Lecture 7: Transport Across Cell Membrane PDF

LECTURE 7:[PHY] 10/10/2024 TRANSPORT ACROSS CELL MEMBRANE Vinoth kumar S 1 Assistant professor Department of physiology  TOPIC OUTCOMES ✓ Describe the mechanism of diffusion of lipid-...

LECTURE 7:[PHY] 10/10/2024 TRANSPORT ACROSS CELL MEMBRANE Vinoth kumar S 1 Assistant professor Department of physiology  TOPIC OUTCOMES ✓ Describe the mechanism of diffusion of lipid- 10/10/2024 soluble and water-soluble molecules. ✓ Describe the properties of various types of gate proteins. ✓ Describe the properties of carrier-mediated diffusion. ✓ Describe the mechanism of primary and secondary active transport. ✓ Describe the functions of the Na+–K+ pump in the body. ✓ Describe the transport of macromolecules across 2 the cell membrane (exocytosis and endocytosis). ICF and ECF is composed of primarily water in which solutes (eg. ions, glucose, amino acids) are dissolved across a selectively 10/10/2024 permeable membrane 3 10/10/2024 4 10/10/2024 5  Lipid bilayer interspersed with large globular protein molecules.  Direct Passage – Lipid soluble substances 10/10/2024 eg. Gases, fatty acids, alchohol, Steroid hormones  Through Transporter – Water Soluble substances eg. Water, ions, glucose, amino acids  Integral Proteins – Transporters Channel Proteins Carrier Proteins 6  CLASSIFICATION OF TRANSPORTS - Mode of transport is classified based on 10/10/2024 energy requirement and usage Diffusion Passive Osmosis Types of transport Primary Active Secondary7 OTHER TRANSPORT MECHANISMS 10/10/2024 Vesicular Transport: I) Exocytosis II) Endocytosis- Pinocytosis, Phagocytosis, Receptor Mediated Endocytosis. III) Transcytosis or Cytopempsis 8 1) PASSIVE TRANSPORT 10/10/2024 Substance move across the cell membrane without any energy expenditure by the cell. Down hill movement – Along the concentration or electrical or pressure gradient 9  DIFFUSION 10/10/2024 Movement of solute molecules from a region of higher concentration to region of lower concentration until equilibrium is reached  Along the Chemical gradient or Electrical gradient or pressure gradient Random molecular movement of molecule – Brownian movement Uses energy of kinetic motion of matter 10 Figure 4-1 START: Initially higher concentration of molecules randomly DIFFUSION move toward lower concentration. Over time, solute molecules placed in a solvent will evenly distribute themselves. Diffusional equilibrium is the result (Part b). 10/10/2024 11  DIFFUSION THROUGH CELL MEMBRANE Simple diffusion Facilitated diffusion 10/10/2024 Occurs through Chemical binding  Lipid bilayer: Lipid with carrier soluble substances proteins  eg. O2, N2, CO2,  eg.Glucose, steroid hormones amino acids  Protein channels: Water Soluble substances 12 eg. Water, ions like Na+, K+ 10/10/2024 13 WATER CHANNELS - AQUAPORINS 10/10/2024 Various isoforms found in various cells 13 Aquaporins have been identified Reabsorption of water from late distal convoluted tubule. 14 ION CHANNELS 10/10/2024 They are Integral proteins – Selective Permeability Channels for K+, Na2+, Ca2+, Cl- Sodium Channel: 0.5 nm. Inner surface negatively charged with gate outside (ECF) Potassium channel: 0.3 nm. no electrical charge with gate inside (ICF) 15 10/10/2024 16  VOLTAGE CHANNEL(GATED) Responds to alteration in membrane potential. 