Download B4 Diffusion and osmosis

Diffusion and Osmosis – Revision Pack (B4)
Diffusion:
Diffusion is the movement of particles in a liquid or gas from an area of high
concentration to an area of low concentration. It happens because of the random
movement of individual particles.
Diffusion explains how molecules like carbon dioxide, water and oxygen can get
into and out of cells via the cell membrane. For example, if a plant is using up
carbon dioxide then the concentration is low, so carbon dioxide will enter via
diffusion.
Leaves are adapted to increase the rate of diffusion of CO 2 and O 2 by:
-
Having a large surface area
Having stomata (pores) which are spaced out
Having gaps between spongy mesophyll cells
The rate of diffusion can be increases by:
-
Having a short distance for molecules to travel
Having a greater surface area for the molecules to diffuse into or out of
Having a steeper concentration gradient (see below)
In this instance, the concentration goes from an
area of higher concentration, to an area of
lower concentration.
To increase the steepness of this gradient, the
difference between the higher concentration
and lower concentration must be greater.
Osmosis:
Osmosis is a type of diffusion. It
needs a semi-permeable
membrane that allows small
molecules like water through, but
doesn’t permit larger molecules
like sugar through.
Osmosis is the movement of
water across a semi-permeable
membrane from an area of high
water concentration (dilute) to
an area of low concentration
(concentrated).
Osmosis happens because of the random movement of water molecules, which are not
restricted (like sugar is) by a semi-permeable membrane. The movement of water molecules
will be from an area where there is lots to an area where there is few.
If we know the concentration of water inside and outside a cell, we can predict the
movement of those water molecules.
Diffusion and Osmosis – Revision Pack (B4)
Water in Cells:
wall)
When lots of water enters a
plant cell, the pressure
pushing on the cell wall is
high. This is a high turgor
pressure, which supports
the cell – stopping it, and
the whole plant, from
collapsing.
When too much water
leaves a plant then it has a
low turgor pressure and the
plant will become limp.
A plant cell that is full of water is
called turgid.
When a plant cell loses water we
call the cell flaccid.
NOTE – when a cell is plasmolysed,
the cytoplasm is pulled away from
the wall.
Animal cells react in the same way as plant cells do towards water loss and water
intake. When too much water is lost, animal cells will shrink and collapse. When too
much water enters an animal cell, the cell will also swell up.
Unlike plant cells, animal cells (like red blood
cells) do not have a supporting cell wall. This
means that when too much water enters an
animal cell, they swell up and burst (image 3)
– this is called lysis.
When too much water leaves an animal cell,
it shrinks into a scalloped shape (image 1) –
this is called crenation.
A normal animal cell is actually more flaccid than it is turgid (image 2).
Diffusion and Osmosis – Revision Pack (B4)
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Diffusion and Osmosis – Revision Pack (B4)
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Diffusion and Osmosis – Revision Pack (B4)
Diffusion and Osmosis – Revision Pack (B4)
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Diffusion and Osmosis – Revision Pack (B4)