biological membranes

• fluid mosaic model-

  fluid- phospholipids and protein molecules are free to move relative to each other within the layer (provide flexibility)

  mosaic - proteins are scattered between the phospholipids

  proteins -

  intrinsic- extend through both layers of bilayer

  extrinsic - proteins partly embedded in the bilayer, they sit in or out of cell
• phospholipids -

  form a bilayer- hydrophobic fatty acid tails that point inwards and hydrophillic phosphate heat point outwards

  • separating the aqueous region on the outside and the aqueous region on inside

  allow lipid soluble and non polar (and water) substances to enter or leave
• phospholipid

  prevent large, water soluble, and polar substances from entering or leaving the cell

  • forms a barrier
  • also makes membrane flexible
• cholesterol - steroid molecule

  small OH group is hydrophillic and interacts with hydrophillic phosphate head the rest interacts with hydrophobic fatty acid tails

  • cholesteral increased packing of membrane reducing fluidity
  • regulate fluidity and maintain stability
  • also helps as a barrier to prevent water and ions passing through easily by reducing permeability
• proteins

  intrinsic - transmembrane proteins that are imbedded through both layers of bilayer. amino acid on external surface is in contact with hydrophobic fatty acid tail as it has hydrophobic R group

  extrinsic - proteins present on one side of the bilayer only. they can be present in either layer so are found on the out or the inside of cell

  (normally hydrophillic R group)
• channel protein-

  used in facilitated diffusion

  • act as pores in membrane and allow specific ions through
  • they can be gated so only open and close in specific circumstances
• carrier proteins

  facilitated diffusion and active transport

  only allow large molecules to pass through

  • molecule binds to protein and it changes shape releasing molecule to other side of membrane
• glycolipid

  • phospholipid with carbohydrate chain
  • cell recognition
  • cell markers and antigen
• glycoprotein

  • proteins with carbohydrate chain
  • receptors
  • cell signalling
  • cell adhesion ( bind cells together to basement membrane)
• jobs of membrane (INSIDE)

  • cell membrane separates cell content from external environment
  • regulates transport of materials into and out
  • cell signalling
  • cell recognition
• cell recognition -

  • white blood cells have different shaped receptors in their plasma membrane that binds to self antigen and non self antigen
  • this allows the white blood cells to distinguish cells as being self or non self. the non self can be dealt with appropriately and be destroyed
• jobs of membrane (OUTSIDE)

  • compartmentalisation- membranes surround organelles separating organelle content and reactions
  • site of chemical reactions
  • provide attachment site for enzymes and pigments
  • form vesicles for the transport of proteins
  • allow cellular compartments to have different conditions
• cell signalling -

  process that leads to communication and coordination between cells so that they can work together and trigger a response
• cell signalling -

  target cells have receptors that are specific shape (tertiary) that is complementary shape to the signalling molecule

  • signalling molecule can be hormone or chemical mediator

  binding of signalling molecule and receptor triggers a response (often activates enzyme)
• cell signalling (insulin)

  1. blood glucose concentration increases. insulin is secreted from pancreas and carried in blood
  2. insulin binds to the receptor on liver or muscle cells
  3. this causes a chemical to be released which causes more glucose transport proteins to be inserted into membrane increasing facilitated diffusion of glucose

  binding of insulin and receptor also activates enzymes that convert glucose into glycogen for storage
• cell signalling (histamine cells)

  1. skin is damaged and mast cells bind to pathogens
  2. mast cells release histamine in local area but not blood so is a chemical mediator
  3. histamine binds to localised cell receptors
  4. triggering an increase blood flow to area and make capillaries more permeable to phagocytes
• cell signalling examples

  • amoeba
  • viruses
  • botox
• diffusion - the net passive movement of molecules down a concentration gradient from high to low concentration

  • passive - require ATP
  • happens in gas and liquid as have kinetic energy
  • continue until concentrations are in equilibrium
• diffusion -

  lipid soluble molecules

  small and non polar

  VERY small and polar (water)
• factors that affect diffusion -

  • temperature
  • concentration gradient
  • stirring motion
  • surface area
  • diffusion distance
  • size of molecule

  FICKS LAW
• facilitated diffusion- net passive movement of polar molecules or ions from an area of high to low concentration down a concentration gradient via a carrier or channel protein
• facilitated diffusion -

  carrier protein - large molecule binds to protein and it then changes shape releasing the molecule to the other side of membrane
• facilitated diffusion -

  channel proteins - transport specific ions and acts as pores. may be gated so only open and close in specific circumstances
• active transport - movement of molecules or ions from a low to high concentration against a concentration using ATP
• active transport - CARRIER PROTEIN

  • to ensure one way flow the molecule binds to the specific shape of protein
  • ATP energy is used and protein changes shape releasing molecule to other side
  • ensuring one way flow
• active transport -

  • carry molecule/ions one way
  • require ATP
  • against concentration gradient
  • faster rate then diffusion
  • molecules/ ions can accumulate inside or outside of cells
• endocytosis - bulk movement of substances into the cell using ATP.

  • cell membrane folds in when in contact with material
  • it engulfs the material (that’s too big to enter through the membrane)
  • membrane then fuses with itself to form a vesicle
  • vesicle pinches off and moves into the cytoplasm

  when solids are transported we call it phagocytosis and when liquids are transported we call it pinocytosis
• exocytosis- bulk movement of substances out of the cell using ATP.

  • vesicle is formed around secretory material
  • vesicle fuses with cell surface membrane
  • releasing secretory material outside of the cell
• osmosis - GCSE definition

  movement of water molecules from a dilute solution to a concentrated solution across a partially permeable membrane
• water potential - pressure of water on a membrane (kilopascals)

  • measure of the tendency of water molecules to diffuse from one place to another
  • the more solutes dissolved in water the lower the water potential because the water molecules will cluster around the solutes and are therefore not free to move. this reduces pressure the water molecules exert on the container
• osmosis - ALEVEL

  movement of water from a high water potential to a low water potential across a partially permeable membrane
• isotonic- the solution surrounding the cell has the same water potential as the cell
• hypertonic- the solution surrounding the cell has a lower water potential then the cell
• hypotonic - the solution surrounding the cell has a higher water potential then the cell
• cells in pure water

  • water moves in by osmosis
  • plant cells - cell wall prevents bursting, providing strength when under pressure (turgid)
  • animal cells- animal cell bursts (haemolysed)
• cells in a hypertonic solution

  • water moves out by osmosis
  • plant cells - membrane pulls away from wall as water leaves (plasmolysed)
  • animal cells - shrinks and appears wrinkled (crenated)
• factors that affect membrane fluidity

  • temperature
  • presence of alcohol - alcohol increases fluidity
  • number of cholesterol molecules - the more cholesterol the less fluidity
  • number of double bonds in fatty acid chain- more double bonds more kinks so can’t pack as closely so more fluid
  • the length of fatty acid chain - shorter the chain the more fluid
• temperatures affec on membrane fluidity-

  • when temp increases phospholipids have more kinetic energy and will move.
  • producing more gaps between phospholipids in membrane as they move further away
  • this increases permeability of the membrane making it easier for particle to cross it
  • carrier/channel/other proteins will also denature at high temperatures
• solvent and membrane structure -

  • water is a polar solvent essential to the formation of bilayer
  • some organic solvents are less polar like alcohol or non polar like benezene
  • these can dissolve membranes and disrupt cells
  • alcohol (non/less polar solvents) make membrane more permeable and fluid
• diffusion -