Transport across membranes

• Describe the fluid mosaic model

  fluid: flexible phospholipid bilayer

  mosaic: proteins scattered through structure
• explain the role of cholesterol and glycolipids in membranes

  cholesterol: steroid molecule - connects phospholipids and reduces fluidity to make bilayer more stable

  glycolipids: cell signalling and cell recognition
• explain the functions of extrinsic and intrinsic proteins in membranes

  extrinsic:

  • binding sites
  • antigens
  • binding of cells
  • involved in cell signalling

  intrinsic:

  • electron carries
  • channel proteins
  • carrier proteins
• name and explain 3 factors that affect membrane permeability

  1. temperature - denatures proteins / phospholipids in membranes
  2. pH - changes tertiary structure of membrane proteins
  3. solvent - may dissolve the membrane
• define osmosis

  the net movement of water molecules across a semi-permeable membrane from and area of high to low water potential
• what is water potential

  the pressure created by water molecules
• define simple diffusion

  • passive process - doesn't require energy
  • the net movement of gas particles from an area of high to low concentration gradient
• define facilitated diffusion

  • passive process
  • specific channel or carrier proteins with complementary binding sites transport large or polar molecules down a concentration gradient
• how do channel and carrier proteins work

  channel: hydrophilic channels bind to specific ions -> one side of the protein closes and the other opens

  carrier: binds to complementary molecule -> shape change releases molecule on the other side of the membrane
• name 5 factors that affect the rate of diffusion

  1. temperature
  2. diffusion distance
  3. surface area
  4. size of the molecule
  5. difference in concentration
• how are cells adapted to maximise the rate of transport across their membranes

  • many carrier / channel proteins
  • folded membrane increases surface area
• Define active transport

  • active process: requires energy for carrier protein to change shape
  • specific carrier protein transports molecules from area of low to high concentration
• Define co-transport

  the movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient
• What effect does the active transport of $Na^+$ ions have on their concentration gradient in epithelial cells?


  It lowers the concentration gradient
• What is the role of co-transport in the absorption of glucose and amino acids in the small intestine?

  It facilitates the movement of glucose and amino acids into cells
• How do $Na^+$ ions move from epithelial cells into the blood?


  By active transport
• How do $Na^+$ ions enter epithelial cells from the lumen of the small intestine?


  Through a co-transport protein by facilitated diffusion
• What happens as $Na^+$ ions diffuse through the co-transport protein?


  They carry glucose into the cell
• What is the concentration of glucose in epithelial cells after $Na^+$ ions have entered?


  High concentration of glucose
• How does glucose pass from epithelial cells into the blood?

  By facilitated diffusion over a different carrier protein
• What type of transport is used for glucose to enter the blood from epithelial cells?

  Facilitated diffusion
• What is the significance of the co-transport mechanism in nutrient absorption?

  It allows efficient uptake of essential nutrients