DIFFUSION

Diffusion and concentration

Why can the student smell the sock from a distance?

The sock can be smelt because sweat and other

molecules are moving away from it and spreading out in
the air. This is called diffusion.
Where is the smell strongest?
The smell is strongest at the sock. The smell becomes
weaker further away from the sock.

2 of 27 © Boardworks Ltd 2012

Moving molecules
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. By contrast, solids cannot diffuse.

3 of 27 © Boardworks Ltd 2012

Diffusion in action: one gas

4 of 27 © Boardworks Ltd 2012

• Diffusion is the net movement of molecules from an area
where they are at a higher concentration to areas where
they are at a lower concentration.
• This is due to the random movement of the molecules.

The difference in the concentration of a

substance between two areas is called
the concentration gradient.
Diffusion in action: two gases

6 of 27 © Boardworks Ltd 2012

Changing concentrations
During diffusion molecules move from an area of high
concentration to an area of low concentration. They are
said to move down a concentration gradient.

Diffusion is a passive
process which means that
no energy is needed.

Molecules diffuse until they

are evenly spaced apart
high low
and equilibrium is reached.
concentration concentration
The rate of diffusion depends on several factors, such as
the distance the particles have to travel and the difference
in concentration.

7 of 27 © Boardworks Ltd 2012

Diffusion: true or false?

8 of 27 © Boardworks Ltd 2012

Why is diffusion important to life?
Diffusion is the main way in which substances move
over short distances in organisms. What substances
need to move?

Oxygen, food and waste products are some of the

substances that move by diffusion.

In animals, how do these vital substances get to where

they are needed?

The substances are

transported in the
bloodstream, from
where they can diffuse
in and out of cells.

9 of 27 © Boardworks Ltd 2012

Diffusion and breathing
Breathing involves the exchange of gases in the lungs; a
process that occurs by diffusion. What happens when
you breathe in?

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?

10 of 27 © Boardworks Ltd 2012

11 of 27 © Boardworks Ltd 2012
Structures of the respiratory system

12 of 27 © Boardworks Ltd 2012

Cross-section through an alveolus
Alveoli are the tiny air sacs at the end of the bronchioles,
in which gas exchange occurs.

deoxygenated blood air in/out

(from body tissues)

alveolus
capillary

oxygenated blood
red blood cell (to body tissues)

13 of 27 © Boardworks Ltd 2012

How are alveoli adapted?
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.

14 of 27 © Boardworks Ltd 2012

What happens in the alveoli?

15 of 27 © Boardworks Ltd 2012

Diffusion and digestion
Carbohydrates, proteins and fats are made up of large
molecules that cannot be readily absorbed by the body.

Digestion breaks down large food molecules into smaller

molecules such as glucose, amino acids and fatty acids
that can be easily absorbed.

In which part of the digestive system is most food absorbed?

16 of 27 © Boardworks Ltd 2012

Diffusion and the small intestine

17 of 27 © Boardworks Ltd 2012

Diffusion and the placenta
The placenta is an organ that develops in the uterus of
female mammals during pregnancy. The umbilical cord
connects the placenta to the fetus.

The placenta enables

nutrients and oxygen to
pass from the mother to the
fetus by diffusion, and waste
substances to diffuse from
the fetus back to the mother.

The placenta can filter out certain molecules and bacteria, but
is unable to stop many harmful substances such as alcohol,
chemicals and some types of virus from reaching the fetus.

18 of 27 © Boardworks Ltd 2012

Diffusion and the placenta
How does the placenta work?
blood to mother
umbilical low in O2/nutrients,
cord high in CO2/waste

placental villi
increase surface
umbilical area for diffusion
artery

umbilical blood from mother

vein high in O2/nutrients,
low in CO2/waste

19 of 27 © Boardworks Ltd 2012

Diffusion and nerves impulses
A synapse is a junction between two neurones across
which electrical signals must pass.
synaptic cleft

neurotransmitter

neurotransmitter nerve impulse

receptor

Neurotransmitter molecules diffuse from vesicles towards

the neurotransmitter receptors, moving from an area of
high concentration to low concentration.

