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WJEC 2.2 ­ Adaptations for gas exchange

Living things need to obtain materials such as carbon dioxide and oxygen from the environment
and remove waste from their cells to the environment.

Requirements may be proportional to volume however diffusion is proportional to surface
area, therefore in large organisms the surface…

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Small Organisms

In small, unicellular organisms (rarely more than 500nm) their external surface area (membrane)
is used for gas exchange, and don't require a specialised gas exchange system because:

o Large SA:Vol ratio

o Short diffusion pathway

o Diffusion alone is able to supply sufficient quantities

o Have a low…

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Earthworms are multicellular, terrestrial animals restricted to damp areas, because they
have a large moist body surface for diffusion (this is a region of potential water loss). They
are adapted for gas exchange by:
Having a large SA:Vol ratio

With a closed circulatory system and blood pigments (Haemoglobin)

Moist skin…

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Water is a dense medium with a low oxygen content so must be forced over the gill
filaments by pressure differences to maintain continuous, unidirectional flow of water.

COUNTER CURRENT MECHANISM - Water flows in the opposite direction to the blood
which is very efficient as equilibrium is reached more…

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1) Water flows into Buccal Cavity
Fish opens mouth
Muscles in buccal cavity floor contract
Buccal cavity floor lowered
Buccal cavity increases in volume and so decreases in pressure
Water flows from high pressure (out of buccal cavity) to lower pressure (in the buccal

2) Water flows from buccal…

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Tracheole tubes are the gas exchange surface which come into contact with every tissue
Muscles in thorax and abdomen contract and relax causing rhythmical movements which
ventilate the tracheole tubes and so maintain a concentration gradient.

Advantages:- +Every tissue directly supplied with oxygen

+No haemoglobin

+Reduced water loss



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Allow water and gas exchange to pass in and out of plant and control these movements by
the guard cells changing shape, thus opening and closing stomata

Guard cells photosynthesise and then respire and release ATP
The ATP is used in active transport to pump potassium ions from surrounding…

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Bronchi To carry air to and from each lung

Bronchioles Small passageways to alveoli

Alveoli Respiratory gas exchange surface

Pleural Reduces friction

Ribs and Alters size of pleural cavity to change
intercostal volume/pressure

Diaphragm Alters size of pleural cavity to change volume/pressure

Exchange of gases

o Millions of…

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The diaphragm moves
down/the rib cage moves
up and out
These actions:

Increase the volume of the
pleural cavity
Decrease pressure inside
Air moves down a pressure
gradient from outside into
the lungs

o Expiration
The diaphragm muscle and external intercostal muscles relax
The diaphragm muscle moves up/the ribcage…


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