EXCHANGE SURFACES
Organisms exchange substances with their environment
Oxygen and carbon dioxide are transferred between cells and the environment during gas exchange
In humans – urea (waste product produced from the breakdown of proteins) diffuses from cells into the
blood plasma for removal from the body by the kidneys
Surface area to volume ratio
How easy it is for an organism to exchange substances
with the environment depends on its surface area to SA of each side = 9mm2
volume ratio (SA: V) (x6 = 54mm2)
SA of each side = 1mm2 Vol = 27mm3
(x6 = 6mm2) The surface area to
Vol = 1mm3 volume ratio is 2:1
The surface area to
volume ratio is 6:1
Multicellular organisms need exchange surfaces Adaptations of exchange surfaces
In single-celled organisms gases and dissolved substances can diffuse Thin membrane – short distance
directly into/out of the cell across the membrane – large surface area Large surface area – more substances at
compared to their volume – enough substances can be exchanged the same time
across the membrane to support the whole volume of the cell In animals, lots of blood vessels – into and
In multi-cellular organisms their surface area is not large enough to out of the blood quickly
support the volume – cells on the outside can get oxygen by diffusion In animals, gas exchange surfaces are
but cells in the organism does not get enough as they are too far away usually ventilated – alveoli – air moves in
from the surface and out
This means they need an exchange surface
EXCHANGING SUBSTANCES
Alveoli – specialised for the diffusion of O2 and CO2 Inside of the small intestines is
Large surface area covered in villi – increase surface area
Moist lining for dissolving gases so food is absorbed in the blood faster
Thin walls Single layer of surface cells
MORE ON EXCHANGING SUBSTANCES
Good blood supply Good blood supply
Leaves
Stomata controlled by the guard cells open and close (when water is low) to exchange oxygen, carbon
dioxide and water vapour – this is its exchange surface
Water vapor escapes from the cells inside the leaf – then escapes by diffusion as there’s less in the air
Adaptations
Flat – increases area of exchange surface – more effective
The walls of the cells inside the leaf form another exchange surface – the air spaces inside the lead
increase the area of this surface so there’s more chance for carbon dioxide to get into the cells
Gills
Gills are the gas exchange surface in fish – water (with oxygen) enters the fish through the mouth and
passes out through the gills – oxygen diffuses from the water into the blood in gills and carbon dioxide out
Each gill is made up of lots of thin plates called gill filaments – big surface area for gas exchange – also
covered in tiny structures called lamellae – increase surface area even more
Lamellae – lots of blood capillaries to speed up diffusion, thin surface layer of cells to minimise distance
Blood flows through lamellae in one direction and water flows over the opposite – maintains a large
concentration gradient between water and blood
Concentration of oxygen is always higher than that in the blood, so as much oxygen as possible diffuses in
Organisms exchange substances with their environment
Oxygen and carbon dioxide are transferred between cells and the environment during gas exchange
In humans – urea (waste product produced from the breakdown of proteins) diffuses from cells into the
blood plasma for removal from the body by the kidneys
Surface area to volume ratio
How easy it is for an organism to exchange substances
with the environment depends on its surface area to SA of each side = 9mm2
volume ratio (SA: V) (x6 = 54mm2)
SA of each side = 1mm2 Vol = 27mm3
(x6 = 6mm2) The surface area to
Vol = 1mm3 volume ratio is 2:1
The surface area to
volume ratio is 6:1
Multicellular organisms need exchange surfaces Adaptations of exchange surfaces
In single-celled organisms gases and dissolved substances can diffuse Thin membrane – short distance
directly into/out of the cell across the membrane – large surface area Large surface area – more substances at
compared to their volume – enough substances can be exchanged the same time
across the membrane to support the whole volume of the cell In animals, lots of blood vessels – into and
In multi-cellular organisms their surface area is not large enough to out of the blood quickly
support the volume – cells on the outside can get oxygen by diffusion In animals, gas exchange surfaces are
but cells in the organism does not get enough as they are too far away usually ventilated – alveoli – air moves in
from the surface and out
This means they need an exchange surface
EXCHANGING SUBSTANCES
Alveoli – specialised for the diffusion of O2 and CO2 Inside of the small intestines is
Large surface area covered in villi – increase surface area
Moist lining for dissolving gases so food is absorbed in the blood faster
Thin walls Single layer of surface cells
MORE ON EXCHANGING SUBSTANCES
Good blood supply Good blood supply
Leaves
Stomata controlled by the guard cells open and close (when water is low) to exchange oxygen, carbon
dioxide and water vapour – this is its exchange surface
Water vapor escapes from the cells inside the leaf – then escapes by diffusion as there’s less in the air
Adaptations
Flat – increases area of exchange surface – more effective
The walls of the cells inside the leaf form another exchange surface – the air spaces inside the lead
increase the area of this surface so there’s more chance for carbon dioxide to get into the cells
Gills
Gills are the gas exchange surface in fish – water (with oxygen) enters the fish through the mouth and
passes out through the gills – oxygen diffuses from the water into the blood in gills and carbon dioxide out
Each gill is made up of lots of thin plates called gill filaments – big surface area for gas exchange – also
covered in tiny structures called lamellae – increase surface area even more
Lamellae – lots of blood capillaries to speed up diffusion, thin surface layer of cells to minimise distance
Blood flows through lamellae in one direction and water flows over the opposite – maintains a large
concentration gradient between water and blood
Concentration of oxygen is always higher than that in the blood, so as much oxygen as possible diffuses in