20 of 27 © Boardworks Ltd 2012

Diffusion in plants
Plants use carbon dioxide and produce oxygen during
photosynthesis. These gases move in and out of the plant
by diffusion.
When the concentration of
carbon dioxide inside the
plant is low, it will diffuse in
from the air, through pores in
the leaves into the plant cells.
If the concentration of
oxygen is high inside the
plant, it will diffuse from the
plant cells through the pores
and into the air.

21 of 27 © Boardworks Ltd 2012

Which process?

22 of 27 © Boardworks Ltd 2012

 https://www.youtube.com/watch?v=
MCvbfqz7ASs&t=115s
• https://www.youtube.com/watch?v=4Eq8rO3f
ABM
Compare the images. Suggest why?

• https://www.youtube.com/watch?v=abeGXKRM1
M0
 What is osmosis?

Osmosis is the movement of

water across a semi-permeable
membrane from an area of a
low concentration of solutes to
an area with high concentration
of solutes.

What are solutes?

 Osmosis is the movement of water across a semi-
permeable membrane from

1. an area of a low concentration of solutes to an area with high

concentration of solutes.
2. an area of a low water potential to an area with high water
potential.
3. an area of a low water concentration to an area with high water
concentration.
4. Towards a dilute solution
5. an area of a high concentration of solutes to an area with low
concentration of solutes.
6. an area of a high water concentration to an area with low water
concentration.
7. an area of a high water potential to an area with low water
potential.
8. Towards a concentrated solution
Dilute vs. concentrated
During osmosis, water molecules diffuse from pure water or
dilute solution to more concentrated solutions.
 Dilute solutions have a low concentration of
solutes ( high water molecules )
 Concentrated solutions have a high concentration
of solutes (less water molecules).

concentrated
pure water dilute solution
solution
27 of 17 © Boardworks Ltd 2012
Osmosis in action

28 of 17 © Boardworks Ltd 2012

What is osmosis?
Osmosis is the diffusion of water molecules from a low
concentration solution to a high concentration solution
across a partially-permeable membrane.
A partially-permeable membrane has holes in it that permit
water molecules through, but are too small to allow larger
molecules through. Osmosis can be demonstrated using
visking tubing filled with a solution and placed in a beaker
of pure water. partially-
permeable
membrane
(visking tubing)

water

glucose
29 of 17 © Boardworks Ltd 2012
Why does the level change?
THINK PAIR SHARE
Start After 4 hours

Document8 - _Hlk479100619
Predicting osmosis

31 of 17 © Boardworks Ltd 2012

 What is osmosis?

Osmosis video

Watch this video carefully answering the

questions on your sheet as we go along…
 Osmosis video
Answers
1. The movement of water across a semi-permeable
membrane.
2. Passive transport
3. Substances that can be dissolved in a solvent
4. Both ways (but no net movement)
5. A high solute concentration compared to something
else
6. Hypotonic – hypertonic
7. Water would move into the red blood cells and they
would swell (and possibly burst)
8. Isotonic
9. Root hair cells have a higher concentration of solutes
 What is osmosis?

On your worksheets there are six examples

of water and solutes separated by a semi-
permeable membrane.

Draw an arrow to show which direction

the water would move (net movement).

Stick these in your book when finished -

then try the stretch question.
 Draw arrows to show the direction of the (net)
movement of water

A B Key:
C
Water –
Solute -

D E F

Stretch – Once you’ve stuck your completed sheet in your book, underneath
explain in which example osmosis would occur most rapidly and why?
 movement of water
Answer sheet

A B Key:
C
Water –
Solute -

D E F

Stretch – Once you’ve stuck your completed sheet in your book, underneath explain in which example
osmosis would occur most rapidly and why? Osmosis would occur most rapidly in E because the higher the
difference in concentration of the two solutions the higher the osmotic pressure
Water will leave
Distilled water
the potato chip (P)
(low solute
and move into the
concentration) will
solution as the
move into the
solution has a high
potato chip(Q)
concentration of
which has a higher
solute. So the chip
solute
will shrivel and
concentration. The
become smaller.
chip will swell and
become larger.
 


