cell membranes

Cards (28)

  • Functions of cell membranes
    - Controls what substances enter and leave the cell
    - Holds the contents of the cell
    - Allows different conditions to be established between the inside and outside the cell
    - Only allows certain molecules through it (described as partially permeable)
  • Fluid-mosaic model
    -Fluid = molecules can move around within the membrane
    - Mosaic = due to the pattern produced by the proteins embedded throughout the membrane
  • Phospholipids
    - Makes the membrane flexible
    - Allow lipid-soluble and small, non-polar molecules to diffuse through
    - Prevents water soluble substances crossing the membrane
  • Cholesterol
    - Reduces the fluidity of the membrane by restricting the movement of other molecules
    - Makes the membrane more rigid and less permeable
    - Maintains shape of membrane
  • Functions of proteins in cell surface membrane
    - Transport
    - Enzymes
    - Antigens
    - Receptors
  • Channel proteins

    - Used in facilitated diffusion
    - Forms a channel to allow ions, polar molecules and large molecules to pass through the bilayer
  • Carrier proteins

    - Used in facilitated diffusion and active transport
    - Protein changes shape to allow molecule through
    - Return to normal once molecule has passed through
    - May need energy from ATP to change shape
  • Membrane bound enzymes
    eg disaccharides
  • Receptors
    - Bind complementary molecules to cause a response eg T cell receptors
  • Antigens
    - Used to determine if a cell is foreign to initiate an immune response
  • Glycoprotein
    - Carbohydrate bound to a protein
    - Carbohydrate acts as a recognition site
  • Glycolipid
    - Carbohydrate bound to a phospholipid
    - Carbohydrate acts as a recognition site
  • Effect of temperature on membranes
    - Low temperatures can make membranes less fluid which means membrane permeability becomes low
    - High temperature make lipids more fluid and membrane permeability is increased
    = Molecules vibrate as they have more kinetic energy
    - If temperature is too high it causes too much vibration and this results in rupture of the membrane
  • Diffusion
    - Passive net movement of particles from a region higher concentration to an area of lower concentration, down a concentration gradient
    - Doesn't require extra energy from the hydrolysis of ATP
    - Lipid-soluble molecules can diffuse through the phospholipid bilayer as they dissolve into the phospholipids
    - Water soluble and charged molecules are repelled by the hydrophobic fatty acid tails in the phospholipid bilayer
  • Facilitated diffusion
    - Passive net movement of larger, water soluble or charged molecule, down a concentration gradient, using channel or carrier proteins
    - Channel and carrier proteins are specific = specific shape and charge determine which specific molecules pass through (tertiary structure determines shape)
  • Why does the rate level off at high concentrations with facilitated diffusion?
    - As conc increases, the rate of absorption increases, so the concentration is the limiting factor
    - Then the rate levels off as all the carrier/channel proteins are in use. Number of carrier/channel proteins is now the limiting factor
    - If the graph does not level off, then this suggests the molecule is using simple diffusion through phospholipids
  • Active transport
    - Movement of particles from a region of lower concentration to a region of higher concentration using carrier proteins and energy from the hydrolysis of ATP

    Steps of active transport:
    - Molecule and ATP binds to the carrier protein
    - Carrier protein changes shape and transports the molecule across the membrane against the concentration gradient which requires energy from the hydrolysis of ATP
    - As molecule is released, carrier goes back to its original shape
  • Factors which affect the rate of diffusion
    - Size of the concentration gradient
    - Surface area the diffusion is occurring across
    - Thickness of surface diffusion is occurring through (diffusion pathway)
    - Number of carrier or channel proteins for facilitated diffusion
    - Temperature
  • Osmosis
    - Passive net movement of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient across a partially permeable membrane
  • Water potential
    - It is the pressure created by water
    - The more negative the value, the lower the water potential
    - Units: kPa
    - If solution has high water potential, this means it has a high free water concentration
    - Water potential of pure water is 0
  • Hypertonic, Hypotonic and Isotonic
    Hypertonic - solution has a lower water potential than inside the cell
    Hypotonic - solution has a higher water potential than inside the cell
    Isotonic - solution has equal water potential as inside the cell
  • Hypotonic solution on cells
    Animal cells:
    - Water moves into the cell by osmosis
    - Cell bursts (lysis) when too much water enters (called osmotic lysis)

    Plant cells:
    - Water moves into the cell by osmosis
    - Cell becomes turgid = do not burst due to cellulose cell wall
  • Hypertonic solution on cells
    Animal cells:
    - Water moves out of the cell by osmosis
    - Cell shrivels (crenation)

    PLant cells:
    - Water moves out of the cell by osmosis
    - Cell becomes flaccid
    - If large amounts of water leaves, cell membrane can come away from cell wall (plasmolysis)
  • Isotonic solution on cells
    - No net movement of water in or out of cells, no effect
  • Increasing the rate of diffusion across the membrane
    - Higher temperature = Increased kinetic energy of molecules so faster diffusion
    - Greater surface area = more phospholipids for diffusion and more carrier/channel proteins for facilitated diffusion
    - Higher concentration gradient = as conc difference increases, rate increases
    - More carrier/channel proteins = more proteins for molecules to pass through
  • Factors affecting membrane permeability
    - Temperature = Higher temperature make the phospholipids more fluid by increasing their kinetic energy
    - Very high temperature = Protein channels/carrier proteins denature, so are non-selective allowing any molecules through
    - Alcohol = dissolves phospholipid bilayer
    - Acid = Protein channels/carrier proteins denature, so are non-selective allowing any molecules through
  • Co-transporters
    - Bind two molecules at a time
    - Concentration gradient of one molecule is used to move the other molecule against its own concentration gradient
    - Type of facilitated diffusion
  • Co-transport of glucose/amino acids
    - Sodium ions are actively transported from the epithelial cells into the blood which maintains the sodium ion concentration gradient between the lumen of the small intestine and the epithelial cell

    - Glucose and sodium ions are co-transported from the lumen of the small intestine into the epithelial cells via a carrier protein (facilitated diffusion because sodium ions move down their concentration gradient)

    - Glucose molecules move into the blood via facilitated diffusion, down the concentration gradient