* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download PASSIVE TRANSPORT
Survey
Document related concepts
Light-dependent reactions wikipedia , lookup
Node of Ranvier wikipedia , lookup
Electron transport chain wikipedia , lookup
Photosynthetic reaction centre wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Biochemistry wikipedia , lookup
Magnesium transporter wikipedia , lookup
Western blot wikipedia , lookup
Signal transduction wikipedia , lookup
Metalloprotein wikipedia , lookup
Magnesium in biology wikipedia , lookup
Transcript
The transport through the plasma membrane LESSON NR. 10 - PSYCHOBIOLOGY Diffusion, Osmosis and electrochemical gradient The combination of a solute in a solution is gathered through a process called DIFFUSION For example, milk in tea or ink in water. Role of the thermal agitation Diffusion, Osmosis and electrochemical gradient In DIFFUSION: At the beginning the molecules are all concentrated in one point of the glass, and very rare or absent at other points. With the passage of time, the number of molecules (which move in a completely independent manner) which will tend to move from the area with the highest concentration to the one with the lowest concentration will be greater of the molecules moving in the opposite direction. This type of movement is defined according to the GRADIENT OF CONCENTRATION and brings the solute in the liquid in a BALANCE OF CONCENTRATION (No gradient). The exact same phenomenon can also be observed in two communicating vessels separated by a semipermeable membrane (i.e. that lets pass only a certain type of molecules) Diffusion, Osmosis and electrochemical gradient OSMOSIS: It is a phenomenon in which there is a flow of solvent (usually water) between two solutions separated by a semipermeable membrane; the phenomenon is generally due to concentration differences, and in that case the solvent flows from the less concentrated solution to the more concentrated until equilibrium is reached or osmotic isotonicity Diffusion, Osmosis and electrochemical gradient ELECTROCHEMICAL GRADIENT: If the substance which diffuses through the semi-permeable membrane is provided with electric charge as are for example, the ION, the final condition of balance will not only be influenced by concentration and then by the gradient, but especially by electrostatic total forces that are generated between the two compartments separated by the membrane. The transport through the plasma membrane Now we must apply the concepts outlined above to the plasma membrane, especially the one of the neurons, and then we look at the main player of the movement across the plasma membrane: • The indoor and outdoor salt solutions • The plasma membrane • The trans-membrane proteins The transport through the plasma membrane Bearing in mind the ability of water to create a shell of hydration around the polar molecules, and thus having in mind that all free ions in the intra- and extra-cellular water solutions are hydrated ions. This feature gives the ions a strong hydrophilicity and does not allow the latter to pass through the hydrophobic wall of the phospholipid bilayer, if not through the presence in the special channels formed by the transmembrane protein membrane. The transport through the plasma membrane The transport through the plasma membrane can then be of two types: passive transport: transportation to the concentration gradient, thus based on DIFFUSION and therefore does not require energy consumption Active Transport: Transport AGAINST the concentration gradient which therefore requires the energy consumption PASSIVE TRANSPORT In passive transport, the transfer across the transmembrane of a substance: 1. Follows the concentration gradient of the substance. 2. It reaches a final state of equilibrium characterized by the equality of the substance concentrations in the two compartments separated by the membrane. 3. In the particular case of the free ions and the concentration of the molecules electrically charged it depends on the balance over that electrochemical equilibrium gradient. 4. It does not require an energy expenditure It can be of two different types: passive or simple diffusion facilitated diffusion PASSIVE TRANSPORT: simple diffusion It represents the (rare) mode of entry / exit from the cell of all those molecules for which the plasma membrane is not a barrier. As passive transport, it only relies on the concentration gradient. They are generally non-polar small molecules that are able to go into the spaces left by the phospholipids: gaseous molecules (O2, CO2 and N2) or small hydrophobic molecules (benzene, ammonia, glycerol, urea etc ...) The membrane crossing speed in the simple diffusion is directly proportional to the lipid solubility of the molecule that crosses it. PASSIVE TRANSPORT: facilitated diffusion It is the most common mode of entry / exit from the cell of sugars, amino acids, nucleosides and free ions. It involves typically polar molecules that in order to cross the phospholipid barrier must make use of facilitation/mediation of suitable protein complexes (usually specific to various substances) PASSIVE TRANSPORT: facilitated diffusion These protein complexes bordering on both sides of the membrane can be distinguished in protein-carrier or permease, and channel proteins PASSIVE TRANSPORT: facilitated diffusion The carrier proteins interact directly with the molecules of solute, recognizing