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Daniel Holloway
Contents
1 Exchange of Materials 3
2 Transporting Substances Around the Body 9
3 Microbiology 16
End of Unit Questions 22
Copyright © 2009 Daniel Holloway
Significant contribution Nelson Thornes AQA Science [GCSE Biology]
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Active Transport
Two of the main ways in which diffused substances are transported across cells are osmosis
and diffusion. Diffusion is the movement of a fluidacross a concentration gradient useful to
the cells; osmosis is the movement of wateracross a semi-permeable membrane. However,
sometimes substances need to be transported againsta concentration gradient or
membrane, which is when active transporttakes place.
By active transport, cells are able to move substances from an area oflow concentration to
an area of high concentration. This is what is meant by moving against the gradient. Because
the substances are being transported against a gradient, energyis required for an active
transport systemto carry a molecule across the membrane and then return to its original
position (see below diagram).
The energy required for active transport to take place comesfrom cellular respiration. The
rate of active transportand rate of respiration in cells are closely linked. The process of
respiration releases energy –so in other words, the more respiration happening, the more
active transport is taking place. This is why cells involved in active transport (e.g. root hair
cells and gut lining cells) usually have a lot of mitochondriato provide the energy needed
from respiration.
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Active transport is an important process in plants. The uptake of mineral ions through the
soil requires active transport because the ions are found in very dilute solutions, whereas
the solution inside the plant cells is a lot stronger. This means the ions have to be taken in
against the gradient (from dilute to concentrated). Glucose is moved out of the gut and
kidney into your blood, even though that is against a gradient. Active transport is also used
in marine birds and reptiles, because they consume large amounts of salt when theydrink
water, and as the kidneys cannot get rid of it all, they have salt glandswhich use active
transport. Without the ability for the cells to do active transport, these marine animals
would die, so active transport is essential to their lives.
Gaseous Exchange
We require a constant supply of oxygen to allow for respiration. Breathing in and out takes
in oxygen as a supply for the cells and removes the waste carbon dioxide produced by the
cells. The lungs(found in the thorax) are protected by the rib cage. The lungs are separated
from the digestiveorgans, found in the abdomen, by the diaphragm.
When you breathe in…
your ribs move UPand OUT
your diaphragm flattens
air is pulled INTOthe lungs
When you breathe out…
your ribs move DOWNand IN
your diaphragm returns to its
domed shape
air is forced OUT OFthe lungs
The lungshave been adapted especially for
making gas exchange more efficient. They are
made up of clusters of alveoli, which are tiny air
sacs with large surface areas, and are kept
moist. They also have a rich blood supply, which
maintains a concentration gradient in both
directions. Oxygen constantly being removed
from the blood and carbon dioxide constantly
entering the lungs means that gas exchange
happens at the highest concentration gradients
to make it rapid and effective.
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The diagram here shows an alveolus(singular of alveoli). It has been adapted to make
gaseous exchange as efficient as possible in the following ways:
sphericalshape gives large surface area
moist surface makes diffusion easy as gases can dissolve
thin walls make diffusion easy
good bloody supply (explained before)
The following table shows the effectiveness of the gas exchange in the lungs:
Air Breathed In AirBreathed Out
Nitrogen, N Approx 80% Approx 80%
Oxygen, O 20% 16%
Carbon Dioxide, CO 0.04% 4%
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The Human Gut
The food we eat is broken down in the gut. It forms simple sugars, such as glucose, amino
acids, fatty acids and glycerol. These products are of no use in the gut, and if they stayed
there would simply be removed with faeces. This is why, via a combination of active
transport and diffusion, the molecules from food enter the bloodstream. This is why food is
broken down duringthe digestion process. After being broken down, the food molecules are
small enough to pass through the walls of the small intestineand into the blood vessels.
They can move this way because there is a very high concentration of food molecules in the
gut, and a very low concentration in the blood, so the process here is diffusion. They move
along a very steep concentration gradient.
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The lining of the small intestine is folded into thousands of tiny villi(above). These greatly
increase the uptake of digested food by diffusion. This is because only a certain amount of
digested food molecules can diffuse over a certain surface area at a given time –so the villi
increase the surface area dramatically.
Diffusion is very rapid and efficient in the gut, because, as with the lungs, it has a rich blood
supply –so digested food molecules are carried away the second it diffuses from one side to
the other. Therefore, a steep concentration gradient is constantly maintained.
Material Exchange in Other Organisms
Fish have protective scales all over their bodies
which prevent them from directly taking in oxygen
from the water –so they have gills, made of very
thin layers of tissue with a rich blood supply. The
gills are thin so there is less surface area for the
gas to diffuse across. Fish do not need to worry
about keeping the gills moist, considering the live
in water.
Gills do not work in air, and “suffocate” out of water –because if they are not kept moist
constantly the gills stick together and there isn’t enough surface area for the fish to get
enough oxygen from the air to survive.
