Diffusion is used by cells to take in
useful substances and remove waste products.
It usually involves diffusion through a
membrane, water movement by osmosis and also active transport e.g.
useful substances e.g. food molecules
like amino acids and sugars, mineral ions, water taken in by osmosis,
waste products e.g. carbon dioxide from respiration, urea
(poisonous) from breakdown of proteins in humans - diffuses
from cells into blood plasma and transferred to the kidneys prior to
excretion - one examples of waste removal,
gas exchange usually involves taking
oxygen into cells for respiration and passing out carbon dioxide to the environment.
Know and understand that many organ systems are specialised for exchanging
The ease with which an organism
can exchange substances with the environment depends on the organisms
surface area to volume ratio AND you can extend this idea to an organ
itself e.g. the lungs.
In single-celled microorganisms
gases and dissolved substances can often diffuse directly into and out of the cell
through the cell membrane.
This is very efficient because a
single cell has a large surface area to volume ratio membrane -
large surface area relative to the volume of the cell.
Therefore the single-celled organism
has no trouble in exchanging sufficient materials with its environment.
Know that the size and complexity of an organism increases the
difficulty of exchanging materials.
One reason for this increased
difficulty in exchanging materials is that the distance from the exchange
surface is getting further away from where the nutrients and oxygen are
needed and the waste to be removed.
Know that gas and solute exchange surfaces in humans and
other multi-cellular organisms are adapted to maximise effectiveness -
they don't have the obvious surface/volume ratio single-celled organisms
Multicellular organisms have a
smaller surface area to volume ratios compared to a single celled
This surface area is NOT sufficient to provide efficient rates
of diffusion of substances in and out of the organism without
significant adaptation through evolution - some examples are
described and explained on this page.
It is essential that the
transfer processes of moving sugars, amino acids, oxygen etc. into cells and
the removal of waste products, can happen as efficiently as possible.
Therefore exchange surfaces have
evolved to maximise the rate of transfer of wanted substances into, and unwanted chemicals out of, multicellular organisms.
To increase the probability
(efficiency - effectiveness) of
exchange the exchange surface needs to be ...
a large surface area to increase
diffusion rate eg alveoli in lungs, villi in intestine
a thin layers of tissue (cells) so diffusion
distance and times
are short - cell membranes are usually quite thin,
and lots of thin blood vessels to
bring in essential molecules for life and carry waste molecules away eg the
thin bronchiole tubes in the lungs.
Animals have an efficient blood
supply with lots of blood vessels including thin capillaries which
have a particularly large surface to volume ratio - this allows fast
diffusion in either direction.
Animals need an efficient gaseous exchange
ventilation system to take in (including oxygen) and give out air
(including waste carbon dioxide) - in the lungs the tiny pockets
called alveoli greatly increase the gas exchange surface.
Substance exchange problems for
The larger a multicellular
organism, the more difficult it is to exchange substances.
Cells deep in the body are some
distance to the surrounding environment - air or water.
Larger organisms have low
surface to volume ratio reducing exchange efficiency.
Therefore, through evolution,
instead of exchange through an outer membrane ('skin') multicellular
organisms have developed specialised exchange organs
including an equally specialised exchange surface.
BUT, specialised organs are not
enough on their own to serve a relatively large body, you also need
specialised transport systems to convey substances to and
from the body cells e.g. to provide nutrients or remove waste
In animals the
transport system is the circulatory system - blood
and also gaseous exchange
in lungs, the lengthy digestive system and the excretory
system - and all systems must work in harmony with each
page being written
In plants, transport is
effected through the xylem and phloem vessels.
Transport and gas exchange in plants,
transpiration, absorption of nutrients etc.
A mathematical 'extra' on
The greater the surface area the
greater the possible rate of material transfer.
The most compact shape to give the
lowest surface area/volume ratio is a sphere, but that's not
very practical for the working of many specialised cells, tissues or
organs - but very good for single-celled organisms!
However, systems in living
organisms that involve transfer of substances, do need as large a
surface area as possible within the volume the 'system'
To this end, many organs have
evolved to give the maximum surface area as possible
within the volume the 'system' occupies.
