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
- 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
- The gas exchange systems of multicellular organisms are adapted to increase the surface area available for the exchange of gases e.g.
- 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
- This is achieved by e.g.
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.