2.5 - biological membranes

• What are the 4 main roles of biological membranes?

  Partially permeable barrier between the cell and its environment AND between organelles and the cytoplasm within cells

  
  

  Sites of chemical reactions

  
  

  Sites of cell communication (cell signalling)
• How/ why are biological membranes sites of chemical reactions?

  May contain chemicals that signal to other cells

  
  

  Membrane bound receptors on self surface are for these! (Which drugs and hormones may bind to)

  
  

  SO: site of cell signalling!
• How/ why are biological membranes sites of cell recognition?

  Have antigens for cell recognition on cell surface - so immune system recognises it as self and doesn't attack it
• Biological membranes and enzymes


  Involved in specific metabolic pathways e.g. villi
• What is the role of membranes within cells?

  Separates the organelle contents from the cell cytoplasm (DNA - protected by nuclear envelope)

  
  

  Transport inside cells - vesicles
• Role of membranes within cells - 2 examples

  Mitochondria - have fooled inner membrane called criste

  Gives large SA for some reactions of aerobic respiration, and contains membranes needed for this

  
  

  Chloroplasts - have inner membranes (thylakoid membranes) which house chlorophyll. On membranes - some photosynthesis reactions occur
• 2 conditions in order for a substance to pass through a biological membrane

  Small and non polar
• What substances can diffuse through biological membranes?

  V small ones - can simply diffuse through CM between structural molecules (CO2, H2O, O2)

  
  

  Other substances - need transport proteins

  
  

  NB some can dissolve in lipid layer and simply pass through
• Diagram of phospholipid bilayer

  
• Why is the fluid mosaic model fluid and a mosaic?

  Fluid - molecules move constantly

  Mosaic - lots of diff things!
• What is the structure of phospholipids?

  2 hydrophobic fatty acid chains (tails) and 1 hydrophilic phosphate group
• What is the function of phospholipids? (membranes)


  Acts as barrier to most water-soluble subtances subtances

  
  

  SO - water-soluble molecules (sugars, amino acids, and proteins) can't leave or enter
• What is the structure of transport proteins?

  Span whole lipid bilayer

  
  

  Membrane spanning region - interacts with hydrophobic region
• What is the function of transport proteins?

  Create hydrophilic channels to allow ions and polar molecules through
• What is the function of carrier proteins?

  Change shape (after binding to ion or polar molecule) to transport a substance across a membrane

  
  

  Involved in active and passive transport
• What is the function of channel proteins?


  Pore like

  
  

  Outer layer: hydrophilic R groups in contact with water based solute with ions - protects from hydrophobic tails of membrane

  
  

  Aquaporins - for water
• What is the structure of cholesterol?

  Lipid
• What is the function of cholesterol in membranes?

  Stablilise membrane fluidity

  
  

  Lower temps - inc fluidity so stops from becoming too rigid by stopping tails from packing too closely

  
  

  Higher temps - stops it becoming too fluid by binding hydrophibic tailes causing the phospholipids to pack closely together

  
  

  Increase mechanical strength of membrane (without would break down and burst)
• What is the structure of Glycolipids?

  Carbohydrate chain attached to lipid on cell surface
• What is the structure of glycoproteins?

  Charbohydrate chain attached to protein on cell surface
• What is the function of glycoproteins and Glycolipids?


  Receptor sites p bind with certain substances on cell's surface, and there are 3 main types...

  
  

  1. Signalling receptors for hormones and neurotransmitters

  2. Receptors involved in endocytosis

  3. Receptors involved in cell adhesion and stabilisation (carb bit forms H bonds with water molecules surrounding cell)

  
  

  Cell recognition sites/ cell markers or antigens for cell-to-cell recognition
• How do higher temperatures influence membrane structure?

  Phospholipds gain KE so become more fluid - reduces effectiveness of cell membranes SO - permeability increases (and polar molecules can pass through)
• How do higher temps influence rate of diffusion?

  Higher temps - diffusion faster due to increased KE
• Is the effects of higher temps reversible or not reversible on membrane structure and permeability?

  Reversible - if temps decrease lipids will return to their normal levels of fluidity
• What happens to biological membranes when temperatures goes to high?

  Above 40 degrees - proteins in membrane start to denature so no longer forms an effective barrier

  
  

  SO - permeability increases meaning that substances can pass through freely

  
  

  IRREVERSIBLE!
• What happens to cell membranes when temp decreases?

  KE decreases -> fluidity decreases

  
  

  SO - membrane becomes more rigid as phospholipids pack closer together
• What happens to cell membranes at 0 degrees and below?

  Ice crystals in membranes start to pierce the bilayer (e.g. frostbite)
• What are some responses to decreases in temperature? (Membranes) CHECK THIS

  Change components of cell membrane - helps organisms adapt to colder environments

  
  

  Types of fatty acid chains - help prevent membrane from becoming too rigid (unsaturated - cant pack as close due to 'kinks')

  
  

  More cholesterol- prevents close packing of phospholipids
• How do solvents affect membrane structure and permeability?

  Organic solvents (e.g. ethanol) damage membranes as they dissolve lipids, thus disrupting membrane structure.

  
  

  Loss of structure - increases fluidity SO is more permeable!!

  
  

  At v high concentrations - membrane is dissolved
• What is diffusion?

  The net movement of molecules down a concentration gradient (area of high concentration to low concentration)
• Is diffusion passive or active?

  Passive as no energy required, instead relies on KE of molecules
• Describe diffusion

  High concentration - increases collisions and so molecules spread further from each other

  
  

  So - more then move to an area of lower concentration until they become evenly dispersed (are then at equilibrium as there is no net movement)
• What molecules can diffuse by simple diffusion?

  Small and non polar molecules (e,g, O2 and CO2

  
  

  Lipid soluble molecules - can diffuse even if larger as can dissolve in lipid bilayer (e.g. steroid hormones - testosterone, progesterone, oestrogen etc)
• What is facilitated diffusion?


  Movement of molecules down a concentration gradient across a partially permeable membrane via protein channels or carriers. Doesn't involve ATP.
• How do small polar molecules diffuse?

  Are insoluble in lipid bilayer (as can't interact with hydrophobic tails)

  
  

  SO - need channel proteins (water filled pore through center with hydrophilic R groups facing inwards)
• How do larger polar molecules diffuse?

  Too large for channel proteins

  
  

  SO - need carrier proteins (molecule bonds and protein changes shape allowing molecule to pass through to other side of protein)

  
  

  PROTEINS ARE SPECIFIC!!!
• Is facilitated diffusion active or passive?

  Passive (down its concentration gradient with the help of a carrier protein)
• Can water diffuse?

  Yes - doesn't need channel protein as is small

  
  

  BUT - is inefficient process so in order to move lots at once (e.g. in collecting duct of nephron) aquaporins are required (water specific channel proteins)
• What factors affect rate of diffusion?

  Temp, diffusion distance, size of diffusing molecule, concentration gradient, and surface area
• How does temp affect the rate of diffusion?

  Higher temp -> molecules gain KE -> diffusion rate increases