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AQA GCSE Biology ­ Unit 3 summary notes
Dissolved substances
Dissolved substances move by diffusion and by active transport.
Diffusion (covered in more detail in unit 2):
The movement of particles in a liquid or gas from an area where they are in higher
concentration to an area where they are in lower concentration.
The greater the difference in concentration, the faster the rate of diffusion.
2 examples of diffusion through the cell membrane:
o Oxygen required for respiration diffuses into cells.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
If animal cells are placed in a solution that has a lower solute concentration than the cytoplasm,
then water will enter the cell by osmosis until it bursts.
This is why it is vital that we maintain the concentration of our body fluids at
an equal solute concentration to our cells' cytoplasm.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
Specialised exchange surfaces
Many organ systems are specialised for exchanging materials.
The effectiveness of an exchange surface is increased by:
o having a large surface area
o being thin, to provide a short diffusion path
o Animals can also maintain high concentration gradients:
by having an efficient blood supply.
for gaseous exchange by being ventilated.
Gas and solute exchange surfaces in humans and other organisms are adapted to
maximise effectiveness.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
The lungs are in the upper part of the body (thorax).
They are protected by the ribcage.
They are separated from the lower part of the body (abdomen) by the diaphragm.
The breathing system takes air into and out of the body.
Oxygen from the air diffuses into the bloodstream.
Carbon dioxide can diffuse out of the bloodstream into the air.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
The moist surface helps oxygen to dissolve so that it can diffuse into the cells.
The capillaries are very close to the alveoli so that the gases have a short distance to
diffuse.
Concentration gradients are maintained by:
o The blood removing oxygen from and bringing carbon dioxide to the lungs.
o Breathing, which replenishes oxygen and removes carbon dioxide.
Ventilation (breathing)
The movement of air into and out of the lungs is known as ventilation.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
Mechanical ventilators:
o Machines can be used that enable air to move into and out of the lungs:
o Negative pressure machines.
Often referred to as iron lungs.
These are used in cases of paralysis, for example, due to polio.
The patient's thorax is contained inside a metal box that can create a very
low pressure outside the lungs.
This causes the thorax to expand so air enters the lungs
o Positive pressure machines.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
Exchange systems in plants
Absorption of water by roots
The surface area of the roots is increased by root hairs.
Most of the water and mineral ions are absorbed by root hair cells.
Water is absorbed by osmosis.
Most of the mineral ions are absorbed by active transport.
Gas Exchange in the leaf
The surface area of leaves is increased by the flattened shape and internal air spaces.
Most photosynthesis takes place in the palisade cells.…read more

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AQA GCSE Biology ­ Unit 3 summary notes
Transpiration
The process by which plants lose water vapour from the surface of their leaves.
It evaporates into the air spaces in the leaf, and then diffuses out through the stomata.
Transpiration is more rapid in hot, dry and windy conditions:
o Heat causes the water to evaporate quicker.
o Dry conditions increases the water vapour concentration gradient.
o Wind moves the water vapour away from the leaf, maintaining the concentration
gradient.…read more

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