A bit of area/volume maths to
illustrate this idea with cubes of various sizes (6 faces):
A 1 cm cube has a volume
of 1 cm3 (1 x 1 x 1), a surface are of 6 x 1 x 1 = 6 cm2
So the surface area / volume
ratio = 6 / 1 = 6 cm-1
A 2 cm cube has a volume
of 8 cm3 (2 x 2 x 2), a surface are of 6 x 2 x 2 = 24 cm2
So the surface area / volume
ratio = 24 / 8 = 3 cm-1
A 3 cm cube has a volume
of 27 cm3 (3 x 3 x 3), a surface are of 6 x 3 x 3 = 54 cm2
So the surface area / volume
ratio = 54 / 27 = 2 cm-1
You can see clearly that
the smaller (thinner etc.) the 'system' or parts of the 'system'
the greater the surface to volume ratio.
Good examples of this are the
millions of tiny air sacs (alveoli) in the lungs and the thin
multi-layered sections of gills in fishes - both of which are to
do with animal respiration.
This mathematical 'extra'
was 'adapted' from the page on
of plants and animals
and is also appropriate to various points in
Diffusion, osmosis and active transport
Examples of exchange systems are now
described in detail with diagrams.
Gas exchange in
the lungs by diffusion
The lungs are the means of
transferring oxygen from air to the blood stream (blood plasma) and to
remove the waste gas carbon dioxide.
Know and understand that in humans:
The surface area of the lungs is greatly increased by
the alveoli - millions of tiny air sacs of the end of the tiny bronchiole tubes in the lungs
where the gas exchange by diffusion takes place.
Know and understand that the lungs are in the upper
part of the body (thorax), protected by the ribcage and separated from the
lower part of the body (abdomen) by the diaphragm.
should be able to recognise these structures of the lungs on the diagrams
above and on the right.
The ribcage physically protects
the lungs from being easily crushed and damaged.
To increase the efficiency of
gas exchange in the lungs the bronchus divides in two (the bronchi), so each
lung gets a good supply of air. Each bronchus divides and divides into many
bronchioles with a tiny sac at the end of each one - the alveoli - which
considerably increases the area for oxygen and carbon dioxide gas exchange.
Know and understand that the breathing system
takes air into and out of the body so that oxygen from the air can diffuse
into the bloodstream for respiration, and waste carbon dioxide from
respiration, can diffuse out of the bloodstream
into the air.
This gas exchange happens in the
lungs which has millions of tiny air sacs called alveoli at the ends of the
finest bronchiole tubes - a large surface area for gas exchange.
Surrounding the alveoli are many small arteries (fine
capillaries) bringing a good supply of 'dark red' deoxygenated blood to the lungs
- the thin walls of the fine capillaries of the small arteries
mean a short distance to enable faster diffusion rates for the
exchange occurs on the moist membrane surfaces of the alveoli and the fine blood
vessels - the moisture in the membranes is good for dissolving gases and
increases the rate of gaseous diffusion.
When the blood from the rest of
the body arrives at the alveoli in the lungs it contains a
relatively high concentration of carbon dioxide and low
concentration of oxygen.
This maximises the diffusion
concentration gradients for the gas exchange i.e. the blood to
absorb fresh oxygen from the alveoli and the expulsion of carbon
dioxide from the blood in breathing out.
Direction of diffusion
gradients - from high to low concentration:
alveoli ==> blood, favours
oxygen transfer by diffusion through the membranes
blood ==> alveoli, favours
carbon dioxide transfer by diffusion through membranes
Therefore the oxygen diffuses out
of the air into the blood capillaries of the alveoli (from high to
low concentration) and carbon dioxide diffuses out in the opposite
direction from the blood to the air in the lungs (again, from high
to low concentration).
So, oxygen, from breathing in, is transferred from the air in the
alveoli into the fine veins which carry the 'bright red' oxygenated blood away to
where it is needed in the rest of the body. Simultaneously carbon dioxide
diffuses in the opposite direction, from the deoxygenated blood into the alveoli
and breathed out.
The alveoli are well designed by
evolution to perform this gas exchange efficiently.
To increase the
probability to transfer gas molecules ...
... the alveoli have a huge surface area because
of their tiny sac like structure,
the cell membrane lining is moist to
the sac walls are thin to reduce diffusion to time,
excellent blood supply of numerous tiny blood vessels - vein and artery
Know and understand that to make air move into the
lungs the ribcage moves out and up and the diaphragm becomes flatter.
changes are reversed to make air move out of the lungs.
Know the movement of air
into and out of the lungs is known as ventilation.
should be able to describe the mechanism by which ventilation takes place,
including the relaxation and contraction of muscles leading to changes in
pressure in the thorax.
