Lecture 14 Membrane Transport PDF

Membrane transport  Cell Membrane surrounds entire cell and cell organelles Fluid in nature – movement of molecules Phospholipid bilayer – head – polar/hydrophilic tail – nonpolar/hydrophobic Integral –carrier & channel Proteins...

Membrane transport  Cell Membrane surrounds entire cell and cell organelles Fluid in nature – movement of molecules Phospholipid bilayer – head – polar/hydrophilic tail – nonpolar/hydrophobic Integral –carrier & channel Proteins Peripheral-receptors & antigen  Functions of cell membrane  Acts as semi permeable barrier –(selective) oMaintains difference in composition of ICF & ECF & fluid in various organelles oProtects cell from toxic substances oExcretion of waste products oTransport of nutrients  Receives signals from the outside Chemical signals Electrical signals Site for attachment to the neighboring cells  Transport through cell membrane  Membrane transport is essential for cellular life.  As cells proceed through their life cycle, a vast amount of exchange is necessary to maintain function.  Transport may involve the incorporation of biological molecules and the discharge of waste products that are necessary for normal function.  Membrane transport refers to the movement of particles (solute) across or through a membranous barrier.  Membrane transport is dependent upon the permeability of the membrane, transmembrane solute concentration, and the size and charge of the solute.  Solute particles can traverse the membrane via three mechanisms: passive, facilitated, and active transport.  Some of these transport mechanisms require the input of energy and use of a transmembrane protein, whereas other mechanisms do not incorporate secondary molecules.  Transport across cell membrane Transport Mechanisms Passive Active Simple diffusion Primary active transport Facilitated diffusion Secondary active transport Filtration Endo/Exocytosis Osmosis dialysis  Methods of transport Passive Active Diffusion Osmosis Filtration Dialysis Simple facilitated Lipid bilayer Protein Leaky channels voltage gated channels Gated channels Ligand gated I. Simple diffusion - Movement of molecules from higher concentration to lower concentration till equilibrium is reached Diffusion can takes place through: a) Lipid bilayer i) Lipid soluble substances- O2,CO2, alcohols, steriods etc ii) Lipid insoluble – water (through spaces between lipid mol), urea, sugar (less or no permeability) iii) Electrolytes – impermeable – charge on fatty acid chain - Hydrated forms are larger. b) Protein Channels Open/leaky – Na+ channels, K+ channels Gated –channels open under specific conditions Ligand gated Voltage gated Na+ Na+, K+ K+ Ca++, Mutation of ionic channels produce channelopathies –affecting muscle and brain – paralysis or convulsions Factors affecting rate of diffusion Lipid solubility Molecular Molecular size & wt. Temperature Thickness of membrane Membrane related Surface area Concentration gradient Gradients Pressure gradient Electrical gradient Based on the thermodynamics of the system, particles will move from an area of high concentration to an area of low concentration in order to increase the entropy of the cell. Additionally, this particle movement will occur spontaneously as the free energy (Gibbs free energy; ∆G) of the system is negative. Where: Further, the amount of energy consumed or released by the system is as follows2: If ∆G0 particle movement requires the input of energy to move in the desired direction. The properties of the membrane must also be considered when determining the rate of flow of the substrate. Fick’s Law can be used to determine flow rate.  Fick’s law of diffusion – ΔC∙P∙A Q α ──── MW∙ ΔX Q = net rate of diffusion ΔC = conc. gradient of a substance P = permeability of membrane to the sub. A = surface area of a membrane MW = molecular wt. of sub. ΔX = thickness or distance II. Facilitated diffusion : - for larger water soluble mols. - type of passive transport - along the conc. Gradient - carrier mediated transport Mechanism - receptor site on one side  - Rate of transport – Vmax Vmax velocity Simple diffusion Facilitated diffusion Conc. gradient Initially, rate is directly proportional to conc. gradient Till it reaches Vmax ( limitation because of no. of carrier mols. & rate of conformational change) Hormonal regulation by changing #of carriers. - Peculiarities of carrier mediated transport –  specificity,  competitive or noncompetitive inhibition,  saturation,  blocking of receptor  Vmax -Examples – transport of glucose, amino acids, galactose, etc. in the peripheral cells or counter transport of Cl and HCO3 in renal tubules III. Osmosis & osmotic pressure– when two solutions of different concentrations are separated by a semi permeable membrane (impermeable to solute and permeable to water) water mols. diffuse from solution having less conc. To the sol. having higher conc. Osmotic pressure is the minimum pressure applied on the solution with high conc. which prevents osmosis. - depends upon total no. of particles of dissolved solutes rather than type of the particles Osmols or mOsmols - expresses conc. of osmotically active particles 1 osmol = total no. of particles in gram molecular wt. of non diffusible substance per kg. of water Applied - Isotonic, hypotonic & hypertonic solutions Isotonic solution – fluids having osmolarity same as that of plasma ( 290 mOsmols ). Red cells suspended in such solution do not shrink or swell. ( 0.9 % NaCl, 