10/10/2024 17  LIGAND CHANNEL(GATED) 10/10/2024 Binding of a ligand causing conformational change in transport protein. Eg. neurotransmitter (Ach) in N.M.J & synapse , hormones etc. 18  FACTORS AFFECTING RATE OF DIFFUSION FICK’S LAW OF DIFFUSION Rate of diffusion iss 10/10/2024 - directly proportional to - Concentration gradient - surface area of membrane - solubility of substance - inversely proportional to - Thickness of membrane - molecular size of substance  Net rate of diffusion (flux) = Diffusion coefficient (D) x Area of membrane (A) x Cin- Cout Thickness of the membrane (T) 19  FACILITATED DIFFUSION  Carrier mediated diffusion – Specific carrier 10/10/2024 protein  Faster than simple diffusion  Larger water soluble substances that cannot diffuse by simple diffusion  The carrier protein undergoes repetitive configurational change after which the substance carried is exposed to ECF or ICF. 20 CARRIER MEDIATED TRANSPORTS: TYPES OF CARRIER PROTEINS 10/10/2024 Depending on the direction of substance transported they are:  Uniport- GLUT Transporter  transports substance in one direction  Symport Transports more than one substance in one direction  Antiport 21 transports substances in opposite direction  Mediated Transport Systems: A cartoon model of carrier-mediated transport. 10/10/2024 The solute acts as a … and then a subsequent ligand that binds to shape change in the the transporter protein releases the protein…. solute on the other side 22 of the membrane. Figure 4-8 10/10/2024 23 Figure 4-9 In simple diffusion, flux rate is limited only by the concentration 10/10/2024 gradient. In carrier- mediated transport, the number of available carriers places an upper limit on the flux rate. 24  OSMOSIS 10/10/2024  Diffusionof pure solvent from low to high solute concentration through a semipermeable membrane.  Pressure necessary to prevent solvent migration called osmotic pressure.  Osmotic pressure is related to number of osmotically active particles dissolved in the solution not their size, molecular weight.  Types- Exosmosis & endosmosis. 25 10/10/2024 26  10/10/2024 10 microns isotonic solution hypertonic solution hypotonic solution equal movement of water net water movement net water movement into and out of cells out of cells into cells 27  2) ACTIVE TRANSPORT o It requires energy 10/10/2024 o Up-Hill movement. o It requires carrier protein (pumps ) it shows specificity, saturation. o Transport against concentration / Electrical / Pressure gradient o (From lower concentration to higher concentration ) o Substances actively transported o Ionic: Na+, K+, Ca2+, Cl- o Non ionic: glucose and amino acids 28  ACTIVE TRANSPORT 10/10/2024 Source of energy primary secondary Ca pump, Co – transport, Na-K pump Counter H pump transport. 29  PRIMARY ACTIVE TRANSPORT 10/10/2024 Energy directly from hydrolysis of ATP or other high energy phosphate compound.  Sodium – Potassium pump  Calcium pump  H+/K+ pump 30  I) SODIUM POTASSIUM PUMP  Na – k ATPase: Transports 3 Na+ to ECF and 2 K+ to ICF 10/10/2024  Complex of 2 globular protein units – larger α (1,00,000 MW) and smaller β (55,000 MW) subunit.  Binding sites:  α Subunit  ATPase activity  Intracellular binding sites for 3 Na+  Sites for ATP and Phosphate  2 extracellular binging sites for 2 K+ & Ouabain 31 10/10/2024 32 MECHANISM OF OPERATION  Step 1: 10/10/2024  Binding of 3 Na+ ions and ATP to the carrier protein on the inner face.  Phosphorylation – Hydrolysis if ATP to ADP and Pi  Transfer of high energy phosphate group to aspartic acid  Configurational change and Na+ move out.  Step 2:  2 K+ binds to the carrier protein on the outer face  Dephosphorylation aspartic – phosphate bond is hydrolysed 33 +  Second configurational change and 2 K is transported into the cell. Functions of + Na - + K Pump  Maintain cytosolic ionic concentration & cell volume. 