Biology 9700/23 — May/June 2012

Background Concept

Mammalian blood vessels come in three main types: arteries, veins and capillaries. They differ in the structure and thickness of their walls, which is closely tied to their function.

• Arteries carry blood away from the heart at high pressure, so they have thick walls with prominent layers of smooth muscle and elastic fibres.
• Veins carry blood back to the heart at low pressure, so their walls are thinner and contain less muscle and elastic tissue; they often have a visibly larger, more irregular lumen.
• Capillaries are the site of exchange between blood and surrounding tissues. They consist of a single layer of endothelial cells (a one-cell-thick endothelium) sitting on a thin basement membrane, with no smooth muscle or elastic layers. Their lumen is so narrow — typically about 5–10 µm in diameter — that red blood cells (about 7 µm across) often have to deform to pass through in single file.

Understanding the Question

The electron micrograph in Fig. 1.1 shows a cross-section of a blood vessel. We are asked to (i) name the type of vessel and (ii) describe ONE visible feature that is characteristic of it. The mark scheme accepts any one clear, observable feature that distinguishes this vessel from an artery or vein.

Approach

Look at the image and identify the most diagnostic features:

1. The wall is very thin — only a single layer of cells is visible between the lumen and the surrounding tissue.
2. The lumen is just large enough to hold the red blood cell(s); in capillaries the red cells are squeezed against the wall.
3. There is no visible layer of smooth muscle or elastic tissue around the endothelium.

The simplest single observation is that the wall is only one cell thick — this is the textbook defining feature of a capillary.

Step-by-Step Reasoning

• The vessel is not an artery: there is no thick muscular wall, no clearly visible internal or external elastic lamina, and the lumen is irregular rather than circular and held open by a thick wall.
• The vessel is not a vein: veins have a much larger, more irregular lumen, a thinner relative wall, but still show a distinct endothelium plus some smooth muscle; here only endothelium is visible.
• The wall is one cell thick (only endothelium), and the lumen is so narrow that a red blood cell fills most of it — both are diagnostic of a capillary.
• Either of those two features earns the second mark; the cleanest single answer is that the wall is one cell thick.

Key Takeaways

• A capillary is defined structurally by having a wall of a single layer of endothelial cells on a basement membrane — there is no smooth muscle, no elastic tissue, and no outer connective-tissue sheath.
• The very narrow lumen (just wider than a red blood cell) is a useful secondary diagnostic, and it is the structural basis of efficient exchange by diffusion.

Common Mistakes

• Writing "artery" or "vein": these would be wrong because the wall here is far too thin and there is no visible muscle layer.
• Saying "the wall is thin" without more detail: this is true but does not specify what the wall consists of. Better: "wall is one cell thick" or "only endothelium present".
• Confusing the small dark feature D (nucleolus) with the nucleus of the endothelial cell — the mark scheme accepts both, but the more precise answer is nucleolus.

Things to Be Careful About

• The question asks for a visible feature. Do not describe invisible features (e.g. "site of exchange") — that is a function, not an observable structural characteristic.
• Stick to one clear feature; the mark is for the single best, observable point.

Background Concept

Electron micrographs are highly magnified images of very small specimens. The relationship between the size of an object as it appears in a printed image (the image size), its real size (the actual size), and the magnification is:

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

To find actual size, rearrange to:

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

Because magnifications are often very large, and because cells and organelles are usually measured in micrometres (1 µm = 1/1000 mm = 10⁻⁶ m), unit conversion is an essential part of every calculation. The most common error in these questions is forgetting to convert millimetres (in which the image is measured with a ruler) into micrometres (the unit the answer must be in).

Understanding the Question

We are told the magnification of Fig. 1.1 is ×6000 and are asked to calculate the diameter of the lumen along the line X–X, showing working and giving the answer in µm to the nearest whole number. To do this we need to:

1. Measure the length of line X–X on the printed micrograph with a ruler (in mm).
2. Convert that image size from mm to µm.
3. Divide by the magnification to obtain the actual size.

Approach

Use the rearranged magnification formula
actual size = image size / magnification.
Measure the line on the page, convert the image size to the same units you want for the answer, then divide. Keep a careful eye on units — image size in mm must be converted to µm before the answer is given in µm.

Step-by-Step Reasoning

• The mark scheme accepts an image measurement of 39 mm (±1 mm) along X–X, so a measured value anywhere in the range 38–40 mm is treated as correct.
• Convert 39 mm to µm: 39 mm × 1000 = 39 000 µm. (If you measured 38 mm, use 38 000 µm; if 40 mm, use 40 000 µm — both still earn the working mark.)
• Apply the formula:

$$\text{actual size} = \frac{39\,000\ \text{µm}}{6000} = 6.5\ \text{µm}$$

• The question asks for the answer to the nearest whole number, so 6.5 µm rounds to 7 µm.
• For 38 mm: 38 000/6000 = 6.33 µm → 6 µm.
• For 40 mm: 40 000/6000 = 6.67 µm → 7 µm.

Because the mark scheme allows ±1 mm in the measurement, a final answer of either 6 µm or 7 µm is acceptable, with the official answer given as 7 µm.

Key Takeaways

• Always state the formula and show the substitution so the working mark is safe even if the final answer is slightly off.
• Convert units before dividing, not after: a clean sequence is mm → µm → divide by magnification.
• The answer must be quoted to the precision the question requests (here, the nearest whole µm).

Common Mistakes

• Dividing 39 by 6000 without converting mm to µm first. This gives 0.0065 mm, which is the right answer in mm but wrong because the question requires the answer in µm. Always check the unit requested.
• Forgetting to show working. The mark scheme allows the working mark independently of the final numerical mark, so showing the substitution protects a measurement error.
• Rounding incorrectly: 6.5 µm rounds to 7 µm (round half up), not 6 µm.
• Quoting 6.5 µm when the question explicitly asks for the answer "to the nearest whole number".

Things to Be Careful About

• Use a ruler and measure to the nearest mm; the mark scheme tolerates ±1 mm in the measurement.
• Magnification is given as ×6000 — a pure number, with no units, so the image size and actual size must end up in the same units.
• Re-read the unit requested in the question. Here it is micrometres (µm), not millimetres.