Crossing the Membrane

• Cell membrane

  boundary of the cell

  separates internal and external environments of the cell

  regulates the movement of substances in and out of the cell
• Components of a cell membrane

  phospholipid bilayer

  chanel proteins

  carrier proteins
• Phospholipid

  Hydrophilic head ~ likes water

  Hydrophobic tails ~ hates water
• Phospholipid bilayer


  semi-permeable around cell

  non-polar ~ will not allow polar (charged) substances through
• Channel proteins


  straw shaped ~ allows things straight through

  doesn't change shape
• Carrier proteins


  bind to the substance being transported

  change shape, return to original shape once molecule has been transported
• Transport proteins

  protein channel

  carrier protein

  both are specific to the molecule they transport
• Transport across a membrane

  Passive

  Active
• Passive transport


  no energy required

  movement of molecules DOWN a concentration gradient

  (high concentrated area wants to move to a low concentrated area NATURAL)

  diffusion, facilitated diffusion, osmosis
• Active transport


  requires energy

  movement of molecules AGAINST a concentration gradient

  (less concentration moving to a more concentrated area UNNATURAL)

  carrier proteins assist
• Concentration gradient


  the difference between concentration of molecules from one area to another

  • along/down = passive ~ no energy required ~ NATURAL ~ high > low
  • against/up = active ~ energy required ~ UNNATURAL ~ low > high
• Diffusion


  movement of molecules down a concentration gradient

  high concentration area to a low concentration area
• Charge, size & solubility

  small - non polar ~ diffuse through lipid bilayer (oxygen, carbon dioxide)

  large - polar ~ need assistance of transport proteins (glucose, potassium, calcium)
• Polar vs Non-Polar

  Polar = charged

  non-polar = not charged
• Facilitated diffusion


  the movement of molecules through transport proteins

  Occurs when molecules are:

  • large
  • polar/charged
• Osmosis


  the diffusion of water

  water moves in the direction of the area of the higher solute across a semi-permeable membrane
• Types of solutions

  Hypertonic

  Hypotonic

  Isotonic
• Hypertonic


  cells are surrounded by a solution of higher solute concentration than themselves

  the external solution is said to be hypertonic to the cells, causing the water to move out of the cells
• Hypotonic


  cells are surrounded by a solution of lower solute concentration than themselves

  the external solution is said to be hypotonic to the cells, causing water to move into the cells
• Isotonic


  the surrounding fluid and internal fluid are of equal concentration

  the external solution is said to be isotonic
• Lysis


  when animal cells are placed into hypotonic solutions and burst due to osmotic pressure

  (does not occur to plant cells due to cell wall)
• Crenation


  when animal cells undergo shrinkage after being placed in a hypertonic solution
• Turgid


  a plant cell is swollen and filled with water when placed in a hypotonic solution
• Flaccid 


  a plant cell has lost water due to being in a hypertonic solution, therefore the membrane moves away from the cell wall due to th eloss of water
• Facilitated diffusion vs Active transport


  FD is along the concentration gradient (high moves to low)

  AT is against the concentration gradient (low moves to high)
• Bulk transport


  movement of groups of molecules in or out of the cell that are too large to fit through transport proteins

  part of the membrane encapsulates the molecules and forms a vesicle that moves through the cell
• Types of movement through the cell

  Endocytosis

  Exocytosis