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Diffusion 3
Surface Area to Volume Ratio
Heat Loss & Metabolic Rate
Gas Exchange: Single Celled Organisms
Gas Exchange: Fish
Countercurrent Exchange Mechanisms
Gas Exchange: Insects
Gas Exchange: Leaves
Xerophytic Adaptations
Xerophytes Diagram
Here's a tip, generally the question wants a reference to either:
Surface area to volume ratio
The elements of Fick's law
Water potential
The actual passage of gases through the organism (as in first through a
spiracle, then tracheoles etc.…read more

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Remember Fick's law from Unit 1:
Animals have adapted gas exchange surfaces that provide the highest
rate of diffusion to meet their needs of respiration.…read more

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Surface Area to Volume Ratio
The size and surface area of an organism vastly affects the length of
the pathway the gases have to diffuse across.
The larger an organism is, the more cells gases have to diffuse across
to reach the inner cells. As well, larger organisms need more oxygen
to meet their respiratory needs.
calculate the Surface Area to Volume ratio of a cube, first find the
area of one side and multiply it by 6.…read more

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As well their demands for gases will be lower as they are smaller
Always relate questions about size of animals/heat loss to surface area
to volume ratio.
Heat Loss & Metabolic Rate
Metabolic rate is defined as: the rate at which chemical reactions take
place in the body. This is to say, the faster your body carries out
chemical reactions, such as respiration, the faster its metabolic rate.…read more

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Gas Exchange: Single Celled Organisms
These animals have no specialised gas exchange systems such as a
circulatory system; they survive on simple diffusion across their
plasma membrane.
They are able to function like that as they have a high surface area to
volume ratio and it meets their respiratory needs. All their cells are
only a short distance from the exchange surface.…read more

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Gas Exchange: Fish
Fish have a small surface area to volume ratio and therefore require a
specialised gas exchange system in the form of gills and blood.
In fish, water enters the buccal cavity and it is pushed across the gill
lamellae out of the operculum. This ventilates the gills and ensures
there is a constant flow of oxygenated water over the gills.
In relation to Fick's law:
A large surface area is maintained by 2 sets of filaments
attached to a gill arch.…read more

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The concentration gradient is provided by 2 factors. The
continuous flow of blood through the capillaries ensures blood
saturated with O2 is quickly removed. The other mechanism is
that water flows over
the capillaries in the
opposite direction of
blood in the
capillaries; this is
known as a
exchange mechanism
that will we look at in
more detail later.…read more

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As you can see, countercurrent mechanisms are more efficient as
there is continuous diffusion and most of the oxygen diffuses into the
blood, instead of the maximum saturation of 50% in parallel exchange.
It is best represented by this graph:
Gas Exch
Gas Exchange: Insects
Insects need to maintain a balance between efficient gas exchange
and reducing water loss. Because of this, their exoskeleton is made of
waterproof chitin, however now; they cannot use their body for gas
exchange.…read more


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