• 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

• 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