Biology 9700/12 — May/June 2010

Background Concept

The conducting division of the human respiratory system is lined by a sequence of epithelia that become progressively simpler as the airways branch and decrease in diameter:

• Trachea and bronchi are lined by ciliated pseudostratified columnar epithelium with numerous goblet cells. The cilia beat in a coordinated manner to move the mucus layer (produced by goblet cells and submucosal glands) upward towards the pharynx — the mucociliary escalator.
• Bronchioles lose cartilage and submucosal glands. Larger bronchioles are lined by ciliated columnar or ciliated cuboidal epithelium with fewer goblet cells. Smaller (terminal and respiratory) bronchioles have ciliated cuboidal epithelium and may lack goblet cells entirely.
• Alveoli are the gas-exchange surface, lined by an extremely thin simple squamous epithelium (mainly Type I pneumocytes, with Type II pneumocytes that secrete surfactant). This thinness minimises the diffusion distance for O₂ and CO₂.

Understanding the Question

The diagram (Fig. 1.1) shows tall, column-shaped cells with cilia on the apical (luminal) surface and a single goblet cell packed with mucus granules, with an overall epithelial height of 80 µm. The question asks where this ciliated columnar epithelium with goblet cells is found in the respiratory system, by ticking its presence or absence in the trachea, bronchus, all bronchioles and alveolus.

The command word is implicit (identify the correct option), and the mark is awarded only for the option that correctly maps the distribution.

Approach

1. Identify the tissue from the diagram: ciliated columnar epithelium with goblet cells.
2. Recall (or apply) the distribution of this tissue along the respiratory tree.
3. Eliminate options where the tick pattern contradicts the known distribution.

Step-by-Step Reasoning

• Trachea — the trachea is lined by ciliated pseudostratified columnar epithelium with goblet cells, so this must be ticked ✓.
• Bronchus — branches of the trachea, structurally similar; also ciliated columnar with goblet cells ✓.
• All bronchioles — this is the critical distractor. Only the larger bronchioles carry ciliated columnar (or cuboidal) epithelium; the smaller bronchioles are lined by ciliated cuboidal cells and lose goblet cells. Because the option specifies all bronchioles, it cannot be ticked ✗.
• Alveolus — gas exchange occurs across a simple squamous epithelium; no cilia, no goblet cells ✗.

Combining these: trachea ✓, bronchus ✓, all bronchioles ✗, alveolus ✗. Only option B matches this pattern.

Key Takeaways

• Ciliated columnar epithelium with goblet cells lines the trachea and bronchi (and the very largest bronchioles).
• The word "all" in option B's row for bronchioles is a deliberate trap — smaller bronchioles have ciliated cuboidal epithelium and lack goblet cells.
• Alveoli are lined by simple squamous epithelium to keep the diffusion distance short.

Common Mistakes

• Ticking "all bronchioles" because the larger ones do carry ciliated columnar epithelium — the question requires the epithelium to be present in every bronchiole.
• Confusing alveolar Type I pneumocytes with columnar cells — alveoli are squamous, not columnar.
• Selecting the option that has a single tick in the right place without checking the rest of the row.

Things to Be Careful About

• Read the row/column heading carefully: "all bronchioles" means every bronchiole, not just the larger ones.
• Distinguish the conducting zone (trachea, bronchi, bronchioles — ciliated) from the respiratory zone (alveoli — squamous, no cilia).
• The 80 µm height given in the figure is consistent with a tall columnar epithelium, ruling out the much thinner squamous lining of alveoli.