The big idea: The wall of an alveolus has two kinds of cell: thin type I pneumocytes for diffusion and type II pneumocytes that make surfactant.
The points to remember
- Type I pneumocytes are extremely thin and flattened, and cover most of the alveolar wall.
- Their thinness makes the diffusion distance for oxygen and carbon dioxide very short.
- Type II pneumocytes are rounded cells with many secretory vesicles called lamellar bodies.
- Lamellar bodies discharge surfactant into the alveolus, where it forms a thin film on the inner surface.
- The alveolar epithelium is one tissue with two cell types, because its job needs different adaptations.
Remember it as: Type I is thin for diffusion; type II makes surfactant.
Real example: babies born early: Type II pneumocytes start making enough surfactant only late in pregnancy, so babies born very early often struggle to breathe and may be given surfactant through a tube.
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How it is asked: Expect a Paper 1A item matching each type of pneumocyte to its adaptation, often on a micrograph, or a short Paper 2 Explain or Outline.
IB-style questionExplain[3 marks]
Explain why the alveolar epithelium contains two types of pneumocyte.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.