UNIT- II: PERMEATION
INTRODUCTION:
* Nutrients and other substances needed by the cells must penetrate the cell boundary layers before they can be metabolized. The cell wall does not present much of a barrier to small molecules and ions, but it excludes large molecules with a relative molecular mass above 600. The cell boundary component that governs the uptake of most substances into the cell is the cytoplasmic membrane or the cell membrane.
* The transfer of nutrients through the cytoplasmic membrane is usually specific. Only those nutrients are taken up by the cells for which a specific transport system is available.
* Transfer is dependent, with few exceptions, on specific Permeases and Translocases.
* These are membrane proteins and the names indicates that they exhibit some of the properties of enzymes.
* They are substrate specific, may be some times substrate inducible and are produced only under conditions that permit protein synthesis.
* Several types of mechanisms for the uptake of substances into the cell can be distinguished.
* Two of these permit only transport but not accumulation of material transported to the cell.
* On the other hand, several processes "Active transport" lead to intracellular accumulation of the material transported.
Types of Cellular Transport:
Permeation: Permeation includes the processes of:
1. Simple diffusion or Passive diffusion
2. Facilitated diffusion
3. Active transport
4. Group translocation
i) Passive diffusion:
Definition: Transport of metabolites or substances across the membrane along the concentration gradient and without the use of a carrier molecule is called passive diffusion.
* Diffusion occurs down a concentration gradient from a higher concentration to a lower concentration region.
* The concentration gradient disappears as the diffusion proceeds.
* Passive diffusion shows non-saturation kinetics (as carriers are not involved in, so no problem of saturation).
* The rate of diffusion is proportional to the difference in solute concentration on the inner and outer sides of the membrane.
* When equiliberium is reached, the concentration becomes same on both the sides of membrane & the diffusion stops.
* Passive diffusion does not involve stereospecificity [i.e. both L & D isomers can move across the membrane at equal rate].
* It take place as a result of random molecular movement & is a slow process.
* It is not an important mechanism for transport across the cell membrane.
* Diffusion through the membrane can take place through : i. Pores or channels in the membrne or ii. By dissolving in the lipid phase.
* Small polar molecules could cross the membrane through the pores which arise in a random manner and are transitory (not permanent).
* Larger molecules cannot penetrate the pores.
* Solute molecules can cross the membrane by dissolving in the lipid bilayer & diffusing across the membrane.
* Passive diffusion is governed by the molecular size & lipophilic properties of the material.
* The rate of such transport are low. Uptake of sugars by passive diffusion has never been demonstrated.
* Apparently water, non-polar toxins, inhibitors & other substances that are not part of the normal intracellular environment are taken up by the passive diffusion.
Facilitated Diffusion (FD):
* It resembles simple diffusion in that it does not require energy & takes place along the concentration gradient.
* It differs, however, in being stereospecific, in showing 'saturation kinetics ' and requiring a carrier.
* In FD a substance is transported into the cell along its concentration gradient i.e. towards equilibrium between the external & internal concentration.
* The process in mediated (in most cases) by a substrate specific permeases.
* The rate of transport is governed, over a wide range by substrate concentration in the medium.
* FD is independent of metabolic energy & the nutrient cannot accumulate inside the cell against concentration gradient.
* FD is stereospecific, i.e. only one of the two possible isomer L or D is transported.
* FD shows saturation kinetics, i.e. Increase in the concentration of substance to be transported results in an increase in the rate of transfer up to certain extent only.
* The rate of influx becomes constant & does not increase further with increase in solute concentration .
* Saturation kinetics indicates that a lipid-soluble carrier molecule is involved in the passage of solute across the membrane.
* When all the carrier molecules are occupied with the solute, the rate of influx becomes constant.
* No metabolic energy is required for facilitated diffusion.
Active Transport:
* Accumulation of solute against its electro-chemical potential through an energy - linked process is called as 'Active transport.'
* Active transport system differs from FD system, in that they consist of at least two components.
* One of them is solute specific carrier which acts in a catalytic manner and the other is a continuous supply of metabolic energy.
* A carrier coupled to a source of metabolic energy can transport solute against a concentration gradient or electro-chemical gradient.
* Active transport can maintain a concentration of an ion or a molecule several times higher on one side of the membrane than the other.