in stereospecific manner the solute and thus forming the carrier-solute complex, which in turn through conformational change permits the passage of molecules from one environment to the plasma membrane. They typically carries the metabolites and generic molecules. They can act sometimes (with a right amount of energy) for the active transport PASSIVE TRANSPORT: facilitated diffusion The channel proteins interact with the solute in weaker way and allow the passage of hydrophilic molecules through a pore whose opening and closing are regulated through accurate and poly-specific mechanisms. Carrying more typically ions, these are defined as ionic channels PASSIVE TRANSPORT: facilitated diffusion An important feature of facilitated diffusion is its saturability. In fact, while in the case of the transported molecules by simple diffusion a concentration increment corresponds to the increase of speed of transport, in the facilitated diffusion a concentration increment will lead to a certain point to the saturation of the transport speed level. PASSIVE TRANSPORT: facilitated diffusion A particular case of facilitated diffusion is the one involving the water. In this case it is not the solute to pass from one compartment to another, but the solvent. The passage of water occurs in specific channels called aquaporins. These channels are so frequent within the membrane that the speed of passage resembles that of a simple diffusion. THE ACTIVE TRANSPORT It is said active transport the transfer of a given molecule from the compartment (intra- or extra-cellular) in which the substance has a lower concentration to he one in which it has a higher concentration. Then in the opposite direction to the gradient of concentration (or the electrochemical gradient in the case in which the molecules are provided with electric charge). This type of transport is defined as concentrative and always requires an energy expenditure, and is then mediated by transporters. Depending on the type of energy used by carriers we can distinguish: Primary active transport use of chemical energy and cleavage of ATP to ADP Secondary active transport use of alternative energies, eg. electrochemical gradient for an ion Active transport is the main mode of exchange through the membrane of many cell metabolites (sugars, amino acids, nucleosides) but they can also take advantage of passive transport. Primary active transport use of chemical energy and cleavage of ATP to ADP The transformation of ATP into ADP occurs in specific protein regions called ATP-Binding Cassette, the presence of these sites involves the particular naming of these transporters: ABC. Secondary active transport use of alternative energies, eg. electrochemical gradient for an ion The glucose is transported into the cell through a secondary active transport (energy of the electrochemical gradient of Na + ions) ABSORPTION OF GLUCOSE in the intestinal epithelium The glucose is transported out of the cell through a passive transport by facilitated diffusion (mediated by carrier proteins) Secondary active transport use of alternative energies, eg. electrochemical gradient for an ion In secondary active transport, we can distinguish: Uniport: secondary carriage of one substance that moves by exploiting the difference in electrochemical potential created by the primary conveyor. Co-transport is the simultaneous transport of two ionic species or other solutes and can differentiate into: • Symport: uses the flow of a second solute gradient to move another molecule against gradient with a movement that takes place in the same direction. An example is the glucose simportatore, that cotrasporta second gradient two sodium ions for each molecule of glucose imported into the cell • Antiport: is the simultaneous transport of two ionic species or other solutes that move in different directions through the membrane. One of the two substances is left flowing second gradient, from a high-concentration compartment to a low concentration. This generates the entropic energy needed to drive the other solute against gradient, from low to high concentration. A typical example will be represented by the Na + / K + ACTIVE TRANSPORT: endocytosis AND exocytosis Endocytosis is an encapsulation of materials determined by the intervention of the membrane active process, which changes its shape, surrounding the material to be introduced, and then enclose it in a vesicle that is free within the cytoplasm. The endocytosis requires considerable energy consumption and enables the intake of material with great size. If the material is solid is called phagocytosis, pinocytosis if it is liquid. To take these materials the cell changes its shape, emitting cytoplasmic extensions, called pseudopods, surrounding the material and enclose it in a vesicle, constituted by a portion of the membrane, which is then free within the cell. Endocytosis is a process by which the substances are incorporated into the cell within vesicles which are derived by the introversion of the membrane towards the interior of the cell. Exocytosis is the reverse process, by which the vesicles contained in the cytoplasm merge with the cell membrane, thus releasing outside the cell to their contents. This process occurs, for example, in nerve cells TRANSPORTERS IN THE MEMBRANE Regardless of the different modes of transport set out above, all of the membrane transporters have certain characteristics in common: Are trans-membrane proteins that look out on both sides of the membrane Present on their inner / outer portion of a specific recognition region of the single molecule, called binding site In consequence of the arrival of the molecule to be transported, the protein undergoes a conformational change that allows the passage A LITTLE TEST Passive transport: A. Passive diffusion B. Facilitated diffusion Active Transport: D. Primary transport (against gradient) E. Secondary transport: C. Uniport E. Symport Exocytosis / endocytosis: F.