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Gas Exchange Revision Pack



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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

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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.

The equation for aerobic respiration is:

C6H12O6 + 6O2 -> 6H2O + 6CO2 + ATP

The four different gas exchange surfaces we will look…

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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…

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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…

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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.…

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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…

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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

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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…


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