transport across membranes

• diffusion is the net movement of molecules or ions from a region where they are more highly concentrated to a region where their concentration is lower until evenly distributed
• facilitated diffusion is the movement of charged ions and polar molecules by transmembrane channels and carriers. it is a passive process
• Fick‘s Law :

  • rate of diffusion is proportional to
  • (surface area x concentration difference)/ thickness of membrane
• channel proteins allow specific water-soluble ions to pass through the membrane
• a molecule binds to the carrier protein

  an inorganic phosphate ion (from ATP) phosphorylates the carrier protein, which causes it to change shape, allowing the molecule to pass through into or out of the cell
• osmosis is the movement of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane
• water potential of a solution is always less than zero
• water will move from an area of less negative water potential to an area with a more negative water potential.