Membranes ppt

• Cell surface membrane


  • Hydrophilic heads face outwards & are in contact with water
  • Hydrophobic tails face inwards forming an internal hydrophobic region which excludes water
• Glycoprotein


  Protein with carbohydrate part added
• Glycolipid


  Lipid with carbohydrate part added
• Membranes


  • Strong enough to keep the cell/organelle intact
  • Partially permeable to control which molecules move in/out
  • Flexible to allow a cell to change shape
• Fluid-mosaic model


  • Fluid - phospholipids and proteins are able to move around
  • Mosaic - embedded proteins that vary in structure & size
• Membrane components


  • Channel protein
  • Carrier protein
  • Glycoprotein
  • Glycolipid
  • Phospholipid bilayer
  • Cholesterol
• Cholesterol importance in cell membranes

  • Keeps membranes stable at body temperature (without it, cells could burst open)
  • Reduces membrane fluidity (more cholesterol = less fluid)
  • Reduces permeability by binding hydrocarbon tails together reducing spaces for diffusion
• Diffusion


  1. Net movement of molecules/ions down a concentration gradient (high to low)

  2. No extra energy/no energy from ATP is needed - it is a passive process
• Concentration gradient


  The difference in concentration between two regions e.g high conc. of particles on one end of a membrane compared to the other side

  -The difference between steep/not steep. The steeper the conc. gradient faster diffusion takes place
• Factors affecting rate of diffusion?increasing rates


  -Higher temp=increased KE of ions/molecules so an overall increased rate

  -Higher conc. gradient = increased rate

  -Increased surface area of the membrane (microvilli)= increased rate
• Facilitated diffusion


  Net diffusion down a concentration gradient through a channel or carrier protein
• Carrier and channel proteins are specific to their transport molecule/ion

  • The shape of their binding sites is only complementary to the shape of the molecule they transport
  • The size/structure of their channel will only allow specific ions through
  • The tertiary structure changes shape allowing the specific ions/molecules to pass through
• Concentration of molecule/ion outside cell- facilitated diffusion

  Rate of uptake into cell is limited by number of carrier/channel proteins
• Ways to increase rate of facilitated diffusion

  -Higher temp=increased KE of molecules/ions so overall increases rate

  -Higher conc. gradient= increased rate (up to limiting number of proteins)

  -Increased number of channel/carrier proteins= overall rate increases
• Osmosis


  The net movement of water molecules from an area of higher water potential to an area of lower water potential, through a partially-permeable membrane
• Water potential


  • What is it?-the ability of water molecules to cross a membrane, measured in kiloPascals (kPa)
  • Pure water has the highest water potential of 0 kPa
  • Solutions have a lower water potential than pure water and have negative water potential as solute molecules bind to water molecules reducing the number of free water molecules that are able to diffuse across the membrane
• Water potential


  The greater the amount of solute, the lower the water potential
• Direction of osmosis


  Water moves down a water potential gradient
• Types of cell that won't undergo lysis

  • Plant
  • Algal
  • Fungal
  • Bacterial
  • They all have a cell wall to prevent lysis
• Animal cell in different solutions

  • In hypotonic solution - lyses/bursts
  • In hypertonic solution - shrivels/crenates
  • Plant cell in hypotonic solution - becomes fully turgid, cell wall prevents lysis so doesn't burst
  • Plant cell in hypertonic solution - becomes plasmolysed (plasma membrane & cytoplasm peel away from cell wall) it loses water and becomes flaccid
• Active transport


  • Molecules/ions moved across a membrane against their concentration gradient using specific carrier proteins
  • Requires extra energy from ATP made by mitochondria during aerobic respiration
• Factors limiting rate of active transport

  -Number of carrier proteins

  -Lack of ATP

  -Any factor that reduces the rate of aerobic respiration
• Examples of active transport

  • Absorption of glucose & amino acids in the ileum
  • Absorption of mineral ions by plant roots
  • Excretion of hydrogen ions and urea by kidneys
  • Exchange of sodium and potassium ions in neurones and muscle cells
  • Cells that AT molecules/ions on a large scale have many mitochondria= ATP for energy in respiration)
• Bulk transport


  • When lots of a substance needs to be moved across a cell membrane, bulk transport is used
  • Phagocytosis is the bulk transport of material into the cell (endocytosis)
  • Secretory vesicles empty cell substances out of cells (exocytosis)
  • Involves changes to the membrane shape and vesicles
  • e.g enzymes for extra-cellular digestion & release of hormones from gland cells
• Cell membrane transport

  
• Properties of molecules and ions determine the way they cross membranes

  
• Carrier and channel proteins limit the rate of diffusion across a membrane

  
• Free water molecules can cross a membrane = higher water potential
• Few free water molecules= lower water potential
• Osmosis and water potential

  • Maximum value of water potential is zero
  • If solutes are present in the water they reduce the tendency of the water molecules to move
  • ^This is because some water molecules are attracted to the solute molecules and cannot now cross the membranes freely
  • When solutes are present in solutions or cells the water potential of the solution or cell is lower, so the water potential becomes a negative value
  • The more solute present the more negative the water potential comes
• osmosis

  If there is a steeper water potential then there's a faster rate of osmosis gradient
• Why does starch and glycogen not cause osmosis of water into the cells in which they are stored?

  They are both insoluble so will not affect/ lower the water potential of the cell
• Effect is osmosis on animal & plant cells

  
• Limiting rate of AT

  
• Active transport

  • Molecule/ion bind to the binding site on carrier protein (complimentary shapes)
  • Hydrolysis of ATP releases energy & releases a phosphate group
  • The phosphate group binds to carrier protein causing it to change shape (using energy)
  • Molecule/ion is transported against the conc. gradient
• • Diffusion= (using)molecules/ions high-low conc. down a conc. gradient
  • Facilitated diffusion=(using) molecules/ions through carrier OR channel protein, down a conc. gradient
  • Osmosis=(using)water down a water potential gradient, area of high-low water potential, through a partially permeable membrane
  • Active transport= (using)molecules/ions across a membrane, against conc. gradient using specific carrier proteins only
• Example of active transport

  • A phosphate group from the molecule ATP attaches to the carrier protein, causing it to change shape
  • As the protein changes shape, it 'pumps' the molecule or ion into the cell