movement of substances

• diffusion

  • net movement of particles from an area of high conc to an area of low conc, down a conc gradient
  • diffusion passive so doesnt require energy- continues until equilibrium reached
• simple diffusion

  • diffuse between bilayer
  • small, non-polar molecules(can interact with non-polar tails)
• factors affecting rate of diffusion

  • steepness of conc gradient- greater different means greater difference in number of particles passing in 2 directions= faster
  • temperature- increased KE as higher temps, move faster so increased rate of diffusion
  • Surface area- increased SA means increased number of molecules that can cross at one time, increasing rate of diffusion

  -as cells increase in size, SA:V decreases which slows rate of diffusion as distance too big

  • properties of molecules/ions- large molecules diffuse slower than small, non polar quiker as they are soluble in non polar bilyer
• facillitated diffusion

  • passive
  • for large polar molecules eg glucose or amino acids
  • for ions eg ca+ or na+
  • need the help of transport proteins- channel and carrier
• channel proteins

  • allow charged substances eg ions to diffuse through cell membrane
  • most channel proteins gated
  • fixed shape
• carrier proteins

  • switch between 2 dhapes
  • binding sites of carrier protein to be open to one side of membrane first and then open to other side when protein switches shape
• active transport

  • movement of molecules/ ions through cell membrane from an area of low concentration to high concentration against the concentration gradient- required energy in form of ATP for respiration
  • required carrier proteins- specific to type of molecule/ion
  • energy required to allow carrier to change shape

  -eg inorganic ions form soil through root hair cells

  -eg absorption of products of digestion from small intestines into blood
• endocytosis(active)

  • large molecules eg proteins, lipids, carbs
  • cell surrounds substance with cell membrane
  • membrane engulfs substance and pinches off inside cell to form temporary vacuole
• exocytosis(active)

  • large substance eg hormones/ enzymes
  • vesicles with substance pinch off from sacs of golgi body
  • fuse with cell membrane to be released outside cell
• osmosis

  • net movement of water molecules from area of low solute concentration(weak) to an area of high solute concentration(strong) through a partially permeable membrane
  • down conc gradient
  • water molecules polar however can still move through bilayer due to their small size
• when cells placed in water

  • water moves into cell via osmosis
  • in animal cells- bursting
  • in plant cells- no bursting due to cell wall
• when cells placed in solution

  • water moement out of cell via osmosis
  • in animal cells- shrivel
  • in plant cells- vacuole and cytoplasm shrink but cell wall maintains overall shape