AQA AS Biology Unit 1: Diffusion

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  • Created on: 22-08-13 16:16
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The exchange of substances between cells and the environment occurs in ways that require
metabolic energy and in ways that do not.
Active Transport Requires energy
Passive Transport Does not require energy
To understand diffusion and other forms of passive transport it is necessary to understand
All particles are constantly in motion due to the kinetic energy that they possess
This motion is random, with no set pattern to the way the particles move around
Particles are constantly bouncing off one another as well as off other objects
Diffusion The movement of molecules or ions from a region
of high concentration to a region of low concentration
Rate of Diffusion
There are a number of different factors that affect the rate at which molecules or ions
Concentration The greater the difference in the concentration of
Gradient molecules or ions on either side of an exchange surface,
the faster the rate of diffusion
Area over which The larger the area of an exchange surface, the faster
diffusion takes place the rate of diffusion
Thickness of The thinner an exchange surface, the faster the rate of
exchange surface diffusion
Diffusion is proportional to Surface area x difference in concentration
length of diffusion path

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Simple Lipid Diffusion
A few substances can diffuse directly through the lipid bilayer of the membrane.
The only substances that can do this are hydrophobic (lipid-soluble) molecules such as
water, oxygen and carbon dioxide.
Lipid diffusion is a passive process, no energy is involved and substances can only move
down their concentration gradients.…read more

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Facilitated Diffusion
Facilitated diffusion is a passive process.
It relies only on the kinetic energy of the diffusing molecules.
There is no external input of energy.
It occurs down a concentration gradient.
It occurs at specific points on the plasma membrane where there are special protein
Channel Proteins
These proteins form water-filled channels across the
They allow water-soluble ions to pass through.
The channels are selective, each opening in the presence of
a specific molecule.…read more


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