movement of substances

• Concentration gradient:

  difference in concentration between two regions
• Direction of movement of particles:

  • particles of fluids (liquids and gases) possess kinetic (movement) energy, continually moving about and movement is random
  • particles will move (diffuse) down the concentration gradient and become evenly spaced out after some time

  Diffusion:

  • is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient
• Diffusion across a membrane:

  • permeable membrane allows both the solvent and solutes to pass through it
  • equal concentration of particles on both sides of membrane
• Factors affecting rate of diffusion:

  • concentration gradient
  • diffusion distance
  • surface area to volume ratio
• Diffusion distance:

  • time taken for a substance to move from one point to another depends on the distance between both points
  • the distance through which a substance diffuses 
  • shorter the diffusion distance, less time needed for substance to travel, higher rate of diffusion
• Surface area to volume ratio:

  • rate of movement of a substance across a cell membrane depends on how large the cell membrane is
  • ratio between surface area and volume of a cell
  • larger surface area, higher rate of diffusion 
  • greater surface area to volume ratio, higher rate which substances move in and out