10/10/2024  Electrogenic pump → Maintains Resting membrane potential  Stores energy for secondary active transport.  Protein synthesis: By maintain K+ concentration Activated by: Thyroxine, insulin, aldosterone Inhibited By: Dopamine, Digitalis 34 10/10/2024 36 CLINICAL SIGNIFICANCE 10/10/2024  Ouabain or Digitalis, a drug used for treatment of heart failure.  Mechanism of Action: Inhibits interfere phosphorylation → Inhibiting Na+/K+ ATPase.  Intracellular accumulation of Na+ → Ca+ efflux decreases → Increases myocardial contractibility. 37 II) CALCIUM PUMP OR CALCIUM ATPASE 10/10/2024  Present in membrane of Sarcoplasmic reticulum of muscle cells (SERCA) → Actively pumps Ca+ from Sarcoplasm (Cytosol) into ‘T’ tubules  Ca+ pump in cell membrane actively transports to ECF III) Hydrogen – Potassium ATPASE:  Gastricmucosa : Pumps H+ into gastric lumen in exchange for K+  In Kidneys: Secretes H+ ions in exchange for K+ 38 B) SECONDARY ACTIVE TRANSPORT 10/10/2024  No direct hydrolysis of ATP required  Energy derived secondarily from the energy which has been stored in form of concentration or electrical gradient created by primary active transport.  Hydrolysis of ATP by Na+/K+ pump required indirectly to maintain [Na+] gradient.  Sodium Co Transport  Sodium Counter Transport 39 i) Sodium Co transport:  Na+-Glucose (SGLT) and Na+- Amino acids 10/10/2024  Carrier protein (Symporter) has 2 binding sites  Absorption from the intestinal epithelium into blood ii) Sodium Counter transport:  Carrier protein is a antiport  Sodium – Calcium counter transport – all cells  Sodium – Hydrogen counter transport – in proximal tubules of kidney. 40 Figure 4-14 Secondary Active Transport: 10/10/2024 Cotransport: the ion and the second solute cross the membrane in the same direction. Countertransport: the ion and the second solute move in opposite directions. 41 10/10/2024 42 10/10/2024 43 10/10/2024 44 10/10/2024 45 10/10/2024 46 10/10/2024 47 10/10/2024 48  Diverse examples of carrier-mediated transport. 10/10/2024 49 Figure 4-15 SUMMARY OF MEMBRANE TRANSPORT 10/10/2024 50 VESICULAR TRANSPORT 10/10/2024  Substances are transported across the cell membrane as Vesicles.  Mode of transport for macromolecules.  Energy & Ca++ dependent.  Endocytosis  Exocytosis  Transcytosis 51 I) ENDOCYTOSIS  Invaginationof a portion of cell membrane 10/10/2024 trapping macromolecules from outside forming a vesicle around the substance. 52 PHAGOCYTOSIS  For large insoluble 10/10/2024 particles like dead cell,bacterium, tissue debris etc.  Seenin tissue macrophage & WBC (neutrophil)  Cell engulfing. 53  PINOCYTOSIS  For small soluble substance. 10/10/2024  Visible only under electronic microscope.  Cell drinking.  Eg; Immunoglobin crosses capillary wall, thyroglobulin enters follicular cells. 54  RECEPTOR MEDIATED ENDOCYTOSIS  Clathrin – fibrillar protein coated pits on 10/10/2024 cell membrane.  Dynamin – transports vesicles Eg; Hormones, nerve growth factor etc. 55 II) EXOCYTOSIS  Fusion of vesicle with cell membrane & expulsion of it’s 10/10/2024 contents.  Eg; secretion of enzymes, hormones, Neurotransmitters 56 10/10/2024 57  III) Transcytosis  Vesicular transport within the cell 10/10/2024  Also called cytopempsis. Transport across Epithelia  Paracellular Transport:  Transport through tight junctions of cells  Transcellular Transport:  Transport between the tight junctions of cells 58  REFERENCES ✓ Textbook of Medical Physiology by A.C. Guyton. 10/10/2024 ✓ Review of Medical Physiology by W.F. Ganong. ✓ Boron & Boulpaep Medical Physiology. ✓ Berne & Levy Physiology. ✓ Costanzo's Physiology. 59 ✓ Vander’s Physiology. 10/10/2024 THANK U 60