PHYSIO LEC 2

• The renal tubule is the functional unit of the kidney responsible for reabsorbing water and solutes from the glomerular filtrate, and for secreting hydrogen ions and potassium ions into the filtrate

  
• Loop diuretics like furosemide, ethacrynic acid, and bumetanide inhibit the Na+/K+/2Cl- cotransporter in the thick ascending limb of the loop of Henle, leading to increased excretion of water and solutes in the urine
• Cell surface structures:

  • Apical or mucosal surface faces the lumen of a tubular tract or external environment
  • Basal surface lies adjacent to blood vessels
  • Lateral surfaces face neighboring cells where junctional complexes are found
• Intercellular junctions:

  • Tight junctions (Zonula occludens) form close contacts between cells, preventing leakage
  • Desmosomes (Zonula adherens) hold cells together
  • Hemidesmosomes (focal adhesions) attach cells to the basal lamina
• Gap junctions serve as channels between cells allowing transfer of ions or molecules, important in heart, smooth muscles, and epithelial cells activity
• Factors influencing membrane transport include the size and solubility of molecules in oil (lipid solubility)
• Particles that can readily pass through the cell membrane include polar molecules with molecular mass < 100 daltons, gases like O2, N2, CO2, and lipid-soluble molecules
• Particles that cannot pass through the cell membrane include charged particles like ions (Na+, K+, Ca2+, Mg+, Cl-, HCO3-) and polar molecules with a molecular mass > 100 daltons
• The cell membrane is permeable to gases and small uncharged polar molecules but impermeable to ions and large polar molecules, necessitating membrane proteins for their transport
• Two types of membrane transport of molecules are passive transport (downhill movement towards concentration gradient) and active transport (uphill movement against concentration gradient)
• Types of passive transport:

  • Simple diffusion: movement towards concentration gradient without requiring ATP, influenced by factors like solute concentration, molecule size, and temperature
• Fick's Law describes the diffusion of solute in water, considering factors like diffusion coefficient, cross-sectional area, solute concentrations, and distance
• Facilitated diffusion is a carrier-mediated transport system that moves substances towards their chemical or electrical gradients without requiring energy (ATP)
• Facilitated diffusion shows structural specificity for molecules and saturation kinetics, affecting the rate of transport of molecules
• Structural specificity in facilitated diffusion influences the rate of transport of molecules like L-arabinose, D-xylose, and L-alanine
• Comparison between simple diffusion and facilitated diffusion:

  • Simple diffusion doesn't require carrier proteins, while facilitated diffusion does
  • The rate of transport in simple diffusion increases with concentration differences, while in facilitated diffusion, it increases as the concentration of the substance increases
• Types of diffusion:

  • Simple diffusion: no carrier proteins, rate of transport increases depending on concentration differences, substances like H2O, O2, nitrogen, alcohol, urea
  • Facilitated diffusion: requires carrier proteins, rate of transport increases as concentration of substance increases, substances like glucose, amino acids, galactose, mannose
• Lipids are permeable to:

  • Cations: K+, Na+, Ca2+
  • Anions: Cl-, HCO3-
  • Glucose (hydrophilic)
  • Big molecules (RNA)
• Types of carrier proteins:

  • Uniporter: transports only 1 substance, e.g., glucose transporters (GLUT) in tissues
  • Symporter or Co-Transporter: moves 2 or more substances across the cell membrane in the same direction, e.g., Na-glucose symporter in small intestinal epithelial cells
• Types of protein channels:

  • Ungated Channel Protein: forms a pore in the cell membrane allowing fast transport, e.g., H2O channel protein (aquaporins)
  • Gated Channel Protein: opening & closing of pore regulated by a gate, e.g., Voltage-Gated Channel
• Voltage-Gated Channel:

  • Requirement for opening and closing: electrical potential across the membrane
  • Present in cells like neurons, fat cells, muscle cells
  • Examples: Na channels, K channels, Ca channels
• Ligand-Gated Channel:

  • Requirement for opening: ligand (neurotransmitter) binding to extracellular part of the channel
  • Present in neurons, abundant in dendrites and cell body
  • Examples: Acetylcholine gated Na channels, Gamma amino butyric acid (GABA) gated Na channels
• Ion-Gated Channel:

  • Requirement for opening: presence of specific ion (ion selectivity)
  • Present in different cells in the body
  • Examples: Liver cells, kidney cells
• Stretch-activated Channel:

  • Channels open or close in response to mechanical forces
  • Channels underlie touch sensation in skin and transduction of acoustic vibrations in the ear
• Light-gated Channel:

  • Abundant in the retina of the eye for vision
  • Channels open with light stimulus
• Active Transport:

  • Requires carrier proteins to move substances against their chemical or electrical gradients
  • Requires energy (hydrolysis of ATP)
  • Examples: Na+-K+ pump (Na-K-ATPase), Calcium pump
• Calcium Ion Transport:

  • In muscle cells, active Ca transport maintains low intracellular Ca levels
  • Ca ions are pumped into the sarcoplasmic reticulum and out of the cell membrane
• Solvent Drag:

  • Solvent flows in one direction dragging along some molecules of solute
• Transport Processes of Macromolecules:

  • Exocytosis: release of proteins in secretory granules or vesicles from the producing cell
  • Endocytosis: entry of substances or organisms inside the cell