2.2

• Surface area to volume ratio


  • Large
  • Moist to allow gases to dissolve
  • Thin to provide a short diffusion distance
  • Permeable to gases
• In single-celled organisms, e.g. Amoeba, the surface area is large enough to meet the needs of the organism and therefore materials can be exchanged directly across its thin and permeable cell surface membrane
• As the cytoplasm is constantly moving, the concentration gradient is always maintained
• In larger organisms the surface area to volume ratio decreases, so diffusion across the body surface is insufficient to meet the needs of the organism
• Adaptations of larger organisms


  • A more specialised gas exchange surface
  • A ventilation mechanism that ensures the concentration gradient is maintained across the respiratory surface
• One consequence of maintaining a moist respiratory surface in terrestrial animals is water loss: this is minimised by having internal gas exchange surfaces, called lungs
• Organism adaptations


  • Flatworm: Flattened body to reduce diffusion distance and increase surface area
  • Earthworm: Secretes mucus to maintain moist surface, well developed capillary network
  • Amphibians: Low metabolic rate, network of blood vessels and haemoglobin
  • Reptiles: Moist and permeable skin, well developed capillary network, internal lungs
  • Birds: Efficient ventilation mechanism to increase concentration gradient across lung surface
• Gas exchange in cartilaginous fish


  • Blood and water flow in the same direction (parallel flow)
  • Gas exchange only possible over part of gill filament surface as equilibrium is reached
  • Ventilation mechanism is basic - open mouth to allow water to pass over gills
• Gas exchange in bony fish


  • Blood and water flow in opposite directions (counter-current flow)
  • More efficient system as there is always a higher concentration of oxygen in the water than in the blood
  • More advanced ventilation mechanism
• Human respiratory system


  1. Trachea branches into bronchi, which branch into bronchioles, ending in alveoli

  2. Inspiration (active): External intercostal muscles contract, diaphragm contracts, alveolar pressure decreases, air drawn in

  3. Expiration (passive): External intercostal muscles relax, diaphragm relaxes, alveolar pressure increases, air forced out
• Alveoli adaptations


  • Very large surface area (700 million)
  • Very thin walls (0.1μm)
  • Surrounded by capillaries for short diffusion distance and good blood supply
  • Moist lining
  • Permeable to gases
  • Collagen and elastic fibres allow expansion and recoil
• The alveoli produce a surfactant, which lowers the surface tension preventing the alveoli from collapsing and sticking together, and allows gases to dissolve
• Gas exchange in insects


  • Branched, chitin-lined system of tracheae with openings called spiracles
  • Spiracles can close during inactivity, chitin helps reduce water loss
  • Tracheole tubes come into direct contact with every tissue, supplying oxygen and removing carbon dioxide, no need for haemoglobin
  • Rhythmical movements of muscles in thorax and abdomen ventilate the tracheole tubes, maintaining a concentration gradient
• Gas exchange in plants


  • Obtain oxygen for respiration and carbon dioxide for photosynthesis by diffusion through leaves
  • Waxy cuticle covers leaf surface to reduce water loss
  • Pores called stomata on underside of leaves can open during day to allow gas exchange, close at night or during drought to reduce water loss
• Stomatal opening mechanism


  1. Guard cells photosynthesize, producing ATP

  2. ATP energy used to actively transport potassium ions into guard cells

  3. This triggers starch to malate ion conversion, lowering water potential so water enters by osmosis

  4. Guard cells expand, creating pore between them

  5. Reverse happens at night
• Leaves are thin and flat providing a large surface area to capture light and for gas exchange
• Leaves have many pores called stomata to allow exchange of gases
• Spongy mesophyll cells are surrounded by air spaces that allow gases to diffuse