membrane transport by dr-mudassar-ali-roomi
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membrane transport by dr-mudassar-ali-roomi

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Amna inayat medical college

Amna inayat medical college
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membrane transport by dr-mudassar-ali-roomi membrane transport by dr-mudassar-ali-roomi Presentation Transcript

  • Membrane Transport By Dr. Mudassar Ali Roomi (M.B; B.S., M. Phil.)
  • Extracellular & Intracellular fluid composition is different!! ECF Vs ICF Na+ > Na+ K+ < K+ Cl- > Cl- PO4 --- < PO4 --- Proteins < Proteins • Transport mechanisms are responsible for Differential composition of ECF & ICF.
  • ECF & ICF composition is different!!
  • Lipid barrier of cell membrane & Cell membrane transport proteins:
  • Different types of transport across a selectively permeable membrane: • Diffusion or passive transport: 1. Simple Diffusion 2. Facilitated Diffusion 3. Osmosis • Active transport – Primary Active Transport – Secondary Active Transport 1. Secondary Active Co-transport 2. Secondary Active Counter-transport • Endocytosis 1. Pinocytosis 2. Phagocytosis • Exocytosis
  • What is the Composition of cell membrane?? • Lipid bilayer. • Large no. of protein molecules in the lipid (including many penetrating proteins).
  • Lipid bilayer as barrier against water molecules & water-soluble substances: • Lipid barrier is not miscible with ECF or ICF. • Allows lipid soluble substances to penetrate directly through the lipid substance.
  • Penetrating proteins as transport proteins: Channel proteins: • Have watery spaces that penetrate throughout the molecule. • Allow free movement of water, selected ions or molecules. • Highly selective. Carrier proteins: • Bind with molecules or ions to be transported. • Undergo conformational change. • Leading to movement of substances through the interstices of protein to other side of membrane.
  • Types of Passive transport: 1. Simple diffusion. 2. Facilitated diffusion. 3. Osmosis.
  • DIFFUSION • Movement of substances down the conc. gradient either through opening in cell membrane or in combination with carrier protein, caused by simple kinetic motion of molecules without the use of energy is called diffusion.
  • Simple Vs Facilitated Diffusion: SIMPLE DIFFUSION • Movement of highly permeable molecule from region of high concentration to lower conc. Without the help of carrier protein and without use of energy. • Example: transport of O2 and CO2 across the membrane. FACILITATED DIFFUSION • Movement of substances across the cell membrane in combination with carrier protein towards concentration gradient without utilization of energy. • Example: glucose transport through the GLUT transporters.
  • Some important definitions • OSMOLE: – No. of particles in one mole of un-dissociated solute is called one osmole. – 1 osmole = 6.02 x 1023 particles. • OSMOLALITY: – No. of osmole of solute per kg of water is called osmolality • OSMOLARITY: – Osmole per liter of solution. – In usual practice.
  • OSMOSIS across selectively permeable membrane- “net diffusion” of water: • Process of net movement of water across a selectively permeable membrane, caused by a concentration difference of water is called osmosis.
  • Osmotic Pressure: • Definition: The exact amount of pressure required to stop osmosis is called Osmotic Pressure. • Osmotic pressure is directly proportional to the number of osmotically active particles. **
  • Importance of number of osmotic particles (molar conc.) in determining osmotic pressure: • Each particle in a solution, regardless of its mass, exerts on average the same amount of pressure against the membrane. • K.E = 1 mv2 2 K.E = average kinetic energy, v = velocity, m = mass. If mass is less, velocity is more.
  • Factors affecting rate of diffusion across a selectively permeable membrane: 1. Effect of conc. difference across membrane 2. Velocity of kinetic motion. 3. Effect of temperature 4. No. & size of openings (channels) in the membrane. 5. Lipid solubility of the substance. 6. Water solubility of the substance. 7. Size of molecules. 8. Selective permeability of protein channels. 9. Opening or closing of many protein channels by gates. 10.Effect of pressure difference across membrane 11.Effect of membrane electrical potential (Nernst potential)
  • Effect of conc. difference on net diffusion through a membrane: • The rate at which the substance diffuses inward is directly proportional to the concentration difference of molecules across the membrane
  • Effect of membrane electrical potential on diffusion of ions- the “Nernst Potential” • Electrical potential if applied across the membrane  Electrical charges of ions cause them to move through the membrane, even in the absence of concentration difference. • Conc. difference of ions develops in the direction opposite to electrical potential difference. • Ions keep moving until the 2 effects balance each other. • Definition: At normal body temperature, the electrical difference that will balance a given conc. difference of univalent ions is called as Nernst potential or equilibrium potential. • EMF (mV) = +/- 61 log C1 C2
  • Effect of pressure difference across the membrane: • Pressure inside the blood capillary is about 20 mmHg greater than outside. • So, at arterial end of the capillary fluid is filtered out.
  • Diffusion through the cell membrane: Simple diffusion & Facilitated diffusion • Simple diffusion • Kinetic movement of ions / molecules through a membrane opening / intermolecular spaces without any interaction with carrier proteins in the membrane. • Facilitated diffusion • Requires interaction of a carrier protein. • Carrier protein binds chemically with & shuttles ions / molecules through the membrane.
