Transport Across Cell Membranes

• Define diffusion

  The spreading out of particles of any substances in the form of a solution or gas, resulting in net movement from an area of high concentration to an area of low concentration
• How substances can pass through membranes

  • Active transport
  • Passive transport
• Active transport

  always uses ATP from respiration
• Passive Transport

  Never uses ATP from respiration
• Passive Transport relies on...

  • the random movement of particles
  • Diffusion
• Types of diffusion

  • Simple diffusion
  • Facilitated Diffusion
• Define simple diffusion

  Simple diffusion is where particles simply diffuse through the membrane
• Define facilitated diffusion

  Facilitated diffusion is where particles diffuse through transport proteins in the membrane
• substances that rely on facilitated diffusion to cross the membrane

  • Sucrose
  • Potassium ion
• ways substances can move across the cell-surface membrane into a cell

  • Simple diffusion of small non polar molecules down a concentration gradient
  • Facilitated diffusion down a concentration gradient via protein channel
  • Osmosis of water down a water potential gradient
  • Active transport against a concentration gradient via protein carrier using ATP
  • Co-transport of 2 different substances using a carrier protein
• features of a cell specialised for absorption

  • large number of carrier proteins: for facilitated diffusion or active transport
  • Large number of mitochondria: to release energy for active transport
  • Folded membrane: large surface area
  • Membrane-bound digestive enzymes: maintains concentration gradient for fast absorption
• Membrane structure

  • Phospholipid bilayer: allows diffusion of small, non polar, hydrophobic substances and prevents diffusion of large, polar, hydrophilic substances
  • Carrier proteins: allows for active transport, facilitated diffusion and co-transport
  • Transport proteins: control which particles diffuse by opening and closing in response to signals
  • rate of passive transport depends on the surface area to volume ratio, length of diffusion pathway, how steep the concentration gradient is, number of transport proteins
  • Cholesterol: affects fluidity and rigidity
• Similarities: diffusion and osmosis

  • Movement from a high concentration to low concentration
  • Passive processes
• Contrast: facilitated diffusion and active transport

  • Facilitated Diffusion involves channel or carrier proteins whereas Active Transport only involves carrier proteins
  • Facilitated Diffusion does not use ATP whereas Active Transport uses ATP
  • Facilitated Diffusion takes place down a concentration gradient whereas Active Transport can occur against a concentration gradient
• polar means hydrophillic

  non polar means hydrophobic
• a molecule being non polar means it will dissolve in the hydrophobic phospholipid bilayer