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Edexcel GCSE Biology Topic 1 - Key Concepts in Biology
Edexcel GCSE Biology Topic 2 - Cells and Control
Edexcel GCSE Biology Topic 3 - Genetics
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Exchanging Transport CGP Topic 8
Diffusion is the net movement (overall movement of particles) of particles
from an area of higher concentration to an area of lower concentration. It is
a passive (no extra energy needs to be added) process and the movement
of particles occurs down a concentration gradient (movement from high to
low concentration).
Osmosis is the net movement of water molecules from an area
of high solute concentration (e.g. pure water) to an area of
low solute concentration (e.g. salty of sugary water) across a
partially permeable membrane (lets some things through, but
not others). It is a passive process and the movement of
particles occurs down a concentration gradient.
Active transport is the movement of substances
against their concentration gradient (from a
lower concentration to a higher concentration)
which requires energy.
For example, there are more mineral ions (e.g.
NO3-) in the soil then in root cells. To get more
mineral ions into the root cell needs energy and
uses active transport. Active transport requires
energy, provided by mitochondria carrying out
aerobic respiration)
Concentration Gradients
There is a higher concentration of particles (e.g. Oxygen, water or glucose) at point A.
And a lower concentration of particles at point B.
So the particles move from A to B, passively by diffusion – setting up a gradient
Movement of particles may occur until the concentration is equal everywhere. At this
point there is no longer a concentration gradient
Particles continue to move around randomly, but there is no net movement in any
particular direction
, Movement of Substances
All organisms must take in substances they need for life processes from the environment. They must also
get rid of any waste products from these processes:
• Cells need oxygen for aerobic respiration, which produces carbon dioxide as a waste product. O2
and CO2 move between cells and the environment by diffusion.
• Water enters & leaves cells by osmosis. In animals, dissolved food molecules (e.g. glucose, amino
acids, fatty acids & glycerol) and mineral ions diffuse along with it.
• Urea (a waste product produced by animals from the breakdown of proteins) diffuses from cells
into the blood plasma for removal from the body by the kidneys.
Surface area X Volume - A ratio shows how big one value is compared to another; in this case it is a
measure of an organisms surface area in comparison to its volume. The larger an organism is, the smaller
its surface area is compared to its volume.
Factors Affecting Rate of Diffusion
• Surface area - the more surface there is available for molecules to move across, the faster they can
get from one side to the other.
• Concentration difference (gradient) - substances diffuse faster when there is a big difference in
concentration between two areas
• Distance to move - substances diffuse more quickly when they don't have to go far. The thinner the
exchange surface (e.g. cell membrane) the shorter the ‘diffusion pathway'.
The relationship between the rate of diffusion and the factors that affect it is described by Fick's law. It
states that:
It means that the rate of diffusion increases in proportion to (at the same rate as) the surface area or
concentration difference.
And in inverse proportion to the thickness of the membrane.
As the surface area doubles, the rate of diffusion… Doubles
As the concentration difference trebles, rate of diffusion … Trebles
As the surface area doubles and the concentration difference trebles, the rate of diffusion… Goes up by 6x
As thickness of cell membrane doubles, rate of diffusion… Halves
As thickness of cell membrane halves, rate of diffusion… Doubles
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