Transport Across Cell Membrane

General Instructions-

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2. Include relevant diagrams and tables wherever possible form your standard textbooks

Need for Transport-

• Physiological activities require nutrients, oxygen, water.
• Metabolic wastes need to be removed.

Classification of Membrane Transport-

• Write in Chart form.
• Passive-
  • Diffusion-
    • Simple Diffusion
    • Facilitated Diffusion
    • Non-Ionic Diffusion
  • Osmosis
• Active-
  • Primary Active-
    • Na-K Pump
    • Ca Pump
    • K-H pump
  • Secondary Active-
    • Na Co-transport
    • Na Counter Transport
• Vesicular Transport-
  • Endocytosis-
    • Pinocytosis
    • Phagocytosis
    • Receptor mediated endocytosis
  • Exocytosis
  • Transcytosis
• Other Transport-
  • Across epithelium
  • Ultrafiltration

Passive Transport

• Transport across electrochemical gradient
• Does not require energy
• Diffusion-
  • Passive transport of  molecules down the gradient.
  • Simple Diffusion-
    • Occurs due to random thermal distribution of molecules that redistributes them uniformly throughout the liquid.
    • Does not require carriers.
    • Movement Occurs across both directions, but net movement is only towards lesser concentration.
    • Net movement seizes when there is diffusional equilibrium.
    • Substances transported this way-
      • All lipid soluble substances
      • Lipid soluble gases like CO2, O2, N2
      • Diffusion of gases in alveoli of lungs
      • Water molecules
      • Na+, K+, Ca2+
    • Fick’s Law-
      • Rate of Diffusion(J) is-
        • Directly proportional to-
          • Concentration gradient(△C)
          • Area of membrane(A)
          • Lipid solubility
        • Inversely Proportional to-
          • Thickness of Membrane(T)
          • Size of diffusing particles/molecules
      • J = DA(△C)/T
    • Protein Channels-
      • Tube shaped channels extending across the cell membrane
      • They have two properties-
        • Selective permeability-
          • Each channel permits only one type based on charge, shape, size, etc.
          • Eg. Na- channels, K Channels
        • Gating mechanisms-
          • Channels with gate like extensions of transport protein that open and close as per requirements
          • They are further 3 types-
            • Voltage gated-
              • They respond to electrical potential across cell membranes.
              • Eg. Na voltage gated channels on the outer end of the channels
              • Eg. K voltage gated channels on the inner end of the channels

• Ligand gated-
  • They open when a certain chemical binds with them
  • Eg. Acetylcholine bonds and opens the Acetylcholine channels during nerve impulse transmission
• Mechanical gated-
  • Opened by mechanical stretch
  • Important in cell movement

• Facilitated Diffusion-
  • Substances move down the gradient through a carrier protein
  • ATP is not required
  • Carrier protein undergoes confirmational change during transport.
  • Vmax- Maximum rate of absorption beyond which rate cannot be increased
  • Specificity- A carrier is specific only for a group of substances
  • Facilitated diffusion can be inhibited competitively as well as non-competitively
  • Eg.-
    • Glucose transporters- GLUTs
    • Water transporters- Aquaporins
    • Most of the amino acids
  • Types-
    • Uniport- Only one type
    • Symport-
      • Transport of two substances are linked.
      • Eg. Facilitated diffusion of glucose in renal tubular cells is linked with that of Na.
    • Antiport-
      • One substance is exchanged for another
      • Eg. Na-K exchange
      • Eg. Na-H exchange
• Non-Ionic Diffusion-
  • Substances diffuse easily in non-ionized forms
  • Ionized form cannot diffuse
  • Seen in renal system
  • Eg.-
    • Diffusion of Ammonia
    • Absorption of Salicylates
• Osmosis-
  • Diffusion of solvent molecules from area of solvent’s higher concentration to lower concentration through a semipermeable membrane
  • Osmotic pressure-
    • Affects water movement
    • Force required to stop the movement of water molecules through the semipermeable membrane
    • Depends on number of molecules
    • Does not depend on size of molecules

Active Transport

• Transport of substances against electrochemical gradient by utilisation of ATP.
• Eg.
  • Ionic- Na+, K+, Ca2+, Cl-, I-
  • Non-Ionic: Glucose, Amino acids, Urea
• Primary Active Transport- energy is obtained from breakdown of high energy compounds like ATP. Eg.-
  • Na-K Pump
  • Ca Pump
  • K-H Pump
  • Na-K Pump-
    • Made of two globular proteins
    • 3 Na binding receptors protruding inside cell
    • 2 K binding receptors protruding outside cell
    • Inside part of protein has ATPase activity
    • Acts as electrogenic pump
    • Maintains constant concentration of Na and K ions inside and outside the cells.
    • Mains constant cell volume
  • Ca Pump-
    • Helps in maintaining extremely low levels of calcium in the intracellular fluid
  • K-H Pump-
    • Present in parietal cells of gastric glands and renal tubules
• Secondary Active- energy is derived secondarily from the energy which has been stored in the form of ionic concentration differences between the two sides of a membrane, created in the first place by primary active.
  • Na Co-Transport-
    • Along with sodium, another substance is transported by a carrier protein called symport.
    • Carrier protein for the sodium cotransport has two receptor sites on the outer surface. Among the two sites, one is for binding of sodium and another site is for binding of other substance
    • Eg,-
      • Glucose
      • Amino Acids
      • Chloride
      • Iodine
      • Iron
      • Urate
    • Sodium cotransport of glucose occurs during absorption of glucose from the intestine and reabsorption of glucose from the renal tubule.
    • Sodium cotransport of amino acids also occurs during the absorption of amino acids from the intestine and reabsorption from renal tubule
  • Na Counter Transport-
    • substances are transported across the cell membrane in exchange for sodium ions by carrier protein called antiport
    • Eg. Na-Ca counter transport present In all cells
    • Na-H counter transport in tubular cells
    • Others- Na-Mg, Ca-Mg, Ca-K, etc
• Endocytosis-
  • transport mechanism by which the macromolecules enter the cell.
  • Pinocytosis or Cell Drinking is engulfing of bacteria, antigens, etc.
  • Phagocytosis or Cell Eating is engulfing of Larger bacteria, larger antigens and other larger foreign bodies.
  • Only few cells like Macrophage and Neutrophils can perform phagocytosis.
  • transport of macromolecules with the help of a receptor proteins like Clathrin is called Receptor Mediated Endocytosis.
• Exocytosis-
  • Process by which the substances are expelled from the cell.
  • Substances are extruded from cell without passing through the cell membrane.
  • Reverse of endocytosis
  • Calcium ions are important in exocytosis of neurotransmitters
• Transcytosis-
  • Mechanism in which an extracellular macromolecule enters through one side of a cell, migrates across cytoplasm of the cell and exits through the other side.
  • Receptor protein coating the pits in this process is caveolin
  • Also called, vesicle trafficking or cytopempsis.
  • Eg. movement of proteins from capillary blood into interstitial fluid across the endothelial cells of the capillary.
  • Eg. Transport of Pathogens like HIV virus
• Other Transport-
  • Across epithelium-
    • Involves movement of substance from one side of epithelium to another
    • Occurs in body cavities lined by continuous sheet of cells like GI tract, renal tubules, etc.
    • Requires tight junction and ion channels and transport proteins
  • Ultrafiltration-
    • Occurrence of dialysis under hydrostatic pressure
    • Arterial capillary pressure= 35mmHg
    • Venous Capillary Pressure= 12 mmHg
    • Here in capillary occurs ultrafiltration of all plasma contents except proteins.