Cell Biology/Membranes/Semi-permeability and osmosis

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The membranes of cells are a fluid, they are semi-permeable, which means some things can pass through the membrane through osmosis or diffusion. The rate of diffusion will vary depending on the its: size, polarity, charge and concentration on the inside of the membrane versus the concentration on the outside of the membrane. When something is permeable it means that something can spread throughout, like (The perfume is permeating the room.). Here is a list of some molecules and how they relate to passing through the membrane without assistance, in other words, through diffusion:

Hydrophobic Molecules[edit]

  • O2 - Oxygen
  • N2 - Nitrogen
  • benzene

Small uncharged Polar Molecules[edit]

  • H2O - Water
  • urea
  • glycerol

Large Uncharged Polar Molecules[edit]

  • Glucose
  • Sucrose
  • nucletide


  • H+ - Hydrogen ion
  • Na+ - Sodium ion
  • K+ - Potassium ion
  • Ca2+ - Calcium ion
  • Cl- - Chloride ion

Various substances will pass through the membranes at varying rates through diffusion.