How can a shark
find injured prey so
quickly?
B2 Additional
 To
describe DIFFUSION
 To explain what affects the RATE of
DIFFUSION
 To link diffusion with EXAMPLES in
BIOLOGY
Respiration
 Digestion
 Placenta
 Leaf Cells
 Nerve Impulses

Starter:
 Explain
in terms
of particles how
the smell of the
perfume gets from
one side of the
class to the other
When you have just
sprayed perfume,
the concentration
of perfume particles
in that area is very
high.
Diffusion is the
movement of the
‘smelly’ particles,
through particles of
air, to an area
where they are at a
LOWER
concentration.
Maybe you
can smell
your dinner
cooking
when you
get home...
This is due to
diffusion.
In which states are molecules able to diffuse?
solid (e.g. ice)
liquid (e.g. water)
gas (e.g. steam)
Molecules in liquids and gases are constantly
moving and bumping into each other. This
means that they tend to spread out.
 The net movement of particles from an
area of high concentration to an area of
low concentration
 Due to the random movement of
particles
 A passive process which means that no
energy is needed
LOW
concentration
HIGH
concentration
A difference in concentration
between 2 areas is called a
CONCENTRATION GRADIENT.
What effect might the gradient have
on speed of diffusion?
Rate of Diffusion
Draw both sets of diagrams, write how
the rate of diffusion changes when the
concentration gradient changes
Fast
Steep concentration gradient
Slow
Shallow concentration gradient



Concentration gradient
Temperature
Distance particles must travel
Faster Diffusion When...
 Large concentration gradient
 Higher Temperature
 Short distance
Think – Pair - Share
Draw the diagrams next to each
other and copy the captions
The particles
are separate
when first put
together.
They slowly
start to mix due
to their random
motion.
The particles
are now nearly
fully diffuse.
The particles
are fully
diffuse.
Has diffusion stopped completely?
Explain your answer
Dissolved substances have to pass through the
partially permeable cell membrane to get into or
out of a cell.
Diffusion is one of the processes that allows this to
happen..
What substances are
we talking about?
All living cells rely on diffusion to live.
They use it for:
 Getting raw materials for
respiration (dissolved
substances and gases)
 Removing waste products (eg. from respiration)
 Plants use of photosynthesis (raw materials
in, waste products out)
Examples…
Oxygen in inhaled air diffuses
through the lungs and into the
bloodstream. The oxygen is
then transported throughout
the body.
Carbon dioxide is the waste gas produced by
respiration. Carbon dioxide diffuses from body tissues
into the bloodstream and is exhaled via the lungs.
Where does gas exchange take place in the lungs?
Alveoli are the tiny air sacs at the end of the
bronchioles, in which gas exchange occurs.
deoxygenated blood
(from body tissues)
capillary
air in/out
alveolus
oxygenated
blood (to body
tissues)
red blood cell
Alveoli have several adaptations that help to make
gas exchange very efficient:
 They are very thin – only one cell thick.
 They are covered by a network of fine capillaries,
enabling gases to pass almost directly between
the lungs and bloodstream.
 They are moist, encouraging gas molecules to
easily dissolve.
 They have a large combined surface area,
allowing large amounts of gases to be
exchanged with each breath.
Digestion breaks down large food molecules into
smaller molecules such as glucose, amino acids and
fatty acids that can be easily absorbed.
Small food molecules are usually absorbed in the
small intestine, diffusing across the intestine wall and
into the bloodstream.
capillary network
small intestine
blood vessels
Villi
The small intestine has these things called villi.
The best way of explaining them, is that they look like millions of little sausages.
The villi INCREASE THE OVERALL SURFACE AREA of the small intestine, thus
increasing the volume of substances which can be absorbed by diffusion.
The villi have a very GOOD BLOOD SUPPLY as each contains a capillary, which leads
to the main blood supply, so when substances diffuse, they can go straight to the
bloodstream.
Attached to each villi are thousands and thousands of MICROVILLI.
These are exactly the same as the villi, except a lot smaller.
Again, the purpose of these is to further increase the surface area. Like before, as well,
this increasing the volume of substances which can be absorbed.
So, the main points to remember are:

The villi and microvilli result in a LARGE SURFACE AREA, for maximum absorption.

The villi also give a GOOD BLOOD SUPPLY to absorb the nutrients.
A synapse is a junction between two neurones
across which electrical signals must pass.
synaptic cleft
neurotransmitter
neurotransmitter
receptor
nerve
impulse
Neurotransmitter molecules diffuse from vesicles
towards the neurotransmitter receptors, moving from
an area of high concentration to low concentration.
•The placenta is an organ
that develops in the uterus
during pregnancy.
•The umbilical cord connects
the placenta to the foetus.
•It enables nutrients and oxygen to
pass from the mother to the foetus by
diffusion, and waste substances to
diffuse from the foetus back to the
mother.
•The placenta can stop
certain molecules and
bacteria from diffusing through
•It is unable to stop many
harmful substances such as
alcohol, chemicals and some
types of virus from diffusing
through, reaching the foetus.
How does the placenta work?
umbilical
cord
umbilical
artery
umbilical
vein
blood to mother
• low in
O2/nutrients,
• high in
CO2/waste
placental villi
increase surface
area for diffusion
blood from mother
• high in
O2/nutrients
• low in CO2/waste
carbon dioxide + water  oxygen + glucose
Carbon dioxide diffuses in
through the stomata
 Oxygen and water diffuse
out of the stomata
 During photosynthesis, the level of
CO2 is low inside the leaf
 This creates a big concentration
gradient so CO2 diffuses into the cell
Label
the leaf