Gas Exchange in Organisms (SL IB Biology)

• Cellular respiration is a process occurring in all living cells that releases energy in the form of ATP
  • This energy is released when substrate molecules such as glucose is oxidised
  • Organisms use this energy to perform important life functions such as nutrition and excretion
• Aerobic respiration requires oxygen to occur and it produces carbon dioxide as a waste product
  • Living organisms acquire this oxygen from their environment and release carbon dioxide back into their surroundings
• The process by which these gases are exchanged between living organisms and their environment is called gas exchange
  • This includes oxygen uptake and the release of carbon dioxide by organisms
  • In plants, carbon dioxide will be absorbed and oxygen released during the day as a result of photosynthesis
• Gas exchange takes place by the process of diffusion, the rate of which is determined by the following factors:
  • Size of the respiratory surface - the bigger the surface, the higher the rate of diffusion
  • Concentration gradient
  • Diffusion distance - the shorter the distance, the higher the rate of diffusion
• Small, unicellular organisms such as amoeba have a large surface area compared to the volume of cytoplasm and a short diffusion distance
  • This means that the rate of diffusion is sufficient to supply the organism with enough oxygen to function

Single Celled Organism Diffusion Diagram

Small, unicellular organisms have a large surface area to volume ratio and a short diffusion distance to allow for effective gas exchange to occur

Challenges of gas exchange in organisms

• As an organism increases in size, the challenges of gas exchange become greater
• This is because an increase in size will result in a:
  • Smaller surface area to volume ratio
  • Greater diffusion distance
• Large, multicellular organisms therefore cannot rely on diffusion alone to supply every cell with oxygen
  • Another challenge is that the external surface of these organisms are designed to provide protection to the tissue underneath and is therefore not suitable as a respiratory surface
• The cells of large, active organisms will require more oxygen than smaller, less active organisms in order to meet their metabolic demands
  • These organisms will require specialised organs for gas exchange

• To maximise the rate of diffusion of oxygen and carbon dioxide, gas exchange surfaces require certain properties which include:
  • Permeability in order for gases to move across the surface
  • Thin tissue layer to create a short diffusion distance for oxygen and carbon dioxide
  • Presence of moisture so that gases can dissolve
    • This will facilitate the diffusion of gases across a gas exchange surface
  • Large surface area so that many gas molecules can diffuse across at the same time

• A steep concentration gradient will ensure a high diffusion rate across a gas exchange surface
  • In organisms, this will allow the diffusion of oxygen into the body and the diffusion of carbon dioxide out of the body
• These concentration gradients are maintained in the following ways:
  • A dense network of blood vessels to provide a large surface area for the diffusion of gases
    • Blood provides a good transport medium for both oxygen and carbon dioxide
  • A continuous blood flow in the blood vessels to ensure that oxygen is constantly transported away from the gas exchange surface and carbon dioxide towards them
    • This ensures that oxygen will always diffuse into the blood and carbon dioxide out of the blood in the lungs
  • Ventilation with air in lungs and water in gills to bring oxygen close to the gas exchange surface and to remove carbon dioxide

Alveolus Diagram

The alveolus is the gas exchange surface in humans where a concentration gradient for oxygen and carbon dioxide is maintained