* Active transport is carried out in at least three different ways in bacteria.
* These include group translocation, membrane bound transport system and binding protein transport systems.
* Group translocation system utilize the chemical energy of the modification reaction.
* membrane bound transport systems utilize the proton- motive force across the membrane .
* Binding protein transport system utilize ATP or a related compound.
Mechanism of Active Transport:
* Active transport and group translocation share with facilitated diffusion, the participation of substrate specific protein.
* They differ from FD, however by their dependence on energy.
* When metabolic energy is available, the substrate can be accumulated inside the cell against the concentration gradient.
* The basic difference between active transport and group translocation is the nature of the product that is released inside the cell.
* In active transport, the molecule released into the cytoplasm is identical with that taken up from the medium.
* In group translocation, the molecule is modified during the transport process, for example by phosphorylation.
* Various models have been proposed for active transport, all of these include specific transport proteins in the membrane.
* these have been given names that indicate their presumed functions (permeases, translocase, translocator proteins & carriers).
* Different transport process are distinguished mainly by the way in which the energy necessary to drive the transport process is made available by proton potential, by ATP or by phosphoenol pyruvate.
* Trasnport of many substances, including inorganic & organic ions, as well as sugars, is driven by the proton potential.
* The bacterial cell maintains a proton potential by constantly pumping out protons & other ions (Na+).
* This is mediated by transport proteins located in the membrane, each of these proteins has a specific functions.
* one protein is known, for e.g. to catalyse the simultaneous transport of a sugar molecule and a proton in the same direction. This is referred as a Symport of two or more substances.
* Other transport protiens catalyse the simultaneous transport of two substances in opposite directions, this is called as Antiport.
* The ions that drive the entry of sugar into the cell are probably always H+ or NA+.
* In prokaryotes the H+ coupled symport system seems to predominate, where as in eukaryotes Na+ coupled symport is the rule.
* Active transport driven by a proton potential is probably the most common mechanism for the active uptake of metabolic substrates.
* In addition to transport system that depends on a proton potential, there are others that depends on ATP.
* In these the periplasmic binding proteins play a part.
* ATP-powered pumps (ATPases) couple the splitting, or hydrolysis, of ATP with the movement of ions across a membrane against a concentration gradient.
* ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy released is used to move one or more ions across the cell membrane.
* As much as 25% of a cell's ATP reserves may be spent in such ion transport.
* Examples include: The Na+ -K + ATPase pumps Na+ out of the cell while it pumps K+ in.
* The membrane potential is probably generated by ATP- dependent pumping mechanism, such as the sodium- potassium pump, and the Na+ potential then drives the Na+ nutrient symport.
* All the amino acids can be active transported, sodium- driven symport pumps.
Group Translocation:
* In group translocation , the transported molecule is chemically modified. or in Group translocation , molecules are transported across the cell membrane with chemical modification.
* ex. A sugar, for instance is taken up as such and is delivered inside the cell as sugar phosphate.
* Glucose, fructose, mannose & other carbohydrates are taken up by the phosphoenol pyruvate dependent posphotransferase system (PTS).
* The PTS catalyses the phosphorylation of several sugars.
* The Unique features of this system are that: i. The phosphoryl donor is not nucleotide triphosphate but phosphoenol pyruvate (PEP). ii. The system is very complex, involving many proteins.
* Group translocation involves solute modification. When the solute is modified, its exit by the same solute specific carrier is prevented.
* Group translocation is not strictly active transport, which requires unaltered accumulation of solute.
* Four proteins are known to take part in group translocation.
* Two of these are enzymes (Enzyme I and II) and Two are substrates (Hpr & Factor III)
* E- II is an integral membrane protein; it forms the channel & it catalyses the phosphorylation of sugar.
* The phosphate group, however is not directly transferred from PEP, but is first donated by E- I to a small heat stable protein called Hpr.
* The phosphorylated form of Hpr (Hpr -P) reacts with E - III which is a peripheral membrane protien.
* The phosphate group is then transferred to sugar by the channel protein - E- II (E-II & E- III are specific for each sugar).
* Whereas Enzyme I (E-I) & HPr take part in all PTS mediated sugar transport (translocation).
* A few sugar transport PTS system do not seem to involve enzyme -III.
