Explain how the ultrastructure of a type II pneumocyte (alveolar cell) is adapted for the production and secretion of pulmonary surfactant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Numerous mitochondria supply ATP for the energy‑demanding synthesis and secretion of surfactant components.
An extensive rough endoplasmic reticulum, studded with ribosomes, synthesises the surfactant‑associated proteins.
A prominent Golgi apparatus processes and packages these proteins together with phospholipids into secretory vesicles known as lamellar bodies.
Lamellar bodies, specialised secretory organelles, fuse with the plasma membrane by exocytosis, releasing surfactant into the alveolar fluid to reduce surface tension.
An extensive rough endoplasmic reticulum, studded with ribosomes, synthesises the surfactant‑associated proteins.
A prominent Golgi apparatus processes and packages these proteins together with phospholipids into secretory vesicles known as lamellar bodies.
Lamellar bodies, specialised secretory organelles, fuse with the plasma membrane by exocytosis, releasing surfactant into the alveolar fluid to reduce surface tension.
Examiner tips
- Use the exact terms – mitochondria, RER, Golgi, lamellar bodies, exocytosis.
- Show the flow from synthesis to secretion – energy, protein synthesis, processing, vesicle release.
- Mention surfactant’s role (reduce surface tension).
Common mistakes
- Confusing lamellar bodies with lysosomes; they are specialised secretory organelles, not degradative. Omitting the role of mitochondria or the Golgi apparatus. Using vague phrases like "lots of organelles" instead of specific names.
Mark scheme (4 marks)
- Numerous mitochondria provide ATP for the energy-demanding synthesis and secretion of surfactant components (phospholipids and proteins).
- An extensive rough endoplasmic reticulum (RER) allows large-scale synthesis of surfactant-associated proteins on membrane-bound ribosomes.
- A prominent Golgi apparatus processes and packages surfactant components (proteins and phospholipids) into secretory vesicles called lamellar bodies.
- Lamellar bodies (specialised lysosomes / secretory organelles) fuse with the plasma membrane by exocytosis, releasing surfactant into the alveolar fluid to reduce surface tension.
Key terms in this question
type II pneumocyte · pulmonary surfactant
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cell specialisation questions
- Explain how the ultrastructure of a pancreatic acinar cell is adapted for the se…
- Explain how the ultrastructure of a myelinating Schwann cell is adapted for its …
- Explain how the ultrastructure of a goblet cell in the intestinal epithelium is …
- Explain how the ultrastructure of a proximal convoluted tubule (PCT) cell in the…
- Explain how the ultrastructure of a hepatocyte (liver cell) is adapted for its r…
- Explain how the ultrastructure of a cardiac muscle cell (cardiomyocyte) is adapt…