Why does a permeable membrane only lets certain molecules?
Getting through a permeable membrane depends on a couple different things. Size is one; some large molecules simply can’t fit through openings. Some channels through the membrane are specifically polar or non-polar and will therefore only let their respective type of molecule in. Other molecules bind to active transport proteins, which allow them in (this is how sodium/potassium pumps work). As to why…the membrane is there to keep unwanted stuff out and to regulate how fast other things come in and out. If too much of a certain molecule enters or leaves at a given time, the cell could die (take a look at hypertonic/hypotonic solutions). Factors: Solute concentration inside cell, solute concentration outside cell, and the presence of active transport proteins in the membrane.
