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MOVEMENT ACROSS
MEMBRANES
Role of plasma membrane
• Maintains the cell’s internal
environment by:
– Keeping some substances in and other
substances out
– Allowing the controlled passage of
specific substances from one side of the
cell membrane to the other
– Receiving signals
– Assisting with cell to cell
communication.
Movement across membranes
• The plasma membrane regulates the
movement of molecules into and out
of the cell.
• This movement depends on the
composition of the membrane and
the surface area available for
exchange.
Composition of the membrane
• One of the most important properties of
membranes in their lipid nature.
• This makes them impermeable to:
– most water-soluble molecules
– ions
– polar molecules
• These substances require specific channels
(made from protein molecules) to pass
through the plasma membrane .
The plasma membrane is
differentially permeable
• This means that some
substances can pass
through the plasma
membrane but others
cannot.
• We say that the
membrane is it is
differentially
permeable or
selectively permeable
or semipermeable.
Moving across membranes
• In general, substances pass in and
out of cells by the following
processes:
– Simple diffusion
– Osmosis
– Facilitated diffusion
– Active transport
– Endocytosis and exocytosis
Simple Diffusion
• Defined as the movement of particles (molecules or ions) from a
region where they are at a relatively high concentration to a
region where they are at a lower concentration.
• The difference in concentration between the two regions is called
the concentration gradient or diffusion gradient.
• Diffusion always takes place wherever such a gradient exists and
continues until the particles are uniformly distributed
throughout the system.
• When that happens, equilibrium is said to be reached.
• Diffusion is passive process. It does not require additional energy
and it takes place equally readily in non-living and living systems
Factors influencing rate of
simple diffusion
• In general, the diffusion rate is higher when:
–
–
–
–
the concentration gradient is greater
when heat is applied
when molecules are smaller
when movement occurs through a gaseous medium.
• Chemical properties also influence the rate of simple diffusion.
– The hydrophobic nature of the interior of a plasma membrane means
only small relatively non polar molecules such as O2 and CO2 can
quickly permeate the membrane by simple diffusion.
– H2O is a polar molecule but gets away with simple diffusion because of
its very small size, however, water molecules diffuse at 10,000 times
slower than they would without a membrane being present.
– Ethanol and glycerol are much larger than water but can still use
simple diffusion to cross the lipid bilayer at reasonable rates because
they are non-polar.
Osmosis
• Special type of simple diffusion.
• Requires no input of energy.
• Osmosis is defined as the net movement of a
solvent, usually water, across a differentially
permeable membrane from a weak or dilute
solution (high water concentration, low solute
concentration) to a strong solution (low water
concentration, high solute concentration).
• More simply, osmosis is the net movement of free
water molecules from a dilute solution through a
partially permeable membrane to a concentrated
solution.
Osmosis
• The differentially permeable membrane allows the small water
molecules to move through but not the larger sucrose molecules.
• Because there are more free water molecules in the less
concentrated solution, there will be a net movement of water
from the dilute to the concentrated solution.
• This is osmosis – the diffusion of water along its own
concentration gradient.
Osmosis and cells
• The plasma membrane of a cell is differentially
permeable to water, and therefore, the amount of
water in the external environment will affect the
concentration of the solution within the cell.
• The external environment may be isotonic,
hypertonic or hypotonic compared to the cellular
environment.
– Isotonic solutions
• Solute concentration the same as intracellular fluid.
– Hypertonic solutions
• Solute concentration higher than intracellular fluid,
therefore lower osmotic pressure.
– Hypotonic solutions
• Solute concentration lower than intracellular fluid,
therefore higher osmotic pressure.
Osmosis and cells
Facilitated Diffusion
• Charged particles (ions) and relatively large
molecules, such as glucose, do not readily pass
through the plasma membrane because of their
size or polar nature.
• In the plasma membrane certain proteins assist
such particles to diffuse in or out of the cell.
• This process is called facilitated diffusion.
• Transport protein molecules span the membrane
from one side to the other.
• There are two types of transport proteins:
– channel proteins
– carrier proteins.
Channel Proteins
• Channel proteins are particularly involved with transporting ions
in and out of cells.
• They form a water-filled pore in the membrane.
• The lining of the channel is hydrophilic and so water-soluble
substances such as ions pass through it relatively easily.
• The channels are selective, allowing certain ions to pass through
but not others.
• Channel proteins speed up the rate at which ions diffuse across
the plasma membrane.
• Movement is passive and doesn’t involve the transfer of energy.
• Can can only take place down a concentration gradient (from
high to low concentration).
More about channel proteins
• Some of the channels can open and close
rather like gates.
• These gated channels open only when they
receive an appropriate signal.
• The signal may be:
– mechanical disturbance of the membrane
– a change in the voltage across the membrane
– binding of another molecule or ion with the
protein.
Carrier Proteins
• Combine with the diffusing molecule or ion, which is then carried
across the membrane and deposited on the other side.
• This requires changes in the conformation of the protein.
• Carrier proteins bind their solutes in such a way as to shield the
polar or charged groups from the non polar interior of the
membrane.
• Relationship between the carrier protein and the transported
molecule is specific.
Active Transport
• Involves movement of molecules
or ions against a concentration
gradient, from a region of low
concentration to a region of
higher concentration.
• Only takes place with an input of
energy, generally derived from the
ATP molecules.
• Allows cells to take up nutrients
even when their concentration
outside the cell is very low.
• Allows cells to get rid of
unwanted substances even when
their concentration is much
greater outside the cell.
• Cells that are actively pumping in
substances against the
concentration gradient are found
to contain many mitochondria.
Endocytosis and exocytosis
• In addition to solutes transported by
diffusion and active transport, the cell
also has to transport very large molecules
and aggregates of macromolecules across
the cell membrane.
• Substances such as these enter the cell by
endocytosis or leave the cell by exocytosis.
• Both these processes require energy.
Endocytosis
• Unique to eukaryotic cells.
• Three forms of endocytosis – names are
different but process is essentially the
same
– Pinocytosis
• uptake of fluid and particles < 0.5mm.
– Phagocytosis
• uptake of large particles and debris > 0.5mm.
– Receptor mediated endocytosis
• requires specific binding of protein or other ligand to
a specific receptor on the surface of the plasma
membrane in order to stimulate endocytosis of bound
ligand
Endocytosis
General description of
endocytosis
• Endocytic vesicles are formed by the plasma membrane
ballooning in to form a pocket that contains the material
from the exterior of the cell.
• The pocket begins to pinch off enclosing the extracellular
material and a vesicle is formed.
• The vesicle containing extracellular material separates
from the cell membrane and moves into the cytoplasm of
the cell.
• These vesicles may then
fuse with other vesicles
called endosomes.
• Endosomes may mature
into lysosomes or fuse
with existing lysosomes.
Exocytosis
• Is the process in which substances produced by the cell are
carried in vesicles and released to the exterior of the cell.
• Vesicles that store cellular products for export from the
cell are called secretory vesicles.
• The secretory vesicle moves to the plasma membrane, fuses
with the plasma membrane and the contents of the vesicle
are discharged to the outside of the cell.
• Important substances such as hormones, digestive enzymes
and even toxins are secreted from cells in this way.
• Exocytosis may be constitutive or regulated (i.e. triggered
by a specific signal).
• For example
exocytosis in
neuronal
chemical
synapses is
triggered by
Ca2+.
Exocytosis