Brown's Biology AQA GCSE BIOLOGY 3
AQA GCSE B3 Biology Unit B3.1 Movement of molecules in and out of cells
AQA GCSE BIOLOGY UNIT
for GCSE Triple Award Science or GCSE Biology
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AQA GCSE Science BIOLOGY B3 Unit
B3.1 Movement of molecules in and out of cells
Appreciate that we need to understand how
biological and environmental systems operate when they are working well in
order to be able to intervene when things go wrong.
Appreciate that modern developments in
biomedical and technological research allow us to do so.
Know and understand that the cells, tissues and organs in
plants and animals are adapted to take up and get rid of dissolved
Know that different conditions can
affect the rate of transfer.
Sometimes energy is needed for
transfer to take place - active transport.
You should be able to use your skills, knowledge and understanding to:
evaluate the development and use
of artificial aids to breathing, including the use of artificial ventilators,
evaluate the claims of manufacturers about sports drinks,
evaluate the conditions that affect water loss in plants.
AQA GCSE Science BIOLOGY Unit B3.1.1 Dissolved substances
a) Know and understand that dissolved substances move by diffusion and
by active transport.
b) Know and understand that water often moves across boundaries by
osmosis - a specific case of particle diffusion.
Know that osmosis is the diffusion
or bulk movement of water from a dilute to a
more concentrated solution through a partially
permeable membrane (semi-permeable membrane) that allows the passage of
A partially permeable membrane
has extremely small pores or holes that only allow the tiniest of molecules
like water through.
Other molecules eg even
relatively small molecules like sugars or amino acids and any larger
molecules like proteins or glycogen, will not pass through a semi-permeable
So, in living organisms, only
water gets through and depending on the concentration of dissolved
substance, osmosis can happen in either direction across a partially
Although the water molecules
(and any other particles) are moving around at random, there will be a net
transfer of water in one direction at a time through a partially permeable
... the water will diffuse from a
less concentrated solution to a more concentrated solution i.e. from the
higher concentration of water molecules to a lower concentration of water
molecules across the membrane
This osmosis diffusion can occur in either direction depending on the
relative concentration of the solutes in the cell fluids or tissue fluids
and concentrated solutions e.g. of sugars, will tend become diluted by water
passing through the partially permeable membrane.
You can do simple experiments to
demonstrate this by placing blocks or cylinders of potato into pure water
and a series of sugar solutions (e.g. glucose) increasing in concentration
(increasingly higher molarity mol/dm3).
The potato 'chips' will shrink
and lose mass the more concentrated the sugar solution because the water
will undergo osmosis and diffuse through the partially permeable membranes
of the potato cells to try and dilute the sugar solution.
Use of the terms turgor and plasmolysis is
c) Know and understand that differences in the concentrations of the solutions
inside and outside a cell cause water to move
into or out of the cell by osmosis.
The soft cell wall, or outer
membrane of an animal
cell, acts as a partially permeable membrane.
The water surrounding cells, the
tissue fluid, contains the dissolved molecules the cell needs to survive eg
sugars, amino acids, oxygen, as well as waste carbon dioxide etc.
If the cells are short of water
('partially dehydrated'), the concentration of dissolved substances
increases, so water diffuses through the cell membrane into the cells to dilute the cell
fluids until equilibrium is established. Conversely, if the cell solution is
too dilute, then water will diffuse out from osmotic action across the
semi-permeable membrane of the cell wall.
d) Know that most soft drinks contain water, sugar and ions.
e) Know and understand that sports drinks contain sugars to replace the sugar
used in energy release during the activity.
f) Know and understand that if water and ions are not replaced, the ion / water
balance of the body is disturbed and the cells do
not work as efficiently.
g) Know and understand that substances are sometimes absorbed against a
This means transfer occurs in
the opposite direction to the natural direction of diffusion and osmosis.
Know that this requires the use of
energy from respiration and this process is called active transport.
Know that active transport enables cells
to absorb ions from very dilute solutions.
Active transport is required to
absorb nutrients like amino acids, sugars like glucose etc. from the gut
when the concentration in the gut is lower than their concentrations in the
blood supply, and a healthy body requires these nutrients all the time.
If the concentrations of
nutrients in the gut is higher than that in the blood stream, then the
nutrients will naturally diffuse into the blood stream because of the
direction of the concentration gradient (more concentrated ==> less
If the concentration gradient
flow is in the direction of blood stream (higher) to gut (lower), then
respiration powered active transport must be used to work against the
natural diffusion flow.
