diffusion

• the difference between the concentration of molecules in two places is a concentration gradient
• the molecules carry on diffusing until they are spread out evenly; the diffusion stops once there is no difference between the different concentrations
• diffusion: the net movement of molecules or ions from a region of high concentration to a region of low concentration down a concentration gradient
• waste substance can also be lost through diffusion
• the movement of molecules by diffusion across cell membrane is passive movement as cells do not need to use energy to move the molecules
• the energy for diffusion comes from the kinetic energy of random movement of molecules and ions; the more the molecules, the greater their kinetic energy and therefore their rate of movement
• Factors that affect diffusion:

  • distance molecules have to travel - cell membranes are very thin enabling a shorter distance for diffusion
  • concentration gradient - cells use the substances that diffuses as quickly as possible, so they keep a low concentration gradient in the cytoplasm, this means that molecules keep diffusing into the cell as there is a steep concentration gradient
  • temperature - molecules move faster and collide more often as the temperature increases ; diffusion is faster with higher temperatures
• water is a universal solvent
• main component of transport fluids: blood, xylem sap, phloem sap in plant