transport across cell membranes (unit 2)

• structure of cell membrane


  phospholipid bilayer- contains proteins (intrinsic of extrinsic)

  intrinsic embedded in membrane- arrangement determined by hydrophilic and hydrophobic regions

  extrinsic found on outer/inner surface of membrane

  - basic structure (cell surface membrane and membranes around cell organelles of eukaryotes) is the same

  FLUID MOSAIC MODEL
• fluid mosaic model


  describes arrangement and movement of phospholipids, proteins, glycoproteins and glycolipids in membrane structure

  - cell membranes 'fluid'- phospholipids and proteins move around via diffusion

  - phospholipids mainly move sideways in their own layers

  - many diff proteins in bilayer move around within, some fixed in position

  cell membranes 'mosaic'- scattered pattern produced by proteins in phospholipid bilayer look like mosaic from above
• cell membrane diagram ...
• cholesterol in cell membrane


  helps stabilize the membrane fluidity- restricts movement of other molecules making up membrane
• movement across cell membranes occurs by- (list)


  simple diffusion, facilitated diffusion, osmosis, active transport and co-transport
• diffusion


  across cell membrane- net movement of molecules from high to low conc- down conc gradient

  eventually reach equilibriums. rate depends on factors:

  -conc gradient-steeper

  -temp- more KE faster

  -SA- greater SA more can move- faster

  -properties of molecules- larger are slower. uncharged and charged (cant) diffuse differently. non polar diffuse quicker- soluble in non polar PLB
• facilitated diffusion


  some substances (large polar molecules eg. glucose and amino acids, and ions eg. Na and Cl). and diffuse through PLB

  can only cross with channel or carrier proteins which are highly specific
• channel proteins


  water filled pores

  allow charged substances (ions) to diffuse through CM

  doesn't occur freely- most are gated meaning part of channel protein on inside surface of membrane can move to open/close pore

  allows channel protein to control exchange of ions
• carrier proteins


  can switch between two shales

  causes binding site of carrier protein to be open to one side of the membrane first and then the other side when carrier protein switches shape

  direction of movement of molecules diffusing across membrane depends on relative conc either side of membrane

  net diffusion of molecules in/out of cell is down conc gradient
• osmosis


  net movement of water molecules from high water potential (dilute) to low through partially permeable membrane

  water enters plant CM via osmosis.

  turgid - pressure plasmolysis- CM away from CW

  
  

  in animal cells - in low water potential solution water leaves through by osmosis and cell shrinks and shrivels (hypertonic- outside has higher solute)

  opposite is high WP and water enters- cell bursts (cytolysis)- no cell wall to withstand pressure

  hypotonic ( outside has lower solute)

  constant water potential inside nobodies of animals must be maintained (isotonic)
• active transport


  movement of molecules and ions through CM from low conc to high conc using energy from respiration

  needs carrier proteins (specific)

  energy needed to make carrier protein change shape allowing it to transfer molecules or ions across CM - energy from ATP -hydrolysed to energy
• co-transport


  coupled movement of substances across CM via carrier protein

  combo of facilitated diffusion and active transport

  eg. example of co-transport protein found on CSM of epithelial cells lining mammalian ileum - absorption of glucose

  Na ions and glucose molecules transported into epithelial cells via facilitated diffusion- FD reliant on conc gradient

  active transport of Na ions out of cell into blood helps maintain gradient

  glucose molecules exit epithelial cell and enter blood via facilitated diffusion
• co-transport diagram
• cells adapted for rapid transport across membranes by...


  inc in SA, inc in no. protein channels/carrier molecules in membranes
• specialised cells for diffusion


  root hair cells and epithelial cells of small intestine-

  RH- adapted for absorption of water and mineral ions from soil

  specialised shape to inc SA so greater uptake via osmosis

  thin walls so short diffusion distance

  permanent vacuole with cell sap- more conc than soil water ensuring high water potential gradient

  epithelial- microvilli to inc SA to inc rate of diffusion so products of digestion are greater

  each villus has constant blood supply continually transporting products of digestion away from epithelial cells maintaining conc gradient across epithelial cell exchange surface
• specialised cells for facilitated diffusion


  neurones, muscle cells and kidney cells

  neurones and muscle- involved in transmission of electrical impulses around body

  CM contains channel proteins for Na K and Ca ions

  opening and closing of channel proteins and no. proteins helps w speed of transmission along membranes of neurones and muscle cells

  certain kidney cells- membranes have high no. aquaporins - special channel proteins that allow FD of water through cell membranes

  allow kidney cells to re absorb water stopping it from being unnecessarily excreted by the body
• RP - production of dilution series of solute to produce calibration curve with which to identify water potential of plant tissue


  calibration curve- graph to determine unknown concentration of sample by comparing to set of standard samples with known conc

  water pot- tendency of water to diffuse from one area to another

  1. dilution series of 1M sucrose solution (0.0,0.2,0.4,0.6,0.8,1.0M) dilute w distilled water

  2. 5cm^3 of each dilution into separate test tubes

  3. cork borer to cut 6 potato chips and cut down to same size. dry using paper towel but don't squeeze

  4. weigh before start

  5. potato chip in each test tube and leave for 20,mins

  6. remove and dry gently then weigh

  7. calc % change in mass for each

  conc- potato chips in lower conc will inc mass, vice versa
• RP- effect of named variable on permeability of CSMs

  CSMs are PLB- selectively permeable which is changed by variables

  permeability of membrane measured using beetroot cells (pig betalain). more pigment leaks out of cells when high CSM permeability. measured using colorimeter

  1. cut beetroot into 6-10 identical cubes w scalpel

  2. wipe off any pigment released

  3. TEMP- each cubes into 10ml of distilled water

  4. each test tube in water bath at range of temps (30-80C)

  5. leave samples for 20mins- pigment will leak out

  6. measure absorbance for each solution - higher absorbance means higher pigment conc- therefore more permeable membrane

  conc- as temp inc, permeability of CSM inc. bc proteins in membrane denature as heat damages bonds in tertiary structure, creates gaps in membranes so easier for molecules to pass through

  low temp phospholipids have little energy and packed closely tg to make membrane rigid. causes dec in permeability and restricts molecules from crossing membrane