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MOVEMENT ACROSS MEMBRANES
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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.
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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 doesn’t 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 < 0.5m.– Phagocytosis
• uptake of large particles and debris > 0.5m.– 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