-G. exocytosis ION CHANNEL Ion channels are protein systems that run through the entire thickness of the plasma membrane and mediate the PASSIVE transport of free ions by facilitated diffusion. They can be comprised of a single protein or by a multi-protein complex which includes several sub-units (each indicated by a Greek letter). The various sub-units form a protein complex that interacts directly with the membrane lipids and has in its interior an aqueous pore that opening on both sides of the cell constitutes a passage or gate. ION CHANNEL Being passive transport, the channels can not influence the direction of movement that will always be done according to the concentration gradient, but it can regulate the flow, depending on whether the channel is open or closed. Through these adjustments the ion channels may change the difference of electric intra-and extra-cellular fillers, which normally constitute the resting membrane potential. In the nerve cells (or in general in excitable cells) they are involved in the generation of action potentials (through the passage of hundreds of millions of ions per sec.) The importance of ion channels is finally proven by diseases that result from their failure (channelopathies) which may occur by genetic causes (cystic fibrosis, idiopathic epilepsy, spinocerebellar ataxia etc) or toxic causes (poisons or drugs) ION CHANNEL Ion channels as many other protein compounds can exist with a large variety of isoforms, made of alternatives polypeptide chains, that are originated by post-translational modifications or through editing of mRNAs. This confers highly specific structural and functional characteristics to a channel and / or its isoforms, enabling the cell to obtain specialized functions for the various molecular compounds. ION CHANNEL An important feature of ion channels is their selectivity for a given ion or a given type of ion (anions, cations). This property depends on the structural and electrostatic features, in the aqueous pore. In fact, the mouth of the channel is normally provided with a filter (or ring) of selectivity, composed of amino acids with positive or negative electric charge. Other selectivity factors depend on the size and structure of the aqueous pore, as well as in the molecular mechanism of ion transfer through the pore (Ion stripped of the hydration shell) ION CHANNEL Another very important feature of ion channels refers to control of their opening. In fact, apart from a few channels that are constantly open (resting or leak, respectively the Cl- and K +) channels are normally in the closed mode and open only in the presence of an appropriate stimulus, remaining open for a few msec. Immediately after the opening, many ion channels undergo another conformational change which consists in the occlusion of the aqueous pore and in contemporary inability to open up again, even in presence of the stimulus that normally determines the opening. This inactive or refractory condition is usually maintained for a few msec, after which the ion channel get back in closed mode. ION CHANNEL Since channels dedicated to the passage of the same ion may have different structural organizations, it is preferred to classify ion channels according to the adjustment of their opening mode. Following these criteria the channels are grouped into relatively homogeneous families of channels. We can have: Channels governed by the membrane potential Channels regulated by ligand Channels regulated by other means (mechanical tension, light, etc ...) Ionotropic receptors • Channels governed by the membrane potential They are also referred to as voltage-gated channels or channels regulated by voltage. They give to the excitable cells the ability to vary in a few msec, the intracellular content of electric charges as compared to the outside. The voltage-dependent channels are so called because they open only when the cell membrane potential reaches a certain value, called threshold. This type of channels has a high level of selectivity for the ions or the type of ions that pass through them. • Channels governed by the membrane potential These capabilities, depend on the channel structure, which includes 4 domain of about 300-400 amino acids. Each domain contains 6 different trans-membrane segments (S1 to S6, in the secondary structure of alphahelices) which are arranged in a symmetrical manner so as to form the tetramer that comprises the channel. Of the 6 segments, s5 and s6 make up the opening of the aqueous pore and between them there is a loop of about 20 amino acids which together with the other 3 loops, forms the selectivity filter. The segments s1 to s4 are formed by electrically charged amino acids and form the voltage sensor. Some channels may ultimately have other sub-feature units that mediate very important phenomena for the channel as its correct position on the plasma membrane