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B2.3.8 (HL)—Pneumocytes in alveoli

Alveolar pneumocytes are specialized so type I cells support gas exchange while type II cells secrete surfactant to prevent adhesion.

Syllabus
First assessment 2025
Objective
B2.3.8
Level
HL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Outline

Scoring notes

Common mistake
Reversing the roles of type I and type II pneumocytes.

Recent exam appearances

May 2025Paper1A ["HL"] · TZ115[ 1 ]B2.3.8 (HL)—Pneumocytes in alveoli
May 2024Paper1 ["HL"] · TZ122[ 1 ]B2.3.8 (HL)—Pneumocytes in alveoli
May 2023Paper1 ["HL"] · TZ122[ 1 ]B2.3.8 (HL)—Pneumocytes in alveoli
May 2022Paper1 ["HL"] · TZ123[ 1 ]B2.3.8 (HL)—Pneumocytes in alveoli
May 2019Paper1 ["HL"] · TZ123[ 1 ]B2.3.8 (HL)—Pneumocytes in alveoli
Practice this objective

Coverage 2017–2025 · Updated 15 Jul 2026

Two Pneumocytes Solve Different Alveolar Problems

HL only

Alveolar epithelium contains complementary specialized cells: extremely thin type I pneumocytes form the main diffusion surface, while type II pneumocytes release surfactant.

The attenuated cytoplasm of type I cells minimizes the distance for oxygen and carbon dioxide diffusion. Type II cells contain many secretory vesicles called lamellar bodies, which discharge surfactant into the alveolar lumen.

Surfactant reduces surface tension at the moist alveolar surface, helping prevent collapse and lowering the work needed to reinflate alveoli. Different adaptations require two cell types within the same functional tissue.

During exhalation an alveolus becomes smaller, but surfactant reduces the inward pull of its water lining; at the same time, the very thin type I layer preserves a short gas-diffusion path.

Type II cells are not the principal gas-exchange surface, and type I cells do not secrete most surfactant. Their specializations are complementary rather than interchangeable.

Pneumocytes in alveoli

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Outline.

Command terms

Identify / Outline

What earns marks

Build the answer around this relationship: Type I pneumocytes are thin cells specialized for gas exchange.

Watch for

Reversing the roles of type I and type II pneumocytes.

Representative question

Question 1

[Maximum number: 2]

Outline the function of pneumocytes in the lungs.

Link Specialized Cells To Function

HL only

A strong answer identifies the cell, names the structural adaptation, and states the function it improves. Erythrocytes and PCT cells show exchange and transport adaptations; pneumocytes show diffusion versus secretion; muscles show contraction coordination; gametes show fertilization roles.

  • For exchange, use biconcave shape, microvilli, thin pneumocytes, or surfactant when appropriate.
  • For contraction, use intercalated discs/gap junctions in cardiac muscle and multinucleate myofibril-packed skeletal fibres.
  • For fertilization, use acrosome, midpiece mitochondria, flagellum, nutrient-rich oocyte, zona pellucida, and cortical granules.

Concept essentials

  • Type I pneumocytes are thin cells specialized for gas exchange.
  • Type II pneumocytes secrete pulmonary surfactant.
  • Surfactant helps prevent alveolar walls from sticking together.
  • Phagocytes in alveoli ingest pathogens rather than producing antibodies.