5% glucose ) In Hypotonic solution, RBCs swell and hemolysis may occur. In hypertonic solution, RBCs shrink because water moves out. Gibbs – Donnan Equilibrium Explains difference in the conc. of diffusible ions in two compartments separated by semi permeable membrane, when one compartment contains non diffusible ions Na + Na + Proteins are non Cl - Cl - diffusible anions in A Pr - Conc. Of Na + is more in A as compared to B A B  A B Na+ 30 Na+ 30 - Cl 30 conc. Gradient for Cl - Pr - 30 Na+ 30 Na+- 30 - Cl 15 More –vity in A Cl 15 Pr - 30 electrical gradient Na+ 45 Na+ 15 - - Conc. gradient Cl 15 Cl 15 electrical gradient Pr - 30 Na+ 40 Na+ 20 Cl - 10 Cl - 20 Pr - 30  Explaination – 1) All the solutions are electrically neutral. ( total no. of anions = total no. of cations ) 2) Product of diffusible cations and an anions in both the compartment is equal. ( Na+A x Cl-A = Na+B x Cl-B ) Applied – In ICF, conc. of diffusible K+ is more because of presence of non diffusible Pr - and PO4 - IV. Filtration Filtration is a process in which fluid along with solutes passes through a membrane due to difference in pressures on both sides. e.g. Filtration at capillary V. Dialysis – Separation of larger dissolved particles from smaller particles It is used for elimination of waste products in the blood in case of renal failure. Active transport  Primary active transport  Secondary active transport  Endocytosis  Pinocytosis  Phagocytosis  Exocytosis Peculiarities of active transport 1) Carrier mediated transport 2) Rapid rate of transport 3) Transport takes place against electrochemical gradient ( uphill ) 4) Expenditure of energy by transport protein which incorporates ATPase activity 5) Carrier protein shows specificity, saturation, competitive inhibition, blocking 6) Substances transported – Na+ , K+, H+, Cl -, I - , Glucose, Amino acids I. Primary active transport – Examples - Na+ - K+ pump, Ca++ pump H+-K+ pump - Inner surface of carrier mol. has ATPase which is activated by attachment of specific ions and causes hydrolysis of ATP molecule - Energy released from ATP causes conformational change in the carrier which transports ions to the opposite side. - Example: Na-K pump is one of the major energy using process in the body & accounts for a large part of basal metabolism. -  Na+ -K + pump- electrogenic pump - Attachment of 2K+ on outer side & 3 Na+ on inner side Activation of ATPase 3Na+ Conformational change ATPa 2K+ Efflux of 3 Na+ & influx of 2K+ es Creates high K+ conc. & - vity inside the cell Helps in maintaining cell volume Regulators of Na-K pump – - Increased amount of cellular Na+ conc. - Thyroid hormones increase pump activity by more activity of Na-K ATPase mol - Aldosterone also increases activity of pumps - Dopamine inhibits pump - Insulin increases pump activity - Oubain or Digitalis inhibits ATPase (used when weakness) of cardiac muscle –maintains Ca ions conc. in ICF of cardiac muscle.  II. Secondary active transport Active transport of glucose depends upon conc. gradient of Na+ from ECF to ICF created by utilization of energy Carrier does not have ATPase activity Substance is transported along with Na+ (Na increases affinity of carrier for glucose) basal lumen Na+ K+ Glucose  Na + – K + pump on basal side  Electrochemical gradient for Na + on luminal side  Carrier mediated transport (SGLT-1)of Na+ along with glucose ( or amino acid ) through the apical membrane  Transport of glucose by facilitated diffusion ( GLUT-2 ) through basal side  Types of transporters Uniport Synport Antiport  Extracellular material to be tackled by lysosomes is brought into the cell by endocytosis 3 types pinocytosis phagocytosis All cells Specialised Receptor cells mediated endocytosis Requires ATPase, Ca, microfilaments  Pinocytosis ECF ECF dynamin Membrane deforming coat protein Endocytic vesicle  B. Receptor mediated endocytosis – highly selective process to import imp. specific large molecules. Requires energy & Ca++. e.g. endocytosis of low density lipoproteins Coated pit e.g. endocytosis of viruses such as Clathrin, actin, hepatitis, AIDS viruses & excess iron myosin  C. Phagocytosis Internalization of large multimolecular particles, bacteria, dead tissues by specialized cells e.g. certain types of WBCs ( Professional phagocytes) The material makes contact with the cell membrane which then invaginates. Pseudopodia (temporary projections of the cytoplasm of a cell) are formed. Pseudopodia are used for locomotion and feeding in amoeba and white blood cells. The activation of the polymerization of the actin filaments forces the membrane to extend and form pseudopodia.  bacterium Pseudopodia internalization Phagoso- some Fusion Residual body absorption digestion Phagocytosis Passive transport Active transport No expenditure of Expenditure of energy energy molecules mol. ( ATP ) Takes place along Can take place against conc., electrical, & conc. Gradient pressure gradient Carrier may or may Carrier is always not be required required Rate is proportional to Rate is proportional to conc. difference availability of carrier & Vmax Simple Diffusion Facilitated Diffusion Passive transport Passive transport For small molecules For large molecules No carrier required Carrier mediated Rate of transport is Initially rate is directly proportional to proportional to conc. conc. gradient gradient till Vmax Examples – ( saturation of carriers) Lipid soluble – Examples – O2, CO2, alcohol glucose, amino acids Lipid insoluble – urea, Na+, K+