Gaseous exchange

In the syllabus and the exam

Gaseous exchange is the umbrella concept covering how oxygen moves into an organism and carbon dioxide moves out across a respiratory surface, always by diffusion down a concentration gradient. Across the syllabus this term links the alveolus in humans, the gills in fish, the tracheal system in insects and the stomata plus intercellular spaces in leaves, so exam questions often compare two respiratory surfaces and ask candidates to state shared features that make gaseous exchange efficient, namely a large surface area, a thin wall or membrane, moisture, and a good blood or gas supply maintaining a steep concentration gradient. A frequent slip is confusing gaseous exchange with breathing or ventilation; breathing is the mechanical movement of air into and out of the lungs, whereas gaseous exchange is the actual diffusion of gases between air and blood, and SPM markers expect these two terms to be used precisely and not interchangeably.

Example

Fish rely on gaseous exchange across their gills, while humans rely on gaseous exchange across the alveoli of the lungs.

Related terms

AlveolusDiffusionAerobic respiration

Source:SRC-DSKP-EN