movement of molecules

• what kind of molecules can pass freely through the membranes

  -fat soluble organic molecules

  -this is because the interact with the fatty acid tails
• channel proteins 


  allow movement of some substances across the membrane. molecules of sugar such as glucose are too large and too hydrophilic to pass directly through the bilayer so have to enter and leave via channel proteins
• carrier proteins


  actively move some substances across the membrane
• passive transport doesn't require energy to move molecules across a membrane. molecules move down the concentration gradient
• small molecules can diffuse through the bilayer such as oxygen and carbon dioxide. they are also non-polar so won't be repelled by the hydrophobic fatty acid tails
• larger or polar molecules need to be moved across the membrane via facilitated diffusion.
• passive processes 


  simple diffusion and facilitated diffusion
• simple diffusion


  movement of molecules from an area of high concentration to an are of low concentration down the concentration gradient until an equilibrium is reached
• facilitated diffusion


  requires channel proteins to move molecules across the membrane. channel proteins are specific to one type of molecule and are gated- can be opened or closed
• small, non-polar molecules can move between phospholipids in bilayer. this is because they aren't being repelled by the fatty acid tails. simple diffusion
• facilitated diffusion


  large or polar molecules and substances. hydrophilic so will be repelled by the hydrophobic fatty acid tails that are non-polar
• net movement- overall movement of molecules in 1 direction. no net movement is when there is no overall movement in any one direction
• osmosis


  movement of water molecules down the water potential gradient from an area of high water potential to an area of low water potential across a partially permeable membrane
• pure water has the highest possible water potential. water with solutes dissolved into it has a negative water potential.
• aquaporins


  -specific channel proteins for water molecules
• water can pass through the bilayer despite its polarity- it is a small molecule
• hypotonic


  -high water potential
• hypotonic- animal cells


  -pure water or solutions with a high water potential can cause the cell to swell and eventually burst

  -this is because there is too high pressure for the membrane to cope with. this is because there is no cell wall

  -haemolysis
• hypertonic


  -concentrated- low water potential
• hypertonic- animal cells


  -cells become crenated- shrivel

  -cell lose water from the cytoplasm and therefore shrivel
• isotonic


  -balance between water and solutes. ideal state. no net movement
• hypotonic- plant cells


  -cells become turgid- expands yet won't burst

  -cell wall prevents the cell from bursting- cell contents push against wall. cellulose cell wall prevents it from bursting
• hypertonic- plant cells


  -lose alot of water from cytoplasm and vacuole

  -pulls the membrane away from the wall- gap fills with solutes

  -process is called plasmolysis and the cell becomes plasmolysed
• isotonic- plants


  -cell becomes flaccid- not shriveled but not plump or turgid
• active transport


  -requires cellular energy to move molecules and substances against the concentration gradient

  -ATP required

  -carrier proteins are complementary and can only carry things one way

  -faster rate than diffusion due to the use of energy

  -polar and or large molecules

  -carrier proteins can change shape- conformational shape and can return back
• endocytosis


  -moving things into the cell

  -phagocytosis and WBC
• exocytosis


  -moving substances out of the cell

  -protein production- enzymes, hormones, neurotransmitters
• endo and exocytosis are active processes so require energy to get cytoskeleton to move vesicles around the cell
• endocytosis


  -cell wraps membrane around substance to form a vesicle
• osmosis


  -passive process

  -movement of water molecules from a solution of high water potential to a solution of low water potential through a partially- permeable membrane
• active transport


  -bulk processes such as endocytosis and exocytosis also require energy

  -energy is needed to move vesicles around the cell
• carrier proteins


  -complementary to molecule being transported

  -carry specific molecules in 1 way

  -can carry molecules in opposite direction to concentration gradient

  -much faster rate than diffusion
• active transport 


  -molecule/ion to be transported combines with specific carrier protein

  -ATP transfers a phosphate group to the carrier protein on the inside of the membrane

  -carrier protein changes shape- conformational change

  -molecule/ion is carried to the other side of the membrane

  -when molecule/ion is released carrier protein returns to its original shape
• endocytosis


  -cell wraps plasma membrane around substance to form a vesicle

  -phagocytosis and pinocytosis
• all membranes are partially permeable to water as water is small enough to pass between the phospholipids in the bilayer. some membranes are up to 100x more permeable to water due to the presence of aquaporins- specialised channel proteins complementary to water molecules
• Water potential is the tendency of free water molecules to move from one place to another
• Fat-soluble organic molecules can diffuse through the bilayer but polar molecules require proteins
• channel proteins


  -hydrophilic channels bind to specific ions- one side of the protein closes and the other opens
• carrier proteins


  -binds to complementary molecule- conformational change releases molecule on other side of the protein.
• why can't glucose pass through a plasma membrane


  -phospholipids act as a barrier and glucose too large