Biology 9700/23 — May/June 2010

Background Concept

A transmission electron micrograph (TEM) lets us see the internal ultrastructure of organelles at very high resolution because it uses a beam of electrons (whose wavelength is far shorter than visible light) instead of light. Animal cells contain several membrane-bound organelles, each with a distinctive appearance in TEM:

• The nucleus is the largest organelle, bounded by a double-membrane nuclear envelope perforated with nuclear pores. Inside, the genetic material is dispersed as chromatin — euchromatin (pale and dispersed, transcriptionally active) and heterochromatin (dark and condensed, largely inactive). A small, dense, rounded body within the nucleus is the nucleolus, the site of ribosome subunit assembly.
• Mitochondria are oval-shaped organelles with a smooth outer membrane and a highly folded inner membrane forming cristae. In section, the cristae give them a characteristic striated appearance.
• The rough endoplasmic reticulum (RER) is a continuous network of flattened membrane sacs (cisternae) whose cytoplasmic surface is studded with ribosomes, giving the membrane a dotted or rough texture. RER is the site of synthesis of proteins destined for secretion, for insertion into membranes, or for delivery to other organelles. The smooth ER (SER) lacks ribosomes and looks smooth-walled.

Understanding the Question

Fig. 1.1 is a TEM image of an animal cell. The candidate is shown an image (with a $6\ \text{µm}$ scale bar) and asked to name the three structures indicated by the labels A, B and C. This is a structure-recognition question; the mark scheme accepts the principal alternative names for each structure but explicitly rejects a few close-miss answers.

Approach

For each label, look at the distinctive structural features in the image (size, shape, internal pattern, surface texture) and match them to your knowledge of organelle ultrastructure:

• A is the largest organelle in the image, with a clear envelope and dark internal material — this is the nucleus.
• B is an oval body with internal parallel membranes — this is a mitochondrion.
• C is a stack of flattened sacs whose surfaces are dotted — this is the RER.

Step-by-Step Reasoning

• A — nucleus. The label points into a large rounded organelle on the left of the image, bounded by a clear envelope. The dark granular material inside is chromatin. The mark scheme accepts "nucleus" (the whole organelle) or "euchromatin" (the dispersed chromatin it points into) but explicitly rejects "nucleolus" — a nucleolus would be a discrete, denser, rounded body inside the nucleus, not the diffuse chromatin region shown.
• B — mitochondrion. The label points to an oval structure with internal parallel lines. These lines are the cristae formed by the inner mitochondrial membrane. No other organelle in an animal cell has this combination of an outer envelope and an extensively folded inner membrane.
• C — rough endoplasmic reticulum (RER). The label points to flattened membrane sacs whose surfaces appear covered in small dark dots — the ribosomes. The dot-studded surface is the defining feature that distinguishes RER from SER. The mark scheme accepts "rough endoplasmic reticulum" or "(R)ER" but explicitly rejects "smooth ER".

Key Takeaways

• Recognise the nucleus by its size, its envelope, and the chromatin pattern inside.
• Recognise mitochondria by their double membrane and cristae.
• Recognise RER by the ribosomes (small dots) on its cytoplasmic surface — this is the key feature distinguishing RER from SER.
• Always read the mark scheme's reject list: "nucleolus" is wrong for A, and "smooth ER" is wrong for C.

Common Mistakes

• Writing "nucleolus" for A — this is rejected by the mark scheme. The image does not show a discrete dense body within the nucleus.
• Writing "smooth ER" or just "ER" for C — the visible ribosomes make it specifically RER, and the mark scheme rejects "smooth/S".
• Confusing mitochondria with other round structures (e.g., lysosomes, vesicles) — only mitochondria have cristae.
• Naming individual ribosomes as the structure at C — the candidate should name the whole RER network, not a single ribosome.

Things to Be Careful About

• Note the mark scheme's R (reject) markings and avoid those answers.
• The mark scheme accepts two answers for A ("nucleus" or "euchromatin"); in a written answer either would earn the mark, but "nucleus" is the safer, more general answer.
• Magnification of the image is not relevant here — the structures must be identified by their morphology, not by their size.

Background Concept

The electron microscope offers much higher resolution than the light microscope, but it does so at a cost. The electron beam can travel only in a vacuum (otherwise the electrons would collide with air molecules and be scattered), so the specimen chamber is held under high vacuum. This forces several constraints on how specimens must be prepared and observed.

Specimens must be:

• Dead (because they are fixed, dehydrated and held in a vacuum — they cannot survive these conditions).
• Extremely thin (typically $50$–$100\ \text{nm}$ sections for TEM) because the electrons must pass through them.
• Stained with heavy metals (e.g. uranyl acetate, lead citrate) to scatter electrons and produce contrast — these are toxic.

Other practical limitations of electron microscopy include:

• The instrument is expensive to buy, house and maintain.
• It produces monochrome / black-and-white images (contrast comes from heavy-metal staining, not from colour).
• Operating it requires technical training and stable electrical supplies.
• Specimen preparation is lengthy and can introduce artefacts (features caused by preparation, not present in the living cell).

Understanding the Question

The question asks for one disadvantage of the electron microscope compared with the light microscope. The mark scheme lists many possible answers, of which any one is sufficient for the single mark.

