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Biology Revision Notes:
Diffusion and Active Transport
Diffusion is the movement of particles or molecules (air/ water etc.) from a higher concentrated region
to a lower concentrated region. Diffusion will only happen until there is an equal concentration in both
regions- the substance is equally distributed. Only gases and liquids can diffuse.
Diffusion takes place:
In the alveoli in our lungs as oxygen enters the bloodstream and carbon dioxide leaves it
In the small intestine as amino acids, glycerol and glucose are absorbed into the blood stream
In the bloodstream as oxygen and glucose diffuse out of the capillaries into cells, and carbon
dioxide and water diffuse into the bloodstream
Osmosis is the movement of water molecules from a region of high water concentration to a region of
low water concentration across a semi- permeable membrane. Osmosis
is a type of diffusion, but must be with water, and must be across a
semi- permeable membrane.
An example of an organism which carries out osmosis is amoeba (single
celled) Osmosis is regularly shown in plant cells;
When placed in a salt solution, there is a higher concentration
of water inside the cell than out
Therefore the water moves out of the cell by osmosis
This causes the cell to decrease in size and become flaccid
The salt solution is in a higher concentration outside the cell and
therefore tries to move in by diffusion, so moves in through
the cell wall. However, the solution is too big to move through
the membrane, and this causes the membrane of the cell to become detached from the wall
and the cell becomes plasmolysed
Framework for model answer on osmosis:
State whether the cell has increased or decreased in size (1)
Give the direction of the water (2)
State that it is moving by osmosis (1)
State the concentrations inside and out for every cell shown (1)
If a plant cell is full of water, it becomes turgid, if it has too little water inside, it becomes flaccid.
If an animal cell is too full of water, it bursts. This is called lysis. If it has too little water inside, it will
crenate. This is because animal cells animal cells have no cell walls where plant cells do.
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This is present in red blood cells- if a red blood cell is placed in a high water solution, it will lyse, and if
placed in a low water solution it will crenate.
Active transport is the movement of particles against the concentration gradient region, therefore,
when particles are pumped from a low concentration to a high concentration region. This uses a lot of
An example of where this takes place is in root hair cells of plants.…read more