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