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Single-celled
organisms
They have just a single cell-surface membrane.
This means diffusion can take place quickly and
easily.
Some have a cell wall, but it is completely
permeable and does not affect diffusion at all.
This means that they do not need any
specialised gas-exchange surface.…read more

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Insects
They are terrestrial organisms and so water evaporation can be a problem.
Therefore they have a special system called tracheae.
This is a network of tubes which branch off into tracheoles.
They extend to the tissues, and so they take oxygen to the tissues and take
carbon dioxide away from the tissues.
A steep concentration gradient is maintained within them.
They have spiracles which are the ends of the tracheoles that can be
opened and closed to prevent water loss.
They move muscles in their bodies in order to ventilate.…read more

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Fish
Fish have an impermeable body, so they exchange gas through their gills.
The gills are located behind the head of the fish.
They are made from gill filament, which are like stacked piles.
The gill lamella lies at a right angle to the filament.
Fish take in water through their mouth and force it over their gills. They
remove the oxygen from the water via diffusion and the water leaves the
fish via the slits in the sides of their head.
The oxygen diffuses into the blood via concurrent flow, which allows for a
constant diffusion gradient to be maintained.
Concurrent flow is where the blood flows in the opposite direction to the
water, and so there is always more oxygen in the water than in the blood,
so diffusion continues.…read more

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Plants
Carbon dioxide is needed for photosynthesis and produced in respiration.
Oxygen is needed for respiration and produced in photosynthesis.
However gas exchange is still needed because the products of each
reaction are not always enough.
Plants are very thin, so a lot of oxygen and carbon dioxide can diffuse
straight in.
They also have pores called stomata, which open and close to allow gases
in and out.
Opening the stomata can lead to water loss, so the plants will only open
them occasionally.
They have a lot of air spaces to allow gases in and out easily.…read more

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Mammals
Mammals have a small surface area to volume ratio, so they
need a specialised surface.
They have lungs which have a large surface area to volume ratio
through tiny alveoli.
The diffusion pathway is also very thin to allow quick diffusion of
gases.
The lungs also have movement of the internal and external
medium which maintains a concentration gradient.
The lungs are located inside their bodies to prevent evaporation
by water loss.…read more

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