3.2.3 Transport

• Membrane structure is described as a fluid-mosaic model. Phospholipids and proteins are able to move around, embedded proteins vary in structure and size.

  
• Cholesterol 


  Type of lipid

  Reduces membrane fluidity

  more cholesterol = less fluid

  Reduces permeability - it binds hydrocarbon tails together reducing space for diffusion
• Simple Diffusion

  Net movement of molecules down a concentration gradient.

  Passive process

  Small, non-polar, lipid soluble, not charged molecules
• Facilitated diffusion

  Transport through a channel or carrier protein, net diffusion down a concentration gradient.

  Larger, polar, water soluble, charged molecules
• Carrier and channel proteins are specific to their transport molecule:

  • The shape of their binding sites is only complementary to the shape of the molecule they transport - only this molecule can bind
  • The size/structure of their channel will only allow specific ions through
• Carrier and channel proteins can limit the rate of diffusion
• Osmosis 


  The net movement of water molecules from a region of higher water potential to a lower water potential, through a partially permeable membrane. Pure water has the highest potential of 0KPa
• Effect of osmosis on cells

  A) Bursts - cell lysis

  B) Shriveled

  C) Turgid

  D) Plasmalysed

  
• Active transport

  Molecules moved across a membrane against the concentration gradient using specific carrier proteins. Energy is required, made by the mitochondria during aerobic respiration.

  Larger, polar, water soluble, charged molecules
• Process of active transport:

  • Molecules bind to active site on carrier protein
  • Hydrolysis of ATP releases energy and phosphate group
  • Phosphate group binds to carrier protein causing it to change shape
  • Molecule is transported against gradient