Cellular physiology and muscles

• Hydrophilic = substances that readily dissolve in water, these molecules are polar

  e.g. glucose, Na+, ethanol, many proteins
• Hydrophobic = substances that are insoluble in water, these molecules are non-polar

  e.g. fats, cholesterol
• Amphiphilic = substances that have mixed properties

  e.g. long chain fatty acids, bile salts, phospholipids
• Rate of diffusion depends on:

  • concentration gradient
  • area available for diffusion
  • molecular mass of the solute
  • diffusion coefficient
• Osmosis = the movement of water through a semipermeable membrane that permits the passage of the water but not the solute particles
• Osmotic pressure = a hydrostatic pressure sufficient to stop this flow of water

  = MRT (molarity X universal gas constant X absolute temperature)
• Osmolarity = moles solute particles per litre of solution

  Osmolality = moles solute particles per kg of water

  rather than measuring osmotic pressure directly
• Iso-osmotic = two solutions with the same osmotic concentration

  Isotonic = when intracellular fluid has the same osmolality as the extracellular fluid

  • if a fluid is isotonic, it is iso-osmotic
  • not all iso-osmotic solutions are isotonic
• Fick's law: Amount moved = diffusion coefficient X area X concentration gradient

  • diffusion coefficient is negative as diffusion occurs from high to low
  • diffusion coefficient becomes smaller as molecular size increases as large molecules diffuse more slowly than small ones
• Osmole = unit of osmotic concentration (number of moles solute that contribute to the osmotic pressure of a solution
• Factors determining the direction of transport across the cell membrane:

  • concentration gradient
  • charge of the molecule
  • membrane potential
• Ion channels:

  • passive diffusion of selective ions
  • high capacity for transport
  • named after the principal ion they transport
• Carrier proteins - transmembrane proteins which transport ions and small organic molecules across the plasma membrane by facilitated diffusion
• Uniport = one molecule passes at a time through a carrier protein

  Symport = this molecule passes with a cotransported ion in the same direction

  Antiport = this molecule passes with a cotransported ion in the opposite direction
• Primary active transport uses ATP from hydrolysis

  Secondary active transport uses an electrochemical gradient
• Exocytosis = secretion of molecules

  • constitutive - when cells release components or deliver newly-synthesised membrane proteins
  • regulated - triggered by a chemical or electrical signal, e.g. hormone release
• Endocytosis = absorption of molecules

  • phagocytosis - cell eating
  • pinocytosis - cell drinking
  • receptor-mediated transport - cytoplasm membrane folds inwards to form coated pits