cell membranes

• 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