Adaptations of gas exchange surfaces in: single-celled organisms (across body surface); insects (tracheal system: tracheae, tracheoles, spiracles); fish (gill lamellae and filaments including counter-current principle); dicotyledonous plants (mesophyll and stomata)
Different organisms have evolved specialised structures to exchange oxygen and carbon dioxide efficiently. Each solution suits the organism's size, environment, and metabolic demands.
Real World
Salmon passing through the gills of a fish demonstrate the counter-current principle in action: deoxygenated blood flows opposite to oxygen-rich water, maintaining a diffusion gradient along the entire gill filament.
Exam Focus
For counter-current flow questions, always state that it maintains a concentration gradient along the whole length — not just at one point.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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