Semi permeable membraneA semi-permeable membrane is a membrane which will allow the passage of certain molecules through it by the process of osmosis. The rate of osmotic diffusion depends on the pressure, concentration and temperature of the solvent on either side. Certain polymers are also semi-permeable, and will allow water and small molecules to pass through slowly.
A common example of a semi-permeable membrane is the phospholipid bilayer (plasma membrane) found in biological cells, and also around the mitochondrion inside biological cells. In this case the phospholipid bilayer will let molecules with a low molecular weight (relative molecular mass) through such as glucose (C6H12O6) and water (H20), but not larger and more important molecules such as proteins, dna, or carbohydrates which means that to transport protein or starch material would require them to be catalytically broken down into their monomer subunits.