Gas exchange

• Gas exchange

  The movement of CO₂/O₂ across an exchange surface
• Gas exchange process

  1. CO₂/O₂ move by simple diffusion in opposite directions

  2. They can diffuse across cell membranes because they are small and non-polar (lipid soluble)
• Exchange surfaces adapted for gas exchange

  • Large surface area
  • Short diffusion pathway
  • Good blood supply
  • Ventilation mechanism
  • Permeable
  • Moist (O₂/CO₂ diffuse best in solution)
• Gas exchange in Amoeba

  • Gas exchange occurs over whole membrane creating a short diffusion distance
  • Aquatic so membranes are moist and thin
  • Unicellular organisms are small so have a large SA:Vol and requires little oxygen
• What are the issues for gas exchange in multicellular organisms?

  Larger organisms have a small SA:Vol so diffusion is slow due to larger diffusion pathways
• Why do simple organisms not have specialised gas exchange surfaces?

  They have a low metabolic rate so there isnt a high demand for oxygen and gases can diffuse directly across the skin membrane
• How are flatworms adapted for gas exchange?

  • Flattened shape which increases SA:VOL ratio
  • Always close to a surface which reduces the diffusion distance
• How are earthworms adapted for gas exchange?

  • Elongated shape due to a large SA:VOL ratio
  • Moist skin which Secretes mucus onto surface to allow gases to dissolve
  • Closed circulatory system allows concentration gradients to be maintained
• Closed circulatory system

  Allows oxygen and carbon dioxide to move in opposite directions to maintain concentration gradient
• Why do larger multicellular organisms need specialised gas exchange surfaces?

  They are larger so have a small SA:Vol and have a high metabolic rate so have a higher demand for oxygen.
• Gills in fish

  • Large surface area: Lots of gill filaments which each have lots of lamellae
  • Short diffusion pathway: Filaments and lamellae are made of squamous epithelium
  • Countercurrent flow: Maintains concentration gradient across whole gas exchange surface
• Human breathing system

  • Internal to keep the gas exchange surface moist and protected from damage
  • Trachea has C-shaped rings of cartilage to prevent collapse during pressure changes in ventilation and provides flexibility
  • Mucus traps dust and bacteria
  • Ciliated cells move mucus to the throat to be swallowed
• Pleural cavity

  • Made of 2 pleural membranes
  • Pleural fluid between membranes reduces friction during ventilation
• Alveoli gas exchange surface in mammals

  • Large surface area: Lots of tiny alveoli
  • Short diffusion distance: One cell thick
  • Good blood supply: Lots of capillaries
  • Surfactant reduces surface tension and prevents alveoli collapsing
• Insect gas exchange

  • Tracheal system with openings (spiracles) on the exoskeleton
  • Trachea have rings of chitin to prevent collapse
  • Tracheoles are fluid-filled to maintain concentration gradient
  • Water moves from tracheole into cell by osmosis when metabolically active
• Adaptations of insect gas exchange to reduce water loss

  • Exoskeleton is made of chitin so is waterproof
  • Spiracles can close to reduce water loss
  • Tiny hairs around spiracles trap humid air
• Types of Vertebrate

  • Amphibians (frogs, toads, newts)
  • Reptiles (crocodiles, lizards, snakes)
  • Birds
• Amphibian gas exchange

  • Tadpoles have gills
  • Inactive frogs use skin as exchange surface
  • Active frogs use lungs
• Reptile gas exchange

  • Lungs are surrounded by ribs for protection
  • Lungs have more complex internal structures like alveoli to increase surface area
• Bird gas exchange

  • Need more oxygen for flight
  • Lung ventilation is more efficient
  • Air sacs connected to lungs which act as bellows
  • No diaphragm because flight muscles ventilate the lungs
• Dicotyledon leaf structure

  • Large surface area to catch light
  • Bundle sheath (parenchyma)
  • Xylem transports water, phloem transports photosynthesis products
  • Significance for gas exchange and photosynthesis
• Leaf epidermis and mesophyll

  • Upper epidermis is transparent to allow light penetration
  • Palisade mesophyll is elongated to increase surface area and contain chloroplasts
  • Spongy mesophyll has large surface area and air spaces for gas exchange
  • Lower epidermis has stomata for gas exchange and to reduce water loss
• Stomata

  • Allow gas exchange
  • Controlled by guard cells that open and close the stomata
• Stomata opening and closing mechanism

  1. Guard cells expand unevenly when turgid

  thick inner wall is inelastic and thin outer wall is elastic
• Inhalation

  1. External intercostal muscles contract
  2. Ribs move up and out
  3. Diaphragm contacts, moves down and straightens
  4. Thoracic cavity volume increases
  5. Pressure inside decreases compared to the external environment
  6. Air is drawn in
• Exhalation

  1. External intercostal muscles relax
  2. Ribs move down and in
  3. Diaphragm relaxes, moves up and curves
  4. Thoracic cavity volume decreases
  5. Pressure inside increases compared to the external environment
  6. Air is forced out
• What is inhalation also known as?


  Negative pressure ventilation
• Adaptations for insect gas exchange

  • Large surface area due to lots of tracheoles
  • Short diffusion pathway as it is made of squamous epithelium
  • Moist because the end of the trachole is fluid filled
  • Maintains concentration gradients as oxygen diffuses in and carbon dioxide diffuses out
• Malate hypothesis

  • Photosynthesis ocurs in the chloroplast using light to form ATP
  • Potassium ions move into the cell by active transport against a concentration gradient using ATP
  • The presence of potassium ions causes starch to be converted into malate which is soluble
  • This causes the water potential to decrease
  • This causes water to move into the cell by osmosis, making the guard cell become turgid and open the stomata
• What happens when a insect is metabolically active?

  1. A lactic acid type substance builds up within the cell
  2. This reduces the water potential
  3. Water moves from the tracheole inot the cell by osmosis
  4. This draws more oxygen in as the fluid level has dropped in the tracheole
• Stages of Fish ventilation

  1. Mouth opens and operculum is closed
  2. Buccal cavity floor lowers
  3. Volume increases and pressure decreases inside the buccal cavity
  4. Water moves into the mouth down a pressure gradient
  5. Opercular cavity expands
  6. Buccal cavity floor raises
  7. Pressure inside buccal cavity is higher than the operculum
  8. Water moves from the buccal cavityover the gills and into the operculum
  9. Mouth closes and operculum opens
  10. Sides of the operculum move inwards, incraesing the pressure
  11. Water rushes out through the operculum