  • 2 pathways for simple diffusion: • Through interstices of lipid bilayer if diffusing substance is lipid soluble. • Through watery channels that penetrate all the way through large transport proteins.
  • Diffusion of lipid-soluble substances through the lipid bilayer • The main factor effecting the rate of diffusion through lipid bilayer is lipid solubility of the substance. • Examples of highly lipid soluble substances: 1. Oxygen, 2. nitrogen, 3. carbondioxide, 4. alcohol.
  • Diffusion of water & other lipid-insoluble molecules through protein channels: Rapid penetration through protein channels: • e.g., Water & • other lipid-insoluble (water-soluble & small molecules). Slow penetration: Water-soluble larger molecules. e.g., urea molecule (size is 20 % > water; penetration is 1000 x < water).
  • Diffusion through Protein Channels & Gating of these channels: • Tubular pathways from ECF to ICF. • Simple diffusion from one side of membrane to other across protein channels.
  • two important characteristics of protein channels: 1. Often show selective permeability for one or more specific ions or molecules. 2. Most channels are gated (can be opened or closed by gates).
  • Specificity of protein channels: It is due to certain characteristics which are : 1. Channel diameter 2. Shape of the channel 3. Nature of electrical charges 4. Chemical bonds along their inner surfaces
  • Characteristics of sodium-channel: (specific for sodium ion passage) • 0.3 to 0.5 nm diameter. • Strong Negative charge on inside. • Pull small dehydrated sodium ions inside, pulling sodium ions away from hydrating water molecules. • Once in the channel, sodium ions diffuse in either direction, according to laws of diffusion (down the concentration gradient)
  • Selective permeability of protein channels for potassium ions: Potassium channels: • Slightly smaller channels. • Not negatively charged. • Chemical bonds are different. No strong attractive forces pull sodium ions away from water molecules that hydrate them. • Hydrated form of potassium ion is smaller, which can pass easily through small potassium channel. Sodium channels: • Slightly bigger channels. • Negatively charged on inside. • Chemical bonds are different. Strong attractive forces pull sodium ions away from water molecules that hydrate them. • Hydrated form of sodium ion is bigger, as sodium ion attracts more water molecules. They cannot pass through small potassium channel, resulting into selective permeability for a specific ion.
  • Gating of protein channels Significance: Selective gating of sodium & potassium ions  Control of ion permeability of the channels. Mechanism: Some gates are extensions of transport protein molecule  open and close by conformational change
  • 2 principal ways of opening & closing of gates: Voltage & Ligand gating Voltage gating: • Molecular conformation of the gate or Molecular conformation of the chemical bonds respond to electrical potential across cell membrane. Chemical (ligand) gating: • Gates open by binding of a chemical substance (ligand) with the protein channel  conformational or chemical bonding change in protein molecule that opens / closes the gate.
  • Voltage & Ligand gating Voltage gated: When strong negative charge inside the cell membrane (at RMP): • Sodium gates remain closed. When inside of membrane loses its negative charge: • Sudden opening of sodium gates  massive sodium influx  onset of action potential. When inside becomes positive: • Potassium gates open  potassium efflux  termination of action potential. Chemical / Ligand gated: Example: • Effect of Acetylcholine on acetylcholine channel  gate opens (negatively charged pore of 0.65 nm diameter)  passage of uncharged molecules / positive ions smaller than 0.65 nm. Important at: • Nerve to nerve junction & • Nerve to Muscle junction
  • 4 types of gated channels: LIGAND GATED • Some protein channel gates are opened by the binding of a chemical substance with them. • e.g acetylcholine channels. VOLTAGE GATED. • Some protein channel gates respond to electrical changes across the cell membrane. e.g. sodium potassium channels. PHOSPHORYLATED GATED CHANNELS • When ATP is broken down to ADP a phosphate group is released which attaches to the protein channel causing its phosphorylation leading to opening and closing of these channels. STRETCH OR PRESSURE GATED CHANNELS • Mechanical stretch of membrane results in channel opening.
  • Facilitated Diffusion: • Carrier mediated diffusion. • Carrier facilitates diffusion of the substance to the other side. Examples: Glucose & most Amino Acids. In presence of insulin, glucose transport increases 10-20-fold. Glucose carrying protein has molecular weight of 45,000.
  • Facilitated diffusion Vs Simple diffusion: Facilitated diffusion • Rate of diffusion reaches a maximum (V max), as the concentration of diffusing substance increases & cannot rise greater than V max Simple diffusion • Rate of diffusion varies directly with concentration of diffusing substance (if the channel is open).
  • What limits the rate of facilitated diffusion: • Saturation of carrier molecules. • The rate of transport cannot be greater than the rate at which carrier protein molecule can undergo change back & forth between its 2 states.
  • Primary Active Transport: Sodium-potassium pump:• The sodium potassium pump is a complex of two separate globular proteins. • Smaller protein might anchor the protein complex in the lipid membrane • The larger protein has three specific features that are important for the functioning of the pump: 1. It has three receptor sites for binding sodium ions on the portion of the protein that protrudes to the inside of the cell. 2. It has two receptor sites for potassium ions on the outside. 3. The inside portion of this protein near the sodium binding sites has ATPase activity.