Gas Exchange in Organisms
- 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
Examiner Tip
Make sure that you do not confuse respiration and gas exchange with each other. Respiration is a chemical process occurring in all living cells while gas exchange refers to the diffusion of oxygen and carbon dioxide across a respiratory surface.