MOVEMENT%20ACROSS%20MEMBRANES

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MOVEMENT ACROSS MEMBRANES
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Role of plasma membrane

• Maintains the cells 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.

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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.

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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 .

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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.

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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

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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

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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.

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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.

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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.

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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.

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Osmosis and cells
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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.

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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 doesnt involve the
  transfer of energy.
• Can can only take place down a concentration
  gradient (from high to low concentration).

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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.

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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.

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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.

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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.

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Endocytosis

• Unique to eukaryotic cells.
• Three forms of endocytosis names are different
  but process is essentially the same
• Pinocytosis
• uptake of fluid and particles lt 0.5mm.
• Phagocytosis
• uptake of large particles and debris gt 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

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Endocytosis
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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.

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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.

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Exocytosis