Question

A membrane which limits the type of molecules which will pass through it is called .

Answer 1

semi permeable

Answer 2

also called selectively permeable membrane

Answer 3

The phospholipid bilayer is semipermeable, meaning in cases other than bulk transport-endocytosis&exocytosis-, it does not compromise its own integrity to allow molecules to enter. The molecules that can most easily diffuse through a cell membrane are small, nonpolar molecules such as N2, O2, and CO2. Ions and polar molecules will have a difficult time crossing the membrane even if they are small because of the middle, hydrophobic layer. They can still pass through, just not as easily. Large molecules, however, cannot pass through the membrane due to their size, regardless or their polarization, and so they rely on those proteins embedded in the bilayer to transport them across (or, in some cases, endocytosis, which is when the cell membrane forms a kind of pocket - looks like a little mouth- and just engulfs the molecules). In this way the proteins are the only gap in what you could imagine as a sort of very fine and very picky filter. Larger molecules cannot get through the mesh of the filter and so they need to be recognized and passed through by those helpful little proteins

Read more: http://wiki.answers.com/Q/Why_do_large_molecules_have_difficulty_crossing_the_phospholipid_bilayer#ixzz2Anc9JmOX