Biology WJEC BY2 2.2 Gas exchange

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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 area to volume ratio is much less than in very
small organisms.
Surface Area to Volume Ratio (SA:Vol): The larger the animal, the SMALLER the SA:Vol. The
smaller the animal, the LARGER the SA:Vol e.g. An elephant has a larger SA than a mouse, BUT the
mouse has a larger SA relative to its volume, compared to the elephant!
This relationship can be modelled using cubes as animals:
Cube 1 Cube 2
1cm 2cm
SA of 1 side = 1 x 1 = 1cm2 SA of 1 side = 2 x 2 = 4cm2
SA of 6 sides = 6 x 1 = 6cm2 SA of 6 sides = 6 x 4 = 24cm2
Vol = 1 x 1 x 1 = 1cm3 Vol = 2 x 2 x 2 = 8cm3
SA:Vol = 6:1 SA:Vol Ratio = 24:8
SA:Vol = 3:1
Larger SA:Vol Smaller SA:Vol
If asked to calculate SA:Vol, the volume must ALWAYS be `1' i.e. 1.5:1 and not 3:2, so that the
organism can be compared.

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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 metabolic rate
Amoeba's size and lifestyle in water enables diffusion to supply its needs.
Flatworms have flattened bodies, typically only 0.2 mm thick which decreases the diffusion
distance.…read more

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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).…read more

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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 quickly :
PARALLEL FLOW- blood flows in the same direction as water ­ equilibrium never reached
(cartilaginous fish i.e.…read more

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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 cavity to opercular cavity
Fish closes mouth
Muscles in buccal cavity relax
Buccal cavity floor raises
Buccal cavity decreases in volume and so increases in pressure
Pressure now higher than in the opercular cavity so water…read more

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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.…read more

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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
epithelial cells into guard cells
The potassium ions stimulate enzymes to convert starch to malate
The increase of potassium ions and malate in the guard cells lowers the water potential
And so water moves in…read more

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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 microscopic air sacs called alveoli provide a large SA for gas exchange
o Blood flow through capillary network around each of the Alveolus maintains the steep
concentration gradient, by bringing low O2 concentration blood to…read more

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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 moves down and in
These actions:
Decrease the volume of the pleural cavity
Increase pressure inside lungs
So air moves down a pressure gradient and goes from
inside the lungs to the outside
N.B.…read more


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