As you breathe in, the
intercostal muscles contract expanding the rib cage, and the diaphragm also
contracts making it flatter, both of which increase the volume of the
This has the effect of
decreasing the pressure in the lungs and allowing air to be easily drawn in,
the air will flow in naturally, due to the pressure difference between the
air in the lungs (lower pressure) and the 'outside' air (higher pressure).
In breathing out, the
intercostal muscles relax (ribcage contracts), the diaphragm relaxes and
moves up, so the combined effect is to increase the air pressure in the
lungs and air is expelled.
Artificial ventilators move air
into and out of a persons lungs, where they cannot work unaided. This may be because some injury or medical
condition or undergoing an operation, which prevents them from breathing
This used to be done by a large
'capsule' called an 'iron lung' which encased the whole body of the patient
except for the head.
When the pump temporarily stops,
the ribcage relaxes, contracting the lungs and expelling the air.
This is a much more convenient
method with a wide range of applications, and, it doesn't interfere with the
body's blood supply, but there can be problems if the alveoli (may burst)
can't cope with the artificially increased air supply.
The human circulatory system - heart, lungs, blood,
blood vessels, causes/treatment of cardiovascular disease
Possible practical work
You can use
sensors, eg spirometers, to measure air flow and lung volume
The structure of fish gills
Fish have a single circulatory system in which
deoxygenated blood from the fish's body is pumped to the heart, which then
pumps it through the gills to absorb oxygen from the water and round through
the rest of the body in one continuous loop - just one circuit in operation
(unlike the double circulatory system of mammals).
Gills are the gas exchange system in fishes and
the structure provides a large surface area for oxygen to be absorbed
into the blood stream and waste carbon dioxide passed out.
Water, containing dissolved oxygen, enters the fish
through its mouth and passes out through the gills.
In the gills, oxygen diffuses from the water to the
blood, simultaneously, carbon dioxide diffuses from the blood into the
To make the gas exchange process as efficient as
possible, the surface area of the gills is greatly increased by the presence
of lots of thin plates called gill filaments.
The surface area is increased even more by lots of
tiny thin tissues called lamellae (plural of lamella).
The lamellae of lots of blood capillaries, increasing
the contact area to speed up the diffusion of gases - oxygen or carbon
The lamellae also have a thin layer of surface cells
to minimise the gas diffusion distance.
The blood flows through the lamellae in one direction
and water flows over them in the other direction and this produces a
continuous high concentration gradient between the blood and water.
The concentration of oxygen in the water is always
higher than its concentration in the blood so maintaining a good supply
of oxygen to the blood by diffusion from the water.
I presume the concentration of carbon dioxide is
higher in the blood than in the water, so the waste gas is continually
diffusing out of the blood?
Exchange surface in the small
The small intestine is where dissolved digested
food particles are absorbed from the digestive system into the
bloodstream to supply the cells with the necessary nutrients.
The transfer through the partially permeable
membrane might be by 'natural' diffusion down a diffusion gradient
or by active transport against a diffusion gradient.
The partially permeable membrane regulates the
transfer of substances.
The efficiency of the process is considerably
increased by the structure of the small intestine ..
a single layer of surface cells - short
diffusion time and distance,
a large surface area for absorption,
and a good blood supply from numerous
all of which speed up the process, so read on
for the detail ...
Know and understand that the villi
in the small intestine provide a large surface area with an extensive
network of thin blood capillaries to absorb the products of
digestion by diffusion and active transport.
The tissue lining in the small
intestine is covered with millions of protuberances called villi, which poke
up from the intestine surface into the partially or wholly digested food
The villi consist of a single
layer of cells (thin) on the very large surface area of the intestine.
Both factors considerably speeds
up the food absorption process.
Each villus (of the millions of
villi) has single layer of surface cells and each villus contains a
multitude of fine blood capillaries into which the small digested food
molecules can rapidly diffuse and be absorbed into the body.
A good blood supply is needed to
efficiently carry the digested food away to where they are
The food molecules can
diffuse into the bloodstream down a normal concentration
gradient, but sometimes active transport is required. See
Diffusion, osmosis and active transport
For example ...
When there is a higher concentration of
glucose in the intestine than in the bloodstream, glucose molecules
will naturally diffuse into the blood stream down the
diffusion gradient (concentration gradient from higher to lower
However, if there is a lower concentration of
glucose in the intestine, your body still needs glucose for
respiration, therefore active transport must be deployed.
This uses energy in such a way as to transfer glucose molecules from
the intestine against the natural concentration (diffusion)
Enzymes - structure, functions, optimum conditions,
investigation experiments gcse
biology revision notes
exchange in plant leaves
For plants see
in plants notes