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Biology B2
Revision ­ diffusion and osmosis…read more

Slide 2

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· Diffusion is the gradual movement of particles from places
where there are lots of them to places where there are
fewer of them.
· DIFFUSION is the passive movement of particles from an
area of HIGH CONCENTRATION to and area of LOW
­ Diffusion happens in both liquids and gases ­ that's because the
particles in these substances are free to move about randomly.
­ The simplest type is when different gases diffuse through each
other. This is what happening when the smell of perfume diffuses
through a room.
­ The bigger the difference in concentration the faster the diffusion
rate.…read more

Slide 3

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· Cell membranes:
­ Cell membranes are clever because they hold the
cell together BUT they let stuff in and out as well.
­ Substances can move in and out of cells by
diffusion and osmosis. Only very small molecules
can diffuses through cell membranes though ­
things like glucose, amino acids, water and
­ Big molecules like starch and proteins can't fit
through the membrane.…read more

Slide 4

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· Cell membranes:
­ Just like with diffusion in air, particles flow through the cell
membrane from where there's a high concentration to
where there's a low concentration.
­ They're only moving about randomly, so they go both
ways ­ but if there are a lot more particles on one side of
the membrane there is a net (overall) movement from that
­ The rate of diffusion depends on 3 main things:
· Distance ­ substances diffuse more quickly when they haven't
got far to move.
· Concentration difference ­ substances diffuse faster if there is a
big difference in concentration. If there are lots more particles
on one side, there are more to move across.
· Surface area ­ the more surface there is available for molecules
to move across the faster they can get from one side to the…read more

Slide 5

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· OSMOSIS is the movement of water molecules
across a partially permeable membrane from a
region of high water concentration to an area of
low water concentration.
­ A partially permeable membrane is just one with very
small holes in it ­ so very small molecules (like water)
can pass through them and bigger molecules (like
sucrose) can't.
­ The water molecules eventually pass both ways
through the membrane during osmosis. This happens
because water molecules move about randomly all
the time.…read more

Slide 6

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­ Because there are more molecules on one side,
there's a steady net flow of water into the region
with fewer water molecules.
­ This means the strong sugar solution gets more
dilute. The water acts like it's trying to even up
the concentration either side of the membrane.
­ Osmosis is a type of diffusion ­ passive movement
of water particles from an area of high water
concentration to an area of low concentration.…read more

Slide 7

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