Properties of Gas Exchange Surfaces (Edexcel A (SNAB) A Level Biology)

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Properties of Gas Exchange Surfaces

  • All organisms need to exchange gases with their environment, e.g.
    • Aerobic respiration requires oxygen and produces carbon dioxide as a waste product
    • Photosynthesis requires carbon dioxide and produces oxygen as a waste product
  • The process of gas exchange occurs by diffusion
  • The surface over which this gas exchange takes place is known as an exchange surface; exchange surfaces have specific properties that enable efficient exchange to take place

Surface area to volume ratio

  • The surface area of an organism refers to the total area of the organism that is exposed to the external environment
  • The volume refers to the total internal volume of the organism, or total amount of space inside the organism
  • The surface area of an organism in relation to its volume is referred to as an organism's surface area : volume ratio (SA:V ratio)
  • As the overall size of the organism increases, the surface area becomes smaller in comparison to the organism's volume, and the organism's surface area: volume ratio decreases
    • This is because volume increases much more rapidly than surface area as size increases

11. Principles of surface area _ volume

  • Single-celled organisms have a high SA:V ratio which allows the exchange of substances to occur by simple diffusion
    • The large surface area allows for maximum absorption of nutrients and gases and removal of waste products
    • The small volume within the cell means the diffusion distance to all organelles is short
  • As organisms increase in size their SA:V ratio decreases
    • There is less surface area for the absorption of nutrients and gases and removal of waste products in relation to the volume, and therefore requirements, of the organism
    • The greater volume results in a longer diffusion distance to the cells and tissues of the organism
  • Large multicellular organisms have evolved adaptations to facilitate the exchange of substances with their environment
    • The gas exchange systems of multicellular organisms are adapted to increase the surface area available for the exchange of gases e.g.
      • Alveoli increase the surface area of mammalian lungs
      • Fish gills have structures called lamellae which provide a very large surface area
      • Leaves have a spongy mesophyll layer within which a large area of leaf cell surface is exposed to the air
  • Note that the problem of internal diffusion distance is a separate, though connected, issue solved by the presence of a mass transport system such as a circulatory system

Diffusion pathway

  • The diffusion pathway, or distance, across an exchange surface is very short
  • The surface often contains only one layer of epithelial cells
    • The cells can also be flattened in shape to further reduce the distance across them
  • This means that substances have a very short diffusion pathway

Concentration gradient

  • This is the difference in concentration of the exchange substances on either side of the exchange surface, e.g. between the air inside the alveoli and the blood
  • A greater difference in concentration means a greater rate of diffusion as the gas molecules move across the exchange surface
  • The continued movement of exchange substances away from the exchange surface mean that a concentration gradient is maintained
    • This is achieved by e.g.
      • The alveoli have a good blood supply; this constantly removes oxygen from the capillary side of the exchange surface and supplies carbon dioxide
      • The ventilation system in mammals ensures constant inhalation and exhalation; this supplies oxygen and removes carbon dioxide from the alveoli side of the exchange surface

Examiner Tip

Be careful when discussing surface area; the phrases 'surface area' and 'surface area : volume ratio' cannot be used interchangeably. Larger organisms have a larger surface area than smaller ones (an elephant clearly has a larger surface area than a bacterial cell), but it is the surface area : volume ratio that gets smaller as body size increases.

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding