FORCES PRODUCING MOVEMENTS OF WATER AND OTHER MOLECULES

• Forces producing movements of water and other molecules across membranes or barriers include:
• Diffusion:

  • Process by which a gas or a substance in solution expands due to the motion of its particles to fill all available volume
  • Movement of particles of a substance dissolved in a solvent are in continuous random movement
  • Tends to spread from regions of high concentration to regions of low concentration until uniform throughout the solution
• Solvent Drag:

  • When solvent moving in one direction drags along some molecules of the solute
• Diffusion is affected by electrical charge and permeability of the barrier
• Filtration:

  • Process by which fluid is forced through a membrane or other barriers due to a difference in hydrostatic pressure on the two sides
  • Molecules smaller in diameter than the pores of the membrane pass through with the fluid, while larger molecules are retained
• Osmosis:

  • Movement of solvent molecules across a membrane into a region with a higher concentration of a solute to which the membrane is impermeable
  • Pressure necessary to prevent solvent migration is called effective osmotic pressure of the solution
• Tonicity:

  • Term used to describe the effective osmotic pressure of a solution relative to plasma
  • Isotonic solutions have the same EOP as plasma
  • Hypertonic solutions have greater pressure
  • Hypotonic solutions have lesser pressure
• Active Transport:

  • Ions and larger non-ionized molecules are transported by carrier molecules in the membrane
  • Facilitated diffusion does not require energy when transport is from an area of greater to lesser concentration
  • Active transport requires energy when transport is from an area of lesser to greater concentration
  • Active transport is carried out by "pumps" and energy is supplied by ATP
• Endocytosis/Exocytosis:

  • Hormones and large polypeptides enter the cell by endocytosis and are secreted by exocytosis
  • Exocytosis involves proteins being secreted by cells moving from the endoplasmic reticulum to the Golgi complex, packaged into secretory granules, and then fused with the cell membrane to release contents outside the cell
  • Endocytosis is the reverse of exocytosis, with one form being phagocytosis where the cell engulfs material which is then enclosed in a vacuole mixed with lysosome digestive enzymes