Approach

Recall the practical constraints of electron microscopy listed in the Background Concept and pick the most concrete, specific one to write down. A good answer names the constraint and (briefly) why it is a disadvantage.

Step-by-Step Reasoning

The cleanest, most specific answer is: "only dead specimens can be viewed", because the vacuum and chemical fixation kill the cells before they can be imaged. This is a major limitation in cell biology, where researchers often want to observe living processes.

Other valid choices that would each earn the single mark include:

• (More) expensive to buy, maintain or run
• Requires (more) technical training to operate
• Specimens are more difficult / take longer to prepare
• Produces only black-and-white (monochrome) images
• Cannot be used on coloured specimens or to study coloured processes
• Sensitive to external magnetic fields / vibration
• Not portable

The mark scheme is generous: any single, specific, relevant disadvantage is credited. Vague answers such as "not as good" or "hard to use" without elaboration are not credited.

Key Takeaways

• The electron microscope's main advantage is its very high resolution, but it comes with the constraint that specimens must be dead, fixed, dehydrated and stained.
• Other practical disadvantages are cost, complexity, monochrome imaging, and lengthy preparation.
• Choose one specific, well-stated disadvantage rather than several vague ones.

Common Mistakes

• Writing vague answers such as "hard to use" or "complicated" without specifying why (e.g. "requires technical training").
• Stating a disadvantage of the light microscope instead (e.g. "lower resolution") — that is the advantage of the electron microscope.
• Writing "lower magnification" — this is incorrect; electron microscopes have higher magnification than light microscopes.
• Listing multiple disadvantages in one short answer; the mark is for one well-expressed point.

Things to Be Careful About

• Keep the answer focused on the electron microscope's limitations, not its advantages.
• "Human error" or "less accurate" are too vague to earn credit.
• A specific disadvantage is required — name the practical consequence (e.g. "expensive to buy") rather than just saying "it's not good".

Background Concept

Magnification is the number of times an image is larger than the object:

$$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$$

In a printed image, the scale bar tells you the actual size that a particular length on the page represents. By measuring the scale bar on the printed image with a ruler, you can find the image size, and then divide by the actual size (read from the scale bar's label) to obtain the magnification of the image.

Unit conversion is essential in these calculations. The standard units for cellular measurements are:

$$1\ \text{mm} = 1000\ \text{µm} = 1\,000\,000\ \text{nm}$$

So a measurement in millimetres on the printed page must be converted to micrometres (or nanometres) before it can be divided by the actual size in the same units.

Understanding the Question

Fig. 1.1 includes a scale bar labelled $6\ \text{µm}$. The candidate is asked to calculate the magnification of the image, showing working, and to give the answer to the nearest whole number. Two marks are available: one for the working (and correct unit conversion), and one for the final numerical answer.

Approach

1. Use a ruler to measure the length of the $6\ \text{µm}$ scale bar on the printed page (in mm).
2. Convert this measurement from mm to µm.
3. Substitute into the magnification formula.
4. Round to the nearest whole number.

Step-by-Step Reasoning

• Measure the scale bar. With a ruler, the $6\ \text{µm}$ scale bar on a typical A4 printing of this paper measures approximately $20\ \text{mm}$. The mark scheme allows a tolerance of $\pm 1\ \text{mm}$ on this reading, so any value in the range $19$–$21\ \text{mm}$ is acceptable.
• Convert to µm.

$$20\ \text{mm} = 20 \times 1000\ \text{µm} = 20\,000\ \text{µm}$$

• Apply the magnification formula.

$$\text{magnification} = \frac{\text{image size}}{\text{actual size}} = \frac{20\,000\ \text{µm}}{6\ \text{µm}} = 3333.3$$

• Round to the nearest whole number.

$$\text{magnification} \approx \times\ 3333$$

The mark scheme accepts the following alternatives based on the $\pm 1\ \text{mm}$ measurement tolerance:

Key Takeaways

• The magnification of a printed image is found by measuring a known feature (e.g. a scale bar) and dividing by its labelled actual size.
• Unit conversion is essential: always convert mm to µm (or µm to mm) so that image size and actual size are in the same units before dividing.
• The mark scheme rewards working (a formula or a clear substitution) as well as the final numerical answer.
• Round to the nearest whole number, as the question requests.

Common Mistakes

• Forgetting to convert mm to µm — dividing $20$ by $6$ gives a magnification of only $3.3\times$, which is wildly wrong. The mark scheme awards partial credit (one mark) for a correctly measured scale bar divided by $6\ \text{µm}$ but without correct unit conversion.
• Not showing working — the question explicitly says "Show your working"; a bare numerical answer with no formula or substitution forfeits one of the two marks.
• Rounding errors — the mark scheme wants the answer to the nearest whole number, not to one or more decimal places. An answer such as "$3333.3$" would earn only one mark (the formula mark) but not the final-answer mark.
• Measuring the wrong length — for example, measuring the whole image rather than the scale bar.

Things to Be Careful About

• Always quote the formula for magnification, even if it seems obvious.
• Make sure the units cancel cleanly: the µm in the numerator and denominator cancel, leaving a dimensionless magnification.
• The mark scheme allows a small measurement tolerance, so an answer of $3167\times$ or $3500\times$ (within the $\pm 1\ \text{mm}$ reading range) is fully acceptable.
• Include the multiplication sign ($\times$) in the final answer to make clear it is a magnification.