Transport across membranes

• Structure of cell membrane

  • phospholipids
  • proteins
  • carbohydrates
  • cholesterol
• How is the phospholipid bilayer formed?

  hydrophilic heads are next to cytoplasm/tissue fluid and hydrophobic tails are not in contact with either aqueous solution, forming a bilayer
• Carbohydrates in membrane

  can attatch to proteins and phospholipids. found on outer surface only, involved in cell recognition
• Cholesterol in membrane

  type of lipid that restricts movement of phospholipids. makes membrane more stable and less fluid
• Which molecules can pass through phospholipid bilayer?

  small/lipid soluble by simple diffusion
• What are the two types of protein in membrane?

  intrinsic and extrinsic
• Protein in membrane

  • channels and carriers
  • receptors
  • enzymes
• Methods of transport

  • simple diffusion
  • facilitated diffusion
  • osmosis
  • active transport
  • co-transport
• Simple diffusion

  movement of particles from high to low conc, down conc gradient. passive process (no energy from ATP needed)
• Facilitated diffusion

  movement of particles from high to low conc, down conc gradient. passive process, requires a protein carrier/channel
• Active transport

  movement of particles from low to high conc, against conc gradient. requires energy from ATP and a protein carrier
• Co-transport

  2 substances are moved together using protein carrier
• Describe process of co-transport

  1. active transport of sodium ions from the epithelial cell into the blood lowers the sodium ion concentration inside the cell and generates a concentration gradient 
  2. sodium ions move into the cell from the ileum by facilitated diffusion, carrying glucose molecules along
  3. The glucose concentration inside the epithelial cell increases, and glucose molecules enter the blood by facilitated diffusion
• Osmosis

  movement of water molecules from high to low water potential, down water potential gradient, through a partially permeable membrane
• Water potential depends on two factors:

  • conc of solutes
  • external pressure
• Effect of ethanol on membrane

  dissolves phospholipid bilayer
• Effect of acid on membrane

  denatures proteins
• Method for potato cube osmosis practical

  • ensure all surfaces of cube are exposed to sucrose solution
  • control temperature - take readings at regular intervals using thermometer
  • pat cubes dry using paper towel before measuring mass
  • measure mass of cubes at regular time intervals - every 5 mins
  • measure initial and final mass of cubes
• Method for beetroot colour practical

  • rinse beetroot to remove any pigment released during cutting
  • same volume of water at different temperatures - take multiple readings at regular intervals using thermometer/water bath
  • use colorimeter to measure light absorbance