Osmosis and cells
Plant and animal cells are surrounded by a partially-
permeable plasma membrane. This allows water and other
small molecules to diffuse across.

Plant cells additionally have a strong cell wall surrounding

the membrane which offers support and protection.

plasma
membrane

cell wall

red blood cell

plant cell
39 of 17 © Boardworks Ltd 2012
Osmosis and plant cells

40 of 17 © Boardworks Ltd 2012

Osmosis and plants
Root cells in plants absorb water
through osmosis.

Many fine root hairs increase the

surface area of a plant’s roots,
allowing them to draw in more
water from the surrounding soil.

The presence of water in plant cells

gives the plant structural support

41 of 17 © Boardworks Ltd 2012

Osmosis and animal cells
Animal cells do not have a cell wall. This means they
respond differently to plant cells to the gain and loss
of water.
In dilute solutions, osmosis can
cause animals cells, such as red
blood cells, to swell up and burst.
This is called lysis.

In concentrated solutions, water

loss causes the cells to shrink.
When this happens to red blood
cells, it is called crenation.

42 of 17 © Boardworks Ltd 2012

Osmosis and animal cells

43 of 17 © Boardworks Ltd 2012

Osmosis and animal cells
In order to remain healthy, animal cells need to maintain
an isotonic water balance. This means that the water
concentration both inside and outside the cell is equal.
hypothalamus
The concentration of water
and salt in the blood is
controlled by the kidneys.

The kidneys are controlled

by the portion of the brain
called the hypothalamus.

44 of 17 © Boardworks Ltd 2012

45 of 17 © Boardworks Ltd 2012
Which process?

46 of 17 © Boardworks Ltd 2012

Multiple-choice quiz

47 of 17 © Boardworks Ltd 2012

• SUMMARY

• https://www.youtube.com/watch?v=AxXN-
j6UzOY

48 of 17 © Boardworks Ltd 2012

What is active transport?
Substances can move passively in and out of cells by
diffusion until the concentration on both sides of the cell
membrane reaches an equilibrium.

Substances can continue to move in and out of a cell using

a process called active transport.

During active transport, carrier

proteins in the cell membrane
‘pick up’ particles and move them
against the concentration gradient.

As the name suggests, active

transport requires energy from the
cell, which is created by respiration.

49 of 10 © Boardworks Ltd 2012

What is active transport?

50 of 10 © Boardworks Ltd 2012

Active transport in plants
Plants need to absorb mineral elements such as nitrogen,
phosphorus and potassium from the soil for healthy growth.

When the concentration of minerals in soil is lower than

inside the plant, active transport is used to absorb the
minerals against the concentration gradient.

What would happen if the minerals

plant relied on diffusion to
absorb minerals?

The cells would become

drained of minerals because
they would travel down the
concentration gradient.

51 of 10 © Boardworks Ltd 2012

Active transport in humans
During digestion, the villi in the small intestine absorb the
soluble nutrients. Over time, the concentration of nutrients in
the villi reach an equilibrium with the concentration in the gut.
glucose

Active transport is
used to continue the
transport of the small
amounts of remaining
nutrients against the
concentration gradient.

52 of 10 © Boardworks Ltd 2012

53 of 10 © Boardworks Ltd 2012
• https://www.youtube.com/watch?v=BXTi5tb
nOr0

54 of 17 © Boardworks Ltd 2012

• FACTORS AFFECTING DIFFUSION

• https://www.youtube.com/watch?v=LUPHoh
qlPTU
• https://www.youtube.com/watch?v=lxHMJa
XOzP4

55 of 17 © Boardworks Ltd 2012

56 of 27 © Boardworks Ltd 2012
SA:VOLUME

• https://www.youtube.com/watch?v=huKUJ
sqik2I

57 of 27 © Boardworks Ltd 2012

58 of 27 © Boardworks Ltd 2012
59 of 17 © Boardworks Ltd 2012