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2.2 Adaptations for gas exchange
1. Living things need to obtain materials such as carbon dioxide and oxygen from the environment and
remove waste from their cells to the environment.
Organisms are differently adapted so gas exchange can take place whether it be in
water or on land. In particular oxygen is needed to convert organic molecules into
energy through the process of respiration.
NB: Gas exchange is the process by which oxygen reaches cells and waste products
are removed, don't confuse with respiration which is energy production in cells.
2. Requirements may be proportional to volume however diffusion is proportional to surface area
3. In large organisms the surface area to volume ratio is much less than in very small organisms.
As organisms get larger their volume to surface area increases meaning they cannot
rely on diffusion alone as the diffusion path would be too long.
4. In small, unicellular organisms the surface area to volume ratio is so large that diffusion
through the body surface is sufficient to supply their needs.
An example of this is an amoeba where the cell
membrane acts as the exchange surface, It is thin and
moist so is efficient at its job.
5. Also, distances within the body are small and transport by
diffusion is again sufficient, e.g. Amoeba,
its size and lifestyle in water enables diffusion to supply its
needs. The cell membrane has a large diffusion path
and large surface area to volume ratio.
6. Larger, multicellular organisms may have a surface area to volume ratio which is too small
to supply all their needs.
Multicellular organisms are an aggregation of cells. Cells aggregate together to
increase their size but decreases their surface area to volume ratio and the diffusion
path increases also materials are needed to be exchanged between different organs as
well as the organism and the environment. So diffusion is no longer a viable process of
7. These organisms therefore possess special surfaces for gaseous exchange, gills for
aquatic environments, lungs for terrestrial environments.
Gas exchange surfaces such as the gills of a fish, the alveoli in the lungs of a mammal, the
trachae of an insect and the spongy mesophyll cells in the leaves of a plant are effective
8. These exchange surfaces have particular properties to aid diffusion: large surface area
thin, moist, permeable surface.
In order to achieve the maximum rate of diffusion a respiratory surface must:
have a large surface area compared to volume
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The large moist area for gaseous exchange is a region of potential water loss.
10. Earthworms are multicellular, terrestrial animals restricted to damp areas. A moist body
surface for diffusion, with a circulatory system and blood pigments, increase efficiency of
gaseous exchange sufficient for a slow moving animal.
Simple multicellular animals such as worms have a low oxygen requirement as they
have an extremely low metabolic rate as they move very slowly.…read more
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Fish can be further classified into two groups according to the material that makes up their
Skeletons are made entirely of cartilage.
Normally live in the sea e.g. shark
Characterised by five gill clefts that
open at five gill slits.
Gas exchange involves parallel flow.
Therefore blood travels through the capillaries in the same direction as the sea water.
This is relatively inefficient as a diffusion gradient is not maintained.…read more
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Compared with parallel flow, counter current flow increases efficiency because the
diffusion gradient between the adjacent flows is maintained over the whole surface.
Blood flows between the gill plates under pressure in the opposite direction the blood in the
capillaries. The blood always meets water with higher oxygen content than itself. It removes
80% of the oxygen from the water. This high level of extraction is important as there is 25%
less oxygen in water than in air.