transport across membranes

• simple diffusion

  the net movement of molecules or ions from a region where they are more highly concentrated to where their concentration is lower until evenly distributed
• facilitated diffusion

  is a passive process

  involves channel and carrier proteins

  net movement from regions of high to low concentrations
• channel proteins

  allow specific water- soluble ions through. they are selective each to the presence of a specific ion. the ion binds with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it on the other
• carrier proteins

  span the plasma membrane. when a specific molecule is present it binds with the protein. this causes it to change shape in such a way the molecule is released inside the membrane
• osmosis

  the net movement of water from a region of high water potential to an area of low water potential through a selectively permeable membrane
• active transport

  the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
• co-transport

  1. sodium ions are actively transported out of the epithelial cells, by the sodium-potassium pump, into the blood. this takes place in a carrier protein
  2. this maintains a higher concentration of sodium ions in the lumen that epithelial cells
  3. sodium ions diffuse into the epithelial cells down the concentration gradient through a cotransport protein. as they do this they take glucose molecules or amino acids with them
  4. the glucose/amino acids are able to pass through to the blood via facilitated diffusion