Remember that absorption by
diffusion down the concentration gradient through membranes doesn't require
energy from respiration
h) Know and understand that many organ systems are specialised for exchanging
i) Know that gas and solute exchange surfaces in humans and
other organisms are adapted to maximise effectiveness.
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 and unwanted chemicals.
To increase the probability 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 layer so diffusion times
are short - cell membranes are usually quite thin,
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,
j) Know that the size and complexity of an organism increases the
difficulty of exchanging materials.
k) Know and understand that in humans:
the surface area of the lungs is increased by
the alveoli - tiny sacs of the end of the tiny bronchiole tubes in the lungs,
the surface area of the small intestine is
increased by villi.
l) 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
This considerably speeds up the
absorption process because the villi massively increase the effective
digested food absorbing surface area of the small intestine.
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 into, a good blood supply is needed to
efficiently carry the digested food away to where they are
AQA GCSE Science BIOLOGY Unit B3.1.2 Gaseous exchange
a) 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 on a diagram.
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.
b) 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. Surrounding the alveoli are small arteries (fine
capillaries) bringing 'dark red' deoxygenated blood to the lungs. The gas
exchange occurs on the membrane surfaces of the alveoli and the fine blood
vessels. Here, 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 and simultaneously carbon dioxide
goes 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, they have a huge surface area because
of their tiny sac like structure, the cell membrane lining is moist to
dissolve gases, the sac walls are thin to reduce diffusion to time and an
excellent blood supply of numerous tiny blood vessels - vein and artery
c) Know and understand that to make air move into the
lungs the ribcage moves out and up and the diaphragm becomes flatter.
AQA GCSE Science BIOLOGY Unit B3.1.3 Exchange systems in plants
a) Know and understand that in plants:
enters leaves by diffusion and then diffuses into cells where photosynthesis
most of the water and mineral ions are absorbed
b) Know and understand that the surface area of the
roots is increased by root hairs and the surface area of leaves is increased
by the flattened shape and internal air spaces.
The fine root hairs considerably
increase the surface area of the roots for absorbing water and minerals.
Root hair cells are very
elongated combining into fine hair-like structures, which greatly increases surface
contact with the soil from which most of the plant's water and mineral
intake are absorbed.
As long as the water
concentration is higher in the soil, the root hairs will naturally absorb
water by osmosis.
The concentration of minerals in
the root hair cells is higher than in the moisture surrounding the roots, so
a problem, because the plants cells would naturally lose essential mineral
ions by osmotic diffusion, not good!
Therefore, active transport
systems must be used by the plant to counteract the natural direction of
diffusion from a high mineral concentration in the plant cells to a low
mineral concentration in the soil moisture.
Energy from respiration is
usually required to absorb minerals into the roots from the soil moisture by
working against the concentration gradient.
Beneath the apparently flat
surface of a leaf is quite a porous layer of air spaces between the outer
layers of cells - particularly on the underside of leaves - quite often the
lower surface of a leaves feel rougher and 'roughness' means a more
disrupted surface of a larger surface area.
c) Know and understand that plants have stomata
(tiny pores or holes) to obtain
carbon dioxide from the atmosphere and to remove oxygen produced in
Carbon dioxide can diffuse in
through the stomata and oxygen can diffuse out.
Since carbon dioxide is being
used up in photosynthesis, the concentration gradient enables more carbon
dioxide to diffuse in through the stomata.
Water, and the oxygen produced
in photosynthesis diffuse out.
d) Know and understand that plants mainly lose water
vapour from their leaves.
Know that most of the loss of water vapour takes place
through the stomata.
Know that evaporation is more rapid in hot, dry and windy
Know that if plants lose water faster than it is replaced by the roots,
the stomata can close to prevent wilting.
The process of water movement from the roots
through the xylem and out of the leaves is called transpiration.
e) Know that the size of stomata is
controlled by guard cells, which surround them.
The size of the opening of the
stomata must be controlled by the guard cells or a plant might lose too much
water and wilt.
Water will diffuse out and
evaporate away much faster in less humid-drier, hotter or windier weather
The guard cells will respond to
the ambient conditions ie close up the stomata if the rate of water loss is
to great for water to be replenished from the roots.
keywords: gcse AQA active transport osmosis
biological & environmental systems exchange surface large area thin efficient
blood supply gaseous exchange ventilation lungs alveoli small intestine villi
Unit B3.1 Movement of molecules in and out of cells Dissolved substances Gaseous
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