A level Biology 9.1 The gas exchange system - Exam style question

Question

Fig. 4.1 is a scanning electron micrograph showing the tissue that lines the bronchi in the gas

exchange system.
Fig. 4.2 is a transmission electron micrograph of a horizontal section made at the position indicated
by the two arrows in Fig. 4.1.

Scanning Electron Micrograph of Bronchial Lining

Transmission Electron Micrograph of Cilia

(a) (i) Name the cells labelled A and B in Fig. 4.1.

(ii) Describe how the tissue shown in Fig. 4.1 is adapted to its function in the gas exchange system.

(b) (i) The structures labelled X in Fig. 4.1 have a characteristic internal appearance, as seen in Fig. 4.2. Describe the internal appearance of the structures labelled X.

(ii) Explain how Fig. 4.2 shows that each of the structures labelled X are intracellular.

(c) Stem cells are found in the lining of the bronchi. Describe the function of centrioles and explain how they are involved in the cell cycle of a stem cell.

▶️ Answer/Explanation

Solution

(a)(i) A: Ciliated epithelial cell; B: Goblet cell.

Explanation: In Fig. 4.1, A is identified as a ciliated epithelial cell due to the presence of hair-like cilia, while B is a goblet cell, which secretes mucus to trap particles.

(a)(ii) The tissue is adapted for gas exchange by: (1) Goblet cells secreting mucus to trap pathogens, (2) Cilia moving mucus upward to clear airways, and (3) Maintaining a clean surface for efficient gas exchange.

Explanation: The mucus traps dust and microbes, while cilia sweep the mucus away from the lungs, ensuring unobstructed airflow.

(b)(i) The structures X (cilia) show a 9+2 microtubule arrangement—nine outer doublets and two central single microtubules.

Explanation: This arrangement is characteristic of cilia and flagella, providing structural support and enabling movement.

(b)(ii) Fig. 4.2 confirms X as intracellular because each cilium is enclosed by the cell membrane, indicating it is part of the cytoplasm.

Explanation: The membrane continuity in the micrograph shows the cilia are extensions of the cell, not external structures.

(c) Centrioles organize spindle fibers during cell division. In stem cells, they replicate in the S/G2 phase, move to poles in prophase, and regulate spindle formation for mitosis.

Explanation: Centrioles ensure accurate chromosome segregation by forming the mitotic spindle, critical for stem cell proliferation and tissue repair.

A level Biology 9.1 The gas exchange system – Exam style question – Paper 2

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