Biology unit 3 notes with questions (AQA)

Colourful B3 notes for AQA


active transport


exchanging materials

ventilating the lungs

exchange in the gut


Exam Qu and answers from past papers

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  • Created on: 21-05-13 09:00
Preview of Biology unit 3 notes with questions (AQA)

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The solutions inside the cells are separated from those outside the cells by the cell
Membranes which only let some types of particles through are called partially
Partially permeable membranes let water flow across them.
A dilute solution of sugar contains a high concentration of water (the
solvent). It has a low concentration of sugar (the solute).
A concentrated sugar solution contains a relatively low concentration of
water and a high concentration of sugar.
Water moves from a high concentration of water molecules (in a dilute solution) to a
less concentrated solution of water molecules (in a concentrated solution) across the
membrane of the cell.
This special type of diffusion, where only water moves across a partially permeable
membrane , is called osmosis.
high low
The concentration in your cells has to stay the same for them to work properly.
The concentration outside your cells may be very different to the concentration inside
This can cause water to move into or out of the cells by osmosis.

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Osmosis in animals
The solutions inside the cells are separated from those outside the cells by the cell
Membranes which only let some types of particles through are called partially
Partially permeable membranes let water flow across them.
If a cell uses up water in its reaction , the cytoplasm becomes more concentrated.
More water moves in by osmosis.
If the cytoplasm becomes too dilute, water leaves the cell by osmosis.…read more

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Active Transport
Dissolved substances need to move in and out of cells.
Substances move by diffusion along a concentration gradient.
Sometimes the substances needed by a cell have to be moved against a
concentration gradient, or across a partially permeable membrane.
This is known as active transport.
low concentration ---------> high concentration
Cells can absorb ions from very dilute solutions.
It takes energy for the active transport system to carry a molecule across the
membrane and then return it to its original position.…read more

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Exchanging Materials - the lungs
In many larger organisms there are special surfaces where has and solute exchange
take place. They are adapted to be as effective as possible.
The effectiveness of an exchange surface can be increased by :
· having a large surface area
· being thin, which provides a short diffusion path
· having an efficient blood supply. This moves the diffusing substances away and
maintains a concentration gradient.…read more

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Ventilating the lungs
For a gas exchange system to work efficiently you need a steep
concentration gradient.
Humans move air in and out of the lungs regularly.
They maintain a steep concentration gradient of both oxygen in and carbon
dioxide out.
This is known as ventilating the lungs or breathing.
It takes place in your specially adapted breathing system.
The breathing system
The lungs are in the thorax and are protected by the ribcage.
They are separated from the abdomen by the diaphragm.…read more

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When you breathe in:
· muscles contract
· volume of thorax increases
· pressure in thorax decreases
· air enters lungs
When you breathe out:
When the intercostal muscles relax , your ribs drop down and in again.
When the diaphragm relaxes it curves back up into your thorax.
The volume of your thorax gets smaller.
This increases the pressure inside your chest so the air is squeezed and
forced out of the lungs.…read more

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Exchange in the gut
Absorption in the small intestine
Digested food molecules move from inside your small intestine into your blood
stream by a combination of diffusion and active transport.
The digestive food molecules are small enough to move freely through the
walls of the small intestine into the blood vessels.
They move in this direction because there is a very high concentration of
food molecules in the gut and a much lower concentration in the blood.…read more

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The loss of water vapour from the surface of plant leaves is known as
Water is lost through the stomata , which are opened and closed by the
guard cells to let in carbon dioxide for photosynthesis.
When the stomata are open plants loose water vapour through them as well.
The loss of water vapour is called evaporation.
As water is lost through the surface of the leaves, more water is puled up
through the xylem to take its place.…read more

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Controlling water loss
Leaves have a waxy , waterproof layer to prevent uncontrolled
water loss.
Most stomata are found on the bottom of leaves. This protects
them from the direct light and energy from the sun reducing the
time that they are open.
If a plant looses water too quickly it wilts. This reduces the
surface area for evaporation.
The stomata close which stops photosynthesis and most water
loss.…read more

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