Surfaces for Exchange
Exchange
• In groups
– discuss what is meant by the word “exchange”
– Apply the word exchange to a biological concept
– Exchange takes place over surfaces
• Write down features of a good exchange surface
• Which processes are used in the exchange of
substances
• Where does exchange occur in organisms and what is
exchanged?
Learning Outcomes
• Explain, in terms of surface area:volume ratio,
why multicellular organisms need specialised
exchange surfaces and single-celled organisms
do not.
• Explain the need for transport systems in
multicellular organisms in terms of size, level
of activity and surface area to volume ratio.
What do you need to know?
Adaptations for gas exchange allow an
increase in body size.
Small animals exchange gases across their
general body surface.
Comparison of amoeba, flatworm and
earthworm.
Gas Exchange
• All living things take gases from their
environment and return gases to it.
• Organisms which respire aerobically take in
oxygen and give out carbon dioxide
• Photosynthesizing plants take in carbon
dioxide and give out oxygen.
• Many Organisms have specialized surfaces
through which this gas exchange occurs.
Size, Surface area and gas exchange
• Organisms – oxygen in, carbon dioxide out
• Exchange of gases takes
place via diffusion
• The part where this
happens is the gas exchange
Surface.
Size, Surface area and gas exchange
The amount of oxygen needed by an organism is
determined by:
• The amount of living cells
• The rate they need to respire
Requirement of oxygen is related to volume of
an organism and the rate depends on the
surface area.
Note: make sure you understand Fick’s Law from BY1
Which organism has the highest
volume?
Which has the most surface available
for gas exchange?
Small organisms e.g. Amoeba
Amoeba has a very high surface area to volume
ratio
Small organisms get all the oxygen they need by
diffusion through their body surface.
Large Organisms e.g. Elephant
The surface area to volume ratio is much smaller
The external surface area of the body is not
large enough for diffusion of oxygen through it
to take place rapidly enough to supply cells with
the oxygen it needs.
For Example
These usually have ‘extra’ surfaces that are
designed for gas exchange
For example: the human lungs
Progress Question
• Very small organisms such as the amoeba do
not have specialised gas exchange systems.
• Mammals are large, multicellular organisms
and have a complex gas exchange system.
• Explain why the mammal needs such a
system when an amoeba does not.
Progress Question - suggestions
• Why do we need gas exchange?
– Oxygen is needed for respiration
– Body needs to get rid of waste carbon dioxide.
• How do simple animals take in the oxygen
they need?
– Diffusion through the surface membranes e.g.
amoeba or flatworm
Gas Exchange surfaces
They are designed so that…
– Large surface area
– Thin as possible
– Create a high gas gradient
for greater diffusion
– Often have a transport
system that takes gases to
and from the gas exchange
site
Why do larger animals need
specialised exchange surfaces?
Why do some organisms need
specialised exchange surfaces?
1) More active so require more food and oxygen
for respiration.
2) Warm blooded – need more respiration to
keep warm.
3) Large organism, small surface area: volume –
surface area is not big enough to supply all
the oxygen.
Gas Exchange Problems For Land
organisms
Problem
1. cells need to be exposed to air in order for
the oxygen to diffuse into the organism.
2. Terrestrial organisms bodies are made of a
high percentage of water.
3. When living cells are exposed to the air,
water molecules evaporate and the cell
dehydrates
This problem is combatted in different
ways
You need to know:
How the flatworm is adapted to the problem
How the earthworm is adapted to the
problem
The insects gas exchange system
The humans gas exchange system
Flatworm
Flatworm
Simple animals that lack specialized exchange
surfaces have flattened, tubular, or thin shaped
body plans, which are the most efficient for gas
exchange.
Flatworms use their outer surfaces as gas exchange
surfaces. They exchange gases exclusively by
diffusion through their body surface.
This is only possible because all cells are localized
relatively near to the exterior since gases diffuse
cell by cell
Earthworm
Earthworms have a series of thin-walled blood
vessels known as capillaries.
Gas exchange occurs at capillaries located
throughout the body as well as those in the
respiratory surface.
Earthworm Adaptation
1. Cells on surface for gas exchange
2. Live in moist conditions to prevent water loss
1. e.g. Live in damp soil to reduce moisture
3. Slimy covering reducing evaporation
Humans
• Gas exchange surfaces inside the body – this
reduces water loss
• Air is brought inside the body to make contact
with the gas exchange surface
• Water proof coating on the outside of the
body
• Breathing system to allow high rate of
exchange of gas
Insect
• Tubes called trachea penetrate inside the
body carrying air to every tissue.
Insect
• Many terrestrial animals have their respiratory
surfaces inside the body and connected to the
outside by a series of tubes.
• Tracheae are these tubes that carry air directly to
cells for gas exchange.
• Spiracles are openings at the body surface that
lead to tracheae that branch into smaller tubes
known as tracheoles.
• Body movements or contractions speed up the
rate of diffusion of gases from tracheae into body
cells.
Essay Question
Describe how gas exchange occurs in different
organisms.
Include 3 different organisms and how gas
exchange varies between them. [10]
Task
Read your partners essay question
In a different colour underline where you
think they will get a mark
Markscheme
A.
B.
C.
D.
E.
Oxygen into the organism
Carbon dioxide out of the organism
Plants opposite direction
Diffusion
The more living cells the more oxygen is
needed
F. Increased surface area increases rate of
diffusion
Markscheme
G. Amoeba have high surface area to volume
ratio
H. Amoeba obtain oxygen by diffusion
I. Amoeba is a single cell organism
J. Humans have lungs
K. Gas exchange surface is inside the body
L. Larger body/volume
M. Needs more oxygen for respiration
Markscheme
O. Insects have trachea
P. Tubes allow gas exchange inside the body
R. Comparison between organisms (up to 3
marks)