scale:

• $200\ \text{µm} = 2 \times 10^{5}\ \text{nm}$
• $8\ \text{nm}$, $20\ \text{nm}$, $2\ \text{nm}$ — all far below $200\ \text{nm}$.

Step-by-Step Reasoning

1. Option A — DNA molecule ($2\ \text{nm}$). DNA is a double helix only about $2\ \text{nm}$ across. This is roughly $100 \times$ smaller than the resolution limit, so it cannot be resolved with a light microscope. (DNA can be made visible indirectly by staining, but the molecule itself cannot be imaged as a separate structure.) — not visible.
2. Option B — Paramecium cell ($200\ \text{µm}$). Paramecium is a ciliated protozoan easily seen at low power with a light microscope; $200\ \text{µm}$ is one thousand times larger than the resolution limit. — visible. ✓
3. Option C — Phospholipid bilayer ($8\ \text{nm}$). The lipid bilayer is far too thin to be resolved by visible light; an electron microscope is needed to see membrane structure. — not visible.
4. Option D — Ribosome ($20\ \text{nm}$). Ribosomes are well below the $200\ \text{nm}$ resolution limit and are routinely shown only in electron micrographs. — not visible.

Only B satisfies the criterion, so the answer is B.

Key Takeaways

• The light microscope resolves structures down to about $200\ \text{nm}$; it cannot resolve smaller objects, no matter how high the magnification.
• Large eukaryotic cells (tens to hundreds of µm) are easily seen; organelles and macromolecules (nm range) are not.
• Resolution, not magnification, is the limiting factor for what can be "seen" with a microscope.

Common Mistakes

• Confusing resolution with magnification. A high magnification that simply enlarges a blurred image does not reveal detail; the limit is set by the wavelength of the illumination.
• Forgetting the unit conversion ($1\ \text{µm} = 1000\ \text{nm}$) and so misjudging whether an object is above or below the $200\ \text{nm}$ threshold.
• Assuming all cell components are visible in a light micrograph — in fact only the larger organelles (nucleus, chloroplasts, large mitochondria in some preparations) can be seen; ribosomes, membranes and DNA cannot.

Things to Be Careful About

• Note the question specifies a natural light source (i.e. visible light); this gives the standard $\sim 200\ \text{nm}$ resolution. If ultraviolet or electron illumination were used, the resolution would be much higher.
• Always check units. $200\ \text{µm}$ is a very different quantity from $200\ \text{nm}$ — the latter would in fact be at the limit of resolution.