3.1-1 Exchange surfaces

• Exchange surfaces in organisms are crucial for the exchange of substances with the environment for survival
• Single-celled organisms like amoeba and Euglena have a large surface area-to-volume ratio, resulting in a short diffusion pathway for exchange
• Multicellular organisms have a smaller surface area-to-volume ratio, requiring specialised exchange surfaces like alveoli in the lungs, gills in fish, villi in the small intestine, and root hairs in plants
• Features of exchange surfaces include:

  • Large surface area
  • Thin barrier
  • Short diffusion distance
  • Fresh supply of molecules on one side to maintain high concentration
  • Removal of molecules on the other side to maintain a steep diffusion gradient
• Surface area is the part of a cell in contact with its surroundings, and as the volume of a cell increases, the surface area also increases, but not to the same extent, resulting in a smaller surface area to volume ratio as the cell gets bigger
• Calculating surface area to volume ratios:

  • Surface area: volume ratio decreases as the cube becomes larger
  • Animals in cold environments have a small surface area to volume ratio
  • Larger multicellular organisms and plants need specialised exchange surfaces
• Effect of surface-area-to-volume ratio on the rate of diffusion:

  • In single-celled organisms, substances can easily enter the cell due to a short distance to be crossed
  • Multicellular organisms have a larger distance for substances to cross due to a higher surface area to volume ratio
  • Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen
• Importance in living organisms:

  • Multicellular organisms need specialised exchange surfaces to facilitate efficient gas exchange due to their larger surface area to volume ratio
• Limitations of this experiment compared to living organisms:

  • The experiment may not fully replicate the complexity and adaptability of living organisms
  • Living organisms have additional factors and systems that influence gas exchange beyond just surface area to volume ratio
• Human respiratory system features:

  • Many alveoli provide a large surface area for gas exchange
  • Alveolus and capillary walls are only one cell thick, reducing the distance for diffusion
  • Alveolar/capillary walls are made of squamous epithelium, which is smooth and flat
  • Capillaries lie close to alveolar walls, ensuring a short distance for diffusion
  • Narrow capillaries also contribute to a short diffusion distance
  • Surfactant reduces cohesive forces
  • Blood from tissues is brought to the lungs to maintain a steep concentration gradient
  • Breathing ventilates the lungs to maintain a steep concentration gradient
• Function of:

  • Intercostal muscles: aid in breathing by expanding and contracting the ribcage
  • Pleural membranes: surround the lungs and reduce friction during breathing
  • Pleural fluid: lubricates the space between the pleural membranes
• Path taken by molecules of oxygen from air into the blood:

  • Oxygen enters through the respiratory system (nose or mouth)
  • Travels down the trachea, bronchi, bronchioles
  • Reaches the alveoli where gas exchange occurs with the blood in the capillaries
• Lung adaptations for efficient gas exchange:

  • Moist surface for easy gas dissolution
  • Good blood supply to maintain a steep concentration gradient
  • Thin walls for shorter distance for gases to travel by diffusion
  • Ventilation helps maintain a steep concentration gradient
• Achieving a steep diffusion gradient in the lungs:

  • Alveoli with thin walls and close capillaries ensure a short diffusion distance
  • Good blood supply and ventilation maintain a steep concentration gradient
• Dissection:

  • Identify structures like the trachea, bronchi, bronchioles, and pleural membrane
  • Cartilaginous rings in the trachea provide support and flexibility
  • Lung tissue floats in water, indicating it holds air even at rest
• Lung tissues:

  • Alveolar walls: made of squamous epithelial cells with capillaries for gas exchange
  • Elastic fibres: provide strength and flexibility for lung expansion and recoil
  • Smooth muscle: contracts to constrict airways, relaxes during exercise
  • Ciliated epithelial tissue: contains ciliated cells and goblet cells for mucus production
  • Cartilage: supports and keeps airways open
• Functions of lung tissues:

  • Alveolar walls: facilitate gas exchange
  • Elastic fibres: provide flexibility for lung expansion and recoil
  • Smooth muscle: controls airway diameter
  • Ciliated epithelium: traps pathogens and dust particles
  • Cartilage: supports and prevents airway collapse
• Distribution of tissues in the lungs:

  • Cartilage: trachea, bronchi
  • Smooth muscle: trachea, bronchi, larger bronchioles
  • Elastic fibres: trachea, bronchi, bronchioles, alveoli
  • Ciliated epithelium: trachea, bronchi, larger bronchioles
• How asbestos leads to disease:

  • Asbestos fibers are inhaled and cause scarring and inflammation in the lungs
  • This leads to conditions like asbestosis and mesothelioma
• Plenary:

  • Alveoli increase lung surface area for gas exchange
  • Gas exchange occurs by diffusion through thin/single layer of cells
  • Surfaces are moist for easy gas dissolution
  • Oxygen enters red blood cells via plasma to be transported away
• Each alveolus has its own capillary network to facilitate gas exchange between air and blood.
• Alveolar sacs are small, thin-walled structures that contain many alveoli.