Diffusion is passive transport, which refers
to the natural, inbuilt motion of particles.
All particles are constantly in motion
due to kinetic energy they possess.
The motion is random
Particles are constantly
bouncing off one another
and other objects
If all particles are at one side, they will bounce and move to
the other side. There will be no movement in the reverse
direction initially, so there will be a net overall movement from
an area of high concentration to an area of lower concentration
Definition
The net movement of molecules or ions from a region where they are
more highly concentrated to one where their concentration is lower
Rate of diffusion
Concentration gradient
Greater the difference
in concentration of
molecules or ions on
either side of an
exchange surface, the
faster the rate of
diffusion
The path from an area of higher concentration
to an area of lower concentration
Particles diffuse down the concentration gradient
Area over which
diffusion takes place
The larger the area of an
exchange surface, the
faster the rate of diffusion
Thickness of exchange surface
The thinner the
exchange surface, the
faster the rate of diffusion
Surface area x difference in concentration/length of diffusion path
Facilitated Diffusion
Explanation
Like diffusion, it
occurs down a
concentration
gradient, but it
occurs at
specificpoints on
the plasma
membrane where
there are special
protein molecules.
These proteins form
water-filled channels
(protein channels)
across the membrane.
The channels are selective, each openin only in the presence of a specific ion.
These channels allow water soluable ions to pass through. Such molecules would usually
diffuse only very slowly through the phospholipid bilayer of the plasma membrane.
An alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane. When
a molecule, eg. glucose, that is specific to the protein present, it binds with the protein. This causes it to
change shape in such a way that the molecule is released to the inside of the membrane. No external
energy is required for this. The molecules move from a region where they are highly concentrated to one
of lower concentration, using only kinetic energy
Osmosis
Definition
The
passage
of water
from a
region
where it
has a
higher
water
potential
to a
region
where it
has a
lower
water
potential
Solutes and water potential
Water potential
The pressure
created by water
molecules. Pure
water is said to
have a water
potential of 0
The addition of a solute to pure water will lower its WP
WP of a solute must always be <0
The more solute added, the lower its WP
Water will move by osmosis from a region of higher (less negative) WP to one of lower (more negative) WP
Active Transport
Definition
The movement of
molecules or ions into or
out of a cell from a
region of lower
concentration to an area
od higher concentration
Explanation
ATP is needed
Used in 1 of 2 ways
To directly move molecules
Using a concentration gradient that has
already been set up by direct active transport.
Also known as co-transport
Against the concentration gradient
Carrier protein molecules are involved
Process is selective, specific substances are transported
Process
Carrier proteins span cell surface membrance and accept molecules or ions to be transported
Molecules or ions bind to receptors on the channels of the carrier protein
Inside the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule, casuing
the protein molecule to change shape and open to the opposite side of the membrane
The molecules or ions are then released to the other side of the membrane
The phosphate molecule is released from the protein and recombines with the ADP to form ATP during respiration