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Exchanging Materials
The general term for when any dissolved substance moves randomly along a
concentration gradient, from high to low across a semi-permeable membrane. This requires
minimal energy.
A specific type of diffusion regarding the movement of water molecules. In all other
aspects, it is exactly as explained for diffusion.
Active Transport:
Sometimes it is necessary for substances to absorbed against a concentration gradient,
so low to high. This requires energy from respiration...
The mitochondria in cells respire to produce energy. The energy is then used so that the
proteins can `carry' the substrate across the membrane.
Sugar and ions, which can pass though cell membranes can also be moved by active
In plants, root hair cells use active transport to gain nitrate ions from the soil (where
concentration is low) and in humans, sugars can be absorbed from the intestine and from
the kidney tubules.…read more

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Exchanging Materials
Exchanging materials in Humans:
Humans have organs which are specialised to help the exchange of material...
Villi in the Small Intestine:
Villi line the walls of your small intestine. Products of digestion are absorbed by both
diffusion and active transport. To increase optimum exchange rate they have a large
surface area, a thin cell wall (one cell thick), a film of moisture and a good blood
supply (network of capillaries).
Thin cell
Network of capillaries
Micro-villi to increase surface area
Blood vessels…read more

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Exchanging Materials
The Breathing System:
The breathing system, which involves the heart and the lungs, takes air into and out
of your body. The main section of your body (above waist, excluding arms and head
of course!); the thorax and abdomen, is split by a thin muscular sheet called the
diaphragm. The thorax above contains the heart and lungs and is protected by the
ribcage. All that you breath in reaches the lungs through the trachea (windpipe),
which has rings of cartilage to prevent it from collapsing. It divides into two tubes
called bronchi and again into many bronchioles.
The lungs
Ribcage Bronchioles…read more

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Exchanging Materials
Alveoli in the Lungs:
The bronchioles continue to divide until they end in air sacs (alveolar sacs) called
alveoli. They are made efficient in the exchange of carbon dioxide and oxygen by a
large surface area, a film of moisture, an excellent blood supply and thin cell walls
(one cell thick).
CO2 diffuses from your blood to your alveoli and O2 diffuses from your alveoli into
your blood. So, your blood swaps carbon dioxide for oxygen to become oxygenated.
Deoxygenated blood
A Single Alveolus and A
Oxygenated blood…read more

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Exchanging Materials
Exchanging Materials in Plants:
Leaves are broad, thin and flat with lots of internal air space which provides a large
surface area, making them efficient for photosynthesis. They have stomata on their
under-surface in order to let carbon dioxide in and oxygen out (by diffusion). This
process in plants is reversed in human respiration.
Photosynthesis also leads to a loss of water vapour in a process called transpiration.
Water loss is inevitable if the plant is to photosynthesise and transpiration in quicker
in hot, dry, windy conditions.
Water vapour is lost in the internal leaf cells by evaporation in the stomata and so
the size of the stomata is controlled by guard cells. If the plants lose water faster than
it is taken up by root hair cells, the stomata closes to prevent wilting and dehydration.
This makes it impossible to photosynthesise if the plant is to close the stomata to
prevent water loss.
The stomata is essentially
the gap that lets gas
exchange occur. The cells
that control this are the
guard cells (pictured).…read more

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A clear presentation with some useful revision tips at the end. As the title shows this is useful for the third unit of AQA Biology on Exchange of materials. Highlighting the key terms is useful. These could be used to make a glossary of key terms for this topic. Definitions of key terms are frequent examination questions.

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