diffusion

• diffusion

  = net movement of particles from a region of higher concentration to a region of lower concentration.

  • passive process and will continue until there's a concentration equilibrium between the two areas.
• • diffusion happens because the particles in a gas or liquid have kinetic energy. This movement is random and an unequal distribution of particles will eventually become an equal distribution.
  • particles move at high speed and are constantly colliding, which slows down their overall movement. So over short distances diffusion is fast, but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place.
• factors affecting rate of diffusion

  -temperature= higher temperature, higher rate of diffusion because the particles have more kinetic energy and move at higher speeds.

  -concentration difference= greater difference in concentration between 2 regions, the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration is larger.

  • simple diffusion= diffusion in the absence of a barrier or membrane.
• diffusion across membranes

  = involves particles passing through the phospholipid bilayer.- can only happen if the membrane is permeable to particles, eg; non-polar molecules like oxygen diffuse freely down concentration gradient.

  • the hydrophobic interior of membranes repel substances with a positive or negative charge, so they can't easily pass through. eg- water.
  • so membranes are described as partially permeable.
• rate of diffusion across membrane

  affected by:

  • surface area= the larger surface area of an exchange surface, the higher the rate of diffusion.
  • thickness of membrane= the thinner the exchange surface, the higher the rate of diffusion.
• facilitated diffusion

  = diffusion across a membrane through protein channels

  • membranes with protein channels are selectively permeable as most protein channels are specific to one molecule/ion.
  • facilitated diffusion can also involve carrier proteins, which change shape when a specific molecule binds.
  • down a concentration gradient.
  • rate is affected by temperature, concentration gradient, membrane surface area and thickness and number of channel proteins present= higher rate of diffusion.