1. Key Definitions

• Respiration: the oxidation of organic molecules (glucose, fat, protein) to release energy as ATP; aerobic (with O₂) or anaerobic (without O₂). Carbohydrate is the most common substrate.
• Gaseous exchange: the movement of respiratory gases (O₂ and CO₂) across a respiratory surface.
• Respiratory surface: an area through which respiratory gases enter/leave, e.g. alveoli, gills, cell membrane.
• Respiratory medium: an environment (liquid or gas) containing the respiratory gases.

Properties of a Good Respiratory Surface

• Thin (short diffusion distance); moist (gases dissolve); large surface area to volume ratio; highly vascularised; permeable, maintaining a steep diffusion gradient.

Why Multicellular Organisms Need Specialised Surfaces

• As animals increase in size, surface area to volume ratio decreases; diffusion alone cannot supply every cell, and higher metabolic rates demand more oxygen — so specialised respiratory surfaces evolve.

Respiratory Pigments

• Coloured molecules that reversibly bind oxygen (a coloured non-protein group linked to a protein).