British Sign Language Glossaries of Curriculum Terms

BSL Environmental Science Glossary - Selective/Partially permeable membrane definition

Definition: A selective or partially permeable membrane is a membrane that will allow some molecules through, but not larger ones. Imagine a tank full of water, with a membrane down the middle, which is full of microscopic holes. Water can diffuse through the membrane, which is equal on both sides. Supposing on the right side, you put in a spoonful of sugar and another with salt. Salt is a small molecule, whereas sugar is a larger molecule. Agitating the solution on the right side of the tank helps the solution diffuse through the membrane. However, only the salt can diffuse through to the other side. The more giant sugar molecules cannot fit through the holes and remain on the right side of the tank. The left side of the tank has only dissolved salt, while the right side has a mixture of salt and sugar, the salt being in the same concentration on both sides.