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Objectives
By the end of this lesson you should be able to
Describe a property of cell membranes which
allows substances to pass across it
Describe the process of diffusion
List factors which affect the movement of
substances across a membrane by diffusion
Pores or
channels
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Oxygen for respiring cells
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Water
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Carbon dioxide for photosynthesising plant
cells
Simple sugars, amino acids
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One way is by diffusion
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Diffusion is the net or overall movement of
molecules from an area of high concentration
to an area of low concentration. It is a passive
process it does not use up energy
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If there is a big difference between two areas
diffusion will take place quickly
The difference between two areas of
concentration is called the
concentration gradient
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Temperature
If the temperature is higher the particles in a
gas or solution move more quickly.
This means diffusion takes place more rapidly
as the random movement of particles speeds
up
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The thickness of the membrane Why?
The surface area of the membrane. Look at
this photo of the lining of the small intestine
Villi in the small
intestine increase
the surface area
How does surface area
Increase rate?
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List 2 things which must be able to pass into
cells.
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What is present in a cell membrane to allow them
to pass across?
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What is the name of the process which lets
substances pass from high to low
concentrations?
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Does this process use up energy?
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What factors affect the rate of this process?
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Set up this model cell and leave it for 30
minutes. Iodine turns black when it mixes
with starch. Hazard –iodine wear goggles
visking
bag has
tiny holes
in it
Boiling tube
containing
iodine solution
and water
Starch solution in the
visking bag. This is
your model cell
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Can you explain the change in colours?
visking
bag
Boiling tube
containing
iodine solution
and water
Starch solution in the
visking bag
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What happens when starch and iodine solution
mix?
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What is present in the visking tubing to allow
small iodine molecules through?
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Why has the inside of the visking tubing model
cell gone black?
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Which particles have moved?
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Why have the starch molecules stayed in the
bag?
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The iodine molecules are very _______ and can
move through the small ----- in visking
tubing. The iodine molecules have moved
from an area of high ________to an area of
______concentration by _________
The starch molecules are too ____ to pass
through the small holes in the visking tubing
so they stay in the model ____