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National 5 –Diffusion
Diffusion is the movement of molecules from a region of high concentration
to a region of low concentration. Diffusion occurs until the molecules are
evenly distributed, i.e. their concentration is the same in all parts of the liquid
or gas e.g.
Concentration gradients
The diagram shows a beaker that contains two
different solutions; water and dye. We can compare
the two solutions by looking at areas 1 and 2. In
area 1 there is a very high concentration of dye
molecules while in area 2 there is a very low
concentration of dye molecules.
Beaker of water
Area 1
Area 2
Dye added to water
very high
concentration
of dye molecules
very low
concentration
of dye molecules
Dye particles begin
to move
high
concentration
of dye molecules
low
concentration
of dye molecules
Water molecules
Dye particles move until
they are evenly spread
Even
concentration
of dye molecules
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The difference in the concentration of dye molecules between areas 1 and 2 is
called the concentration gradient. The dye molecules will diffuse from the
high concentration at area 1 to the low concentration at area 2. When this
occurs the molecules are said to be moving down the concentration gradient.
Diffusion into and out of cells
The cell membrane contains tiny pores and controls the entry and exit of
molecules from the cell.
Diffusion is the main process by which small
molecules move into or out of the cell:
Substances that can
enter the cell by
diffusion
Substances that
can leave the cell
by diffusion
Substances that cannot
enter or leave the cell
by diffusion
Oxygen
Carbon dioxide
Starch
Water
Water
Protein
Dissolved food e.g.
glucose
Fat
Diffusion in organisms
Gas exchange in organisms occurs because of diffusion. In multicellular
organisms (e.g. man) there are organs for gas exchange.
Oxygen concentration
Carbon dioxide
concentration
Lungs
High
Low
Red blood cells returning
from living cells of the body
Low
High
Carbon dioxide diffuses from the blood cells into the lungs and oxygen diffuses
from the lungs into the blood cells.
Oxygen concentration
Living cells of the body
Low
Carbon dioxide
concentration
High
2
Red blood cells coming
from lungs
High
Low
Oxygen diffuses from the blood cells into the body cells and carbon dioxide
diffuses from the body cells into the blood cells.
National 5- Osmosis
Osmosis is a special case of diffusion and can be defined as the movement of
water molecules through a selectively (or semi) permeable membrane from a
region of high water concentration to a region of low water concentration.
Selectively permeable membranes
Cell membranes are said to be selectively permeable because they are selective
about the molecules that they allow to move through them. Small molecules like
water can move through freely and larger molecules like glucose can move
through but slowly.
Large and very large molecules like starch and sucrose
cannot move through cell membranes by diffusion.
Osmosis in action
We can see osmosis in action by setting up the experiments shown below using
visking tubing.
Visking tubing is a bag containing lots of very small holes and
this allows it to behave in the same way as a selectively permeable membrane.
Beaker 1
Water
Results
Beaker 3
Beaker 2
1% salt
solution
Visking tubing
containing 1%
salt
molecules
will
move
or sucrose solution
10% salt
solution
Visking tubing
1% salt it
viskingcontaining
tubing making
or sucrose solution
In beaker 1, water
into the
gain
mass.
In beaker 2, overall there will be no water movement and therefore the mass of
the visking tubing will not change.
In beaker 3, water will move out of the visking tubing making it lose mass.
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Osmosis and cells
Before looking at osmosis and cells the following need to be considered:



a solution of WATER has a HIGH water concentration and a LOW salt/sugar
concentration
a solution of 1% SALT/SUGAR has the SAME water and salt/sugar
concentration as an average CELL
a solution of 10% SALT/SUGAR has a LOW water concentration and a HIGH
salt/sugar concentration
Hypertonic, Hypotonic and Isotonic
When comparing solutions to cells the terms hypertonic, hypotonic and isotonic
are used.
A hypertonic solution has a HIGHER salt/sugar concentration and a LOWER
water concentration than a cell.
A hypotonic solution has a LOWER salt/sugar concentration and a HIGHER
water concentration than a cell.
An isotonic solution has the SAME salt/sugar concentration and the SAME
water concentration as a cell.
Osmosis and animal and plant cells
Red blood
cell
Surrounding
solution
1.
1
2.
3.
Hypotonic
( water)
Result
Effect on cell
Water enters
cell
(HWC to
LWC)
Cell swells and
bursts
(HAEMOLYSIS)
4
2
Isotonic
(1 % salt
solution)
3
Hypertonic
(10 % salt
solution)
Potato cell
Surrounding
solution
1.
1
2.
2
3.
3
Hypotonic
(water)
Isotonic
(1 % salt
solution)
Hypertonic
(10 % salt
solution)
No overall
water
movement
Water leaves
cell
(HWC to
LWC)
No effect
Cell shrinks
Result
Effect on cell
Water
enters cell
(HWC to
LWC)
No net
water
movement
Cell
contents
swell and press
against cell wall.
Cell is TURGID
Water
leaves cell
(HWC to
LWC)
No effect
Cell contents and
vacuole shrink and
membrane
is
pulled away from
cell wall. Cell is
PLASMOLYSED
Both diffusion and osmosis are passive processes that don’t require a lot of
energy. Sometimes some substances enter or leave cells against a concentration
gradient and this does need energy. This is known as Active Transport.
National 5- Active transport
The cell membrane is made up of phospholipids and proteins and is selectively
permeable.
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The proteins embedded in the cell membrane enable molecules or ions to move
against their concentration gradient (i.e. from low concentration to high
concentration). This requires an input of energy in the form of ATP. The ATP
causes a change in the shape of the protein which moves a molecule or ion
across the membrane.
An example of this is: some species of Seaweed e.g. Ecklonia radiata
Liquid Analysed
Sea water
E.radiata cell sap
Ion concentration of elements (mg/litre)
Potassium
Sodium
0.01
0.5
0.59
0.04
Chloride
0.58
0.62
Check point A3
Level 4 E&O
Nat 4
Nat 5
2 Transport across cell membranes
a. The cell membrane consists of lipids and proteins and is selectively permeable.
b. Passive transport is along a concentration gradient and does not require energy.
c. Diffusion in cells as the movement of molecules down a concentration gradient.
d. Osmosis as the movement of water molecules across a membrane in terms of water
concentration.
e. Animal cells can burst or shrink and plant cells can become turgid or plasmolysed in
different solutions.
f. Active transport requires energy for membrane proteins to move molecules and ions
against the concentration gradient
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