Frogs begin their life cycles as tadpoles. All tadpoles
begin with a set of gills to help them survive, seeing
as they spend their entire lives in water –but when
they become adult frogs, they can live on land and
yet breathe in water at the same time. Tadpoles get
their oxygen from diffusion through their gills, in the
same way they lose carbon dioxide along the
concentration gradient. But as the creature grows
into an adult frog, the gills are reabsorbed back into the body –and the gills aren’t there
anymore. We say the tadpole has undergone metamorphosis. An adult frog has moist, thick
skin with a rich blood supply. The majority of its gaseous exchange takes place through the
skin. However, gas exchange can be done through the mouth also, using the frog’s simple
lungs system. In water, allrespiration is done through the skin.
Insects’ muscles require a lot of oxygen because they are so active. But extremely little, or
no, gaseous exchange can take place through the tough outer covering of an insect, so they,
like us, have an internalrespiratory system which supplies oxygen to all the cells which need
it and removes carbon dioxide. Insects have many spiracles, which are tiny openings –they
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open when oxygen is needed and close when it is not. This also prevents water loss, much
like plant stomata. Spiracles lead to a tube system which delivers the oxygen straight to the
tissues where it is needed. Most of the gas exchange takes place in the tracheoles, tiny
tubes which are freely permeable to gases. They are moist and constantly have air pumped
in and out of them by the insect to maintain a steady concentration gradient.
Material Exchange in Plants
All plants require carbon dioxide and water for photosynthesis. The carbon dioxide is
obtained via diffusion through the leaves. Theflattened shape of the leaves increases the
surface area for diffusion to take place across. Leaves are usually flat, too, so that the
distance between the air and the photosynthesising cells is as short as possible.
A problem is that water is always
being lost by evaporation. So
allowing carbon dioxide in will
also lose water vapour. However,
the plant does not need carbon
dioxide all the time, because at
night there is no sunlight –so
photosynthesis cannot take place.
So they have openings known as
stomatawhich can open and close
at specific times to allow carbon
dioxide in and out. Another
adaptation is that they have a waxy cuticlecovering them, which is both gas-proof and
waterproof.
Roots have been adapted for uptake of water and mineral ions. Water is vital for shaping
cells and for photosynthesis. Minerals are needed to make proteins and other chemicals.
The roots themselves are thin and have a large surface area. The root hair cells have also
adapted to increase surface area and increase efficiency ofwater uptake. The cell
membranes of root hair cells have microvilliwhich further increase surface area for
diffusion and osmosis. The distance between here and the xylem(transport tissue for the
water) is minimal, also.
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Transpiration
The loss of water vapourthrough the surface of
the leaves is called transpiration. As water is lost
through the opening in stomata, more water is
pulled up through the xylem to take its place. This
constant movement of water around the plant is
known as the transpiration stream. Because it is
all caused by evaporation, anything affecting
evaporation on a plant will also affect
transpiration. Factors which increase evaporation
will also increase transpiration. Sunny and warm
conditions increase rate of photosynthesis, which
means more carbon dioxide is needed, which
means stomata are opened, which means water is lost –so these conditions also increase
transpiration rate: hot, dry and windy.
Other than having a waxy cuticle, another adaptation to help with the problem of water loss
is that a plant can wilt. This happens when water is being lost faster than it is being gained.
Wilting of leaves involves them collapsing and hanging downwards to prevent much water
loss by minimising the surface area.
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The Circulatory System
The blood circulation system we have is made up of three main
components: blood vessels, the heartand the blood. It is made up
of twodifferent blood systems –a double circulation. The diagram
shows that one transports blood from the heart to the lungs and
back again, the other takes blood around the rest of the body.
Having a double circulation is vital in animals like ourselves because
we are constantly active and in need of a rich blood supply –and
with this system, we are constantly receiving oxygenated blood from
the lungs which is sent around the body in one cycle.
There are three main blood vessels in the system, which have all adapted to carry out
specific functions. The diagram below shows each of them…
The arteries(left) carry blood away from the heart to the organs in the body. This is usually
oxygenated blood, explaining the redtubes. When you feel your pulse, that is the arteries
stretching as blood is forced through them and returning back into their original shape.
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The veinscarry blood towards the heart, usually low in oxygen and hence are deep purple-
red in colour. No pulse in veins, but they do contain valves usually which prevent the
backflow of blood.
The capillariesare found in junctions between the arteries and veins. These are found in
huge networks. The walls are a single cell thick so that substances which need to get out of
the blood and into body cells can easily via diffusion.
The Human Heart
Our hearts are made of two pumps, for the double circulation. These together beat around
seventy times a minute. The walls of the heart are made pretty much entirely from muscle,
which gets oxygen from the coronary blood vessels.
Blood Transport
The liquid part of our blood is called plasma. It transports red blood cells, white blood cells
and platelets. White blood cells (for immune system) and platelets (blood clotting) are not
involved in transporting materials around the blood –it is the red blood cells and plasma.
Blood plasmais a yellow liquid which transports all blood cells and other substances around
the body. Carbon dioxide produced in the organs is carried in plasma back to the lungs. It is
the red blood cells which give blood its red colour.
Urea, a waste product formed in the liver is carried in the plasma to the kidneys. In the
kidneys, urea is removed from the blood and transformed into